Integrative and Comparative Biology Advance Access published December 16, 2013
Integrative and Comparative Biology Integrative and Comparative Biology, pp. 1–9 doi:10.1093/icb/ict100
Society for Integrative and Comparative Biology
SYMPOSIUM
Effect of Body Mass on Hibernation Strategies of Woodchucks (Marmota monax) Stam M. Zervanos,1,* Christine R. Maher† and Gregory L. Florant‡
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E-mail: [email protected]
Synopsis The benefits of mammalian hibernation have been well documented. However, the physiological and ecological costs of torpor have been emphasized only recently as part of a hibernation-optimization hypothesis. This hypothesis predicts that hibernators with greater availability of energy minimize costs of torpor by less frequent utilization of torpor and by maintaining higher body temperatures (Tb) during torpor. In order to further examine the relationship between body mass and other parameters of hibernation, we present data, collected over a 12-year period, on the hibernation patterns of free-living woodchucks (Marmota monax) in southeastern Pennsylvania. Body mass was positively correlated with Tb and negatively correlated with percentage of the heterothermic period spent in torpor. Thus, woodchucks with greater mass exhibited less time in torpor as a proportion of their heterothermic period and at higher Tb than those with lesser mass. This strategy potentially enhances the physiological and physical ability of woodchucks to defend territories, avoid predation, find mates, and complete the reproductive cycle upon emergence from hibernation. Our results further support the hibernation-optimization hypothesis by demonstrating the relationship between body mass and characteristics of torpor and contributing toward a fuller understanding of this concept.
Introduction The ability to enter hibernation in winter enhances survival during shortages of food and water. Nonetheless, the adaptive value of undergoing torpor does not come without physiological and ecological costs, which include reduced sensory and motor function leading to increased potential for predation, reduced protein synthesis, sleep deprivation, and decreased immune response (Humphries et al. 2003b; Munro et al. 2005; Boyles et al. 2007). Mammalian hibernators balance negative aspects of torpor with energy-saving benefits by optimizing utilization of energy during the hibernation season (Boyles et al. 2007). This cost-benefit hypothesis or ‘‘hibernation-optimization hypothesis’’ (Boyles et al. 2007) suggests that to reduce the negative effects of torpor, hibernators with sufficient energy stores spend less time in torpor and more time at euthermic (378C) body temperature (Tb) (Humphries et al. 2003b).
Evidence supporting the hibernation-optimization hypothesis comes both from food-hoarding and fat-storing hibernators. Eastern chipmunks (Tamias striatus), which are food-hoarding hibernators, when provided with supplementary food before entering hibernation spent more time in euthermia and maintained higher Tb during torpor (French 2000; Humphries et al. 2003a). In another study, time spent in torpor was drastically reduced when the natural food supply for free-ranging chipmunks was high (Landry-Cuerrier et al. 2008). Similarly, among little pocket mice (Perognathus longimembris), time spent in torpor was inversely related to the quantity of food (French 1975). Composition of the food supply may also play a role in the expression of torpor (Geiser et al. 1994; Munro et al. 2005). When the diet of free-ranging chipmunks was supplemented with polyunsaturated fatty acid (PUFA), they spent significantly less time in torpor and exhibited higher Tb during torpor than did control
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*Department of Biology, Pennsylvania State University/Berks, PO Box 7009, Reading, PA 19610-6009, USA; †Department of Biological Sciences, University of Southern Maine, Portland, ME 04104, USA; ‡Department of Biology, Colorado State University, Ft. Collins, CO 80523, USA
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hibernation during winter and to commence reproduction in spring (Zervanos et al. 2010). We tested the hibernation-optimization hypothesis by analyzing the relationship between body mass and other parameters of hibernation in a free-ranging population of woodchucks in southeastern Pennsylvania over a 12-year period. Juvenile woodchucks in this population enter hibernation with a body mass of 2–3 kg, whereas adults enter hibernation with body mass of 3–5 kg, thus providing a continuous spectrum of body mass from 2 to 5 kg. We recognize that we have not measured fat mass directly; however, previous work suggests body mass and fat mass are correlated in woodchucks, (Snyder et al. 1961). We expect woodchucks with greater mass to spend a lower percentage of their heterothermic period in torpor and at higher Tb during torpor than those with lesser mass.
Materials and methods Study site The study was conducted over 12 hibernation seasons from October 1998 to March 2010 on the Pieffer Farm of the Pennsylvania State University’s Berks Campus located in southeastern Pennsylvania (75822’ W; 40822’ N). An estimated population of 55 woodchucks inhabits the farm, primarily along the ecotones of woodlots and fields (Zervanos and Salsbury 2003). Collection of data For the first five hibernation seasons (1998–2003), surgically implanted intraperitoneal temperature transmitters were used to monitor hourly Tb from free-ranging woodchucks (Zervanos and Salsbury 2001). During the 2003–2004 to 2009–2010 hibernation seasons, surgically implanted iButton temperature data loggers (MAXIM Inc., TX) were used to measure Tb hourly. All transmitters provided at least 0.28C resolution. iButtons provided 0.18C resolution and were recovered from recaptured animals in April of each year. Success of recovering loggers was 63%. During each hibernation season, three to six temperature transmitters or iButtons were placed 2–3 m into the burrows of our study animals to monitor burrow temperatures. A weather station (Model 900ET, Spectrum Tech., IL) located on the studysite monitored climatic conditions during the study. Over the 12-year period, 37 adult males, 6 juvenile males, 48 adult females, and 7 juvenile females were monitored. Times of immergence and emergence of study animals were documented by placing infrared motion-triggered cameras at entrances to burrows.
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animals that had access only to natural food supplies (Munro et al. 2005). However, some fat-storing hibernators showed the opposite pattern (Geiser and Kenagy 1987; Frank 1992; Florant et al. 1993). Golden-mantled ground squirrels (Spermophilus lateralis) and yellow-bellied marmots (Marmota flaviventris) had longer and deeper bouts of torpor when fed a high PUFA diet compared with a diet of saturated fatty acid (Geiser and Kenagy 1987; Frank 1992; Florant et al. 1993). In some fat-storing hibernators, if available stored fat, as measured by mass, is high, then torpor is reduced and a higher level of torpor Tb is maintained. For example, large mouse-eared bats (Myotis myotis) that were deprived of food spent more time in torpor than did those given food ad libitum (Wojciechowski et al. 2007). In another study, little brown bats hibernating in the warmer portions of caves had higher body mass than did those hibernating in cooler portions (Boyles et al. 2007; Jonasson and Willis 2011). Researchers similarly found that big brown bats (Eptesicus fuscus) with experimentally reduced availability of fatty acid chose significantly lower temperatures for hibernation than did controls (Boyles et al. 2007). These studies suggest that bats minimize the negative effects of torpor by selecting microclimates that allow them to hibernate at higher temperatures based on their level of stored fat. The hibernation-optimization hypothesis is also supported by a study on a non-hibernator, the golden spiny mouse (Acomys russatus). These desert-dwellers use torpor to survive shortages of food and water. When food is supplemented in their natural environment, golden spiny mice spend less time torpid and with fewer bouts of torpor. Therefore, they regulate the amount of torpor based on availability of energy (Levy et al. 2011). Thus, several studies lend support to the hibernation-optimization hypothesis. However, most of these studies have been conducted on either foodhoarders (e.g., chipmunks) or small, fat-storing species (e.g., bats). Woodchucks (Marmota monax) represent some of the largest fat-storing hibernators (Armitage 2003). Woodchucks distributed along a latitudinal gradient from Maine to South Carolina exhibit a direct relationship between latitude and total time spent in torpor (Zervanos et al. 2010). Animals in Maine spend 68% more time in torpor compared to South Carolinean animals. However, total time spent in euthermia does not differ among the three populations. Woodchucks in each population maximize time spent in euthermia, utilizing torpor only at the level needed to survive
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Hibernation strategies of woodchucks
Body mass at immergence was measured each year during October and November. Only data from animals whose body mass was measured over a 3-week period prior to the start of hibernation were used in this study. Analysis of data
Results Ambient and burrow temperatures Mean ambient temperatures during the entire 4month hibernation season did not differ significantly among the 12 years of the study (P ¼ 0.084; Table 1). Mean burrow temperatures also did not differ significantly among the 12 years (P ¼ 0.363; Table 1). Furthermore, yearly ambient and burrow temperatures were not correlated (r ¼ 0.252, P ¼ 0.174). Torpor Mean length of the heterothermic period for adult males and for adult females did not differ among years (P40.05). Over the 12 years, adult males had significantly shorter periods of heterothermy than did adult females (t ¼ 2.69; d.f. ¼ 78; P ¼ 0.017; Table 1). Juveniles did not differ from their adult counterparts. Juvenile males had mean (SE) duration of heterothermia of 108.3 9.4 days (adults ¼ 101.1 4.3 days; N ¼ 6 juveniles and 37 adults; P ¼ 0.271), whereas juvenile females averaged 124.4 5.9 days (adults ¼ 119.3 4.0 days; N ¼ 7 juveniles and 48 adults; P ¼ 0.211). Juvenile males and females did not differ in duration of heterothermia (P ¼ 0.187), but sample sizes were small (N ¼ 6 and 7, respectively). A similar pattern was observed for the dates of first torpor. Males entered their heterothermic period significantly later than did females, on mean ( SE) days-of-year 320.6 5.6 and 305.8 3.6, respectively (t ¼ –2.94; d.f. ¼ 78; P ¼ 0.009). Again, juveniles did not differ from adults. Male juveniles had mean first torpor dates of 326.7 16.5 (N ¼ 6; P ¼ 0.304; compared to adult males), whereas juvenile females averaged 315.8 11.4 (N ¼ 7; P ¼ 0.158; compared to adult females). However, juvenile males
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Data on body temperature were analyzed via an Excel (Microsoft Corporation, Redmond, WA) spreadsheet program. Only data from multi-day bouts of torpor 424 h were used. We defined deep torpor as the period of low stable Tb5158C. To measure duration of deep torpor, we adopted criteria used by Wabmer and Wollnik (1997). Time of entry into deep torpor (end of euthermia) began when Tb fell to 308C. Time of arousal from deep torpor (beginning of euthermia) began when Tb rose to 308C. Duration of torpor was measured from the time of entry into torpor to the time of arousal from torpor. Duration of the interval between bouts of euthermia was the time measured between the end of one bout of torpor to the beginning of the next bout. Mean duration of bouts for each animal was calculated from all durations of bouts during the heterothermic periods. The percentage of the heterothermic period spent in torpor for each animal was determined by summing the total duration of bouts of torpor and the total duration of bouts of euthermia and calculating the percentage based on the total duration of the heterothermic period. We defined ‘‘hibernation season’’ as the period between fall immergence and spring emergence and ‘‘heterothermic period’’ as the interval between the date of the first bout of deep torpor in autumn to the date of arousal from the final bout of deep torpor in spring. First torpor was defined as the date and time when Tb reached 308C during entrance into the first bout of deep torpor. Final arousal was defined as the date and time when Tb reached 308C during arousal from the last bout of deep torpor. Torpor Tb was the mean of the lowest Tb reached during each bout of the heterothermic period. Mean duration of bouts of torpor, mean duration of bouts of euthermia, mean torpor Tb, and mean number of bouts of torpor were calculated for each animal during its period of heterothermy. These four dependent variables were analyzed in separate threeway analyses of variance (ANOVA) where sex, year, and body mass were designated as main effects in a general linear model. The same four dependent variables were used in a two-way ANOVA for age and sex effects. In addition, mean burrow temperatures from each burrow during each hibernation season
were used in a one-way ANOVA to determine year effects. Significance was assigned based on 50.05. Significant results were examined in post hoc tests using the Tukey–Kramer multiple comparisons method. All ANOVA and post hoc tests were performed using Minitab statistical software (Release 13 for Windows, Minitab Incorporated, State College, PA). We compared sexes and age classes using two-sample t-tests. We examined relationships of body mass to bout lengths of torpor, torpor Tb, number of bouts of torpor, and percentage of the heterothermic period spent in torpor by calculating Pearson correlation coefficients and least squares linear regressions (Statistics 9, Analytical Software, Tallahassee, FL). Significance was assigned based on 50.05.
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S. M. Zervanos et al.
Table 1 Mean SE ambient temperatures, burrow temperatures, and lengths of periods of heterothermia for adult woodchucks over a 12-year period at Pieffer Farm, Pennsylvania.
Year
Mean ambient temperature (8C)
Mean burrow temperature (8C) (N)
Duration, in days, of heterothermia in males (N)
Duration, in days, of heterothermia in females (N)
1998–1999
3.4 0.3
7.3 0.2 (4)
109 7.1 (2)
118 1.6 (7)
1999–2000
2.8 0.5
7.7 0.2 (4)
103 8.2 (3)
121 1.8 (7)
2000–2001
4.2 0.4
7.0 0.5 (4)
101 6.6 (3)
113 3.4 (4)
2001–2002
4.3 0.5
7.4 0.6 (3)
103 5.2 (4)
117 2.2 (3)
2002–2003
4.5 0.9
6.7 0.7 (3)
100 7.4 (3)
119 2.8 (4)
2003–2004
3.6 0.7
7.1 0.6 (4)
102 5.8 (4)
121 4.5 (4)
2.6 0.6
7.2 0.4 (6)
99 2.3 (4)
122 8.9 (3)
3.3 0.8
7.1 0.6 (6)
101 4.6 (3)
119 6.5 (4)
2006–2007
3.2 0.6
7.5 0.3 (5)
98 4.9 (3)
128 7.3 (3)
2007–2008
2.8 0.9
6.9 0.5 (4)
109 8.0 (2)
116 6.4 (3)
2008–2009
2.3 0.6
6.3 0.7 (4)
94 7.3 (3)
114 7.6 (3)
2009–2010
6.1 1.1
7.7 0.6 (3)
98 6.6 (3)
117 7.2 (3)
Mean
3.5 1.1
7.1 0.47
101 4.3
119 4.0
Data for mean temperatures were collected from November 1 to February 28 of each year. Sample sizes refer to number of burrows or number of woodchucks.
and juvenile females did not differ significantly from each other (P ¼ 0.102). The dates of last arousal were not significantly different among the four groups (P ¼ 0.630). Adult males ended their period of heterothermia on Day 56.9 4.9 and adult females on 59.1 5.5, whereas juvenile males ended on 62.3 9.9 and juvenile females on 61.3 11.6. During periods of heterothermia, bouts of torpor and euthermia followed the pattern described by Zervanos and Salsbury (2003). We found interactions between mean burrow temperature and the mean durations of bouts of euthermia and of torpor. By using only data from burrow temperatures between 6.58C and 7.58C, interactions became non-significant for both durations (P ¼ 0.196 and P ¼ 0.439, respectively). Sample sizes were reduced by removing animals whose body mass was measured earlier than 3 weeks prior to hibernation. We used these reduced data for all subsequent analyses (16 adult males, 5 juvenile males, 22 adult females, and 7 juvenile females). No significant interactions of sex and year were found for body mass, duration of euthermia, Tb, or percent time spent in torpor (P40.05). Thus, data from all 12 years were combined for further analysis (Table 2). Adult males had shorter duration of bouts of torpor and fewer number of bouts than did adult females (duration of bouts: t ¼ 2.28, d.f. ¼ 35, P ¼ 0.030; number of bouts: t ¼ 4.38, d.f. ¼ 35, P ¼ 0.001; Table 2). Juvenile duration of bouts of
torpor and number of bouts of torpor were not significantly different from those of adult females (P40.05; Table 2). Duration of euthermia bouts did not differ according to age or sex (P40.05; Table 2). However, adults exhibited higher torpor Tb than did juveniles (t ¼ 2.10, d.f. ¼ 18; P ¼ 0.047), and adults tended to spend less percent time in torpor than did juveniles, although the difference was not statistically significant (t ¼ –3.14, d.f. ¼ 28, P ¼ 0.064; Table 2). Body mass prior to immergence and mean duration of bouts of torpor were inversely correlated in males (r ¼ –0.483, N ¼ 21, P ¼ 0.036), but not in females (r ¼ –0.046, N ¼ 29, P ¼ 0.811). In addition, body mass was not correlated with number of bouts of torpor in females (r ¼ –0.146, N ¼ 29, P ¼ 0.441), whereas in males, heavier animals had fewer bouts of torpor (r ¼ –0.465, N ¼ 21, P ¼ 0.045). For both sexes combined, body mass was positively correlated with duration of bouts of euthermia (r ¼ 0.399, N ¼ 50, P ¼ 0.006) and with torpor Tb (r ¼ 0.48, N ¼ 50, P ¼ 0.001; Fig. 1). Finally, body mass was negatively correlated with percentage of the period of heterothermic spent in torpor (r ¼ –0.535, N ¼ 50, P ¼ 0.001; Fig. 1).
Discussion Support of hypothesis To conserve the greatest amount of energy, we would expect a mammalian hibernator to utilize torpor
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2004–2005 2005–2006
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Hibernation strategies of woodchucks
Table 2 Comparison of characteristics (mean SE) of male and female woodchucks during periods of heterothermia, 1998–2010, at Pieffer Farm, Pennsylvania. Males Body mass (kg) Duration of bout of torpor (h)
Females
Juveniles (N ¼ 5)
Adults (N ¼ 16)
Juveniles (N ¼ 7)
Adults (N ¼ 22)
2.15 0.16a
4.18 0.10b
2.23 0.13a
3.90 0.12b
128.40 6.65b
154.04 9.94a
152.97 7.53a
a
a
42.86 4.67a
152.32 15.37a a
Duration of bout of euthermia (h)
32.32 6.19
40.55 5.18
Torpor Tb (8C)
10.23 0.72a
12.32 0.47b
9.91 0.76a
11.27 0.35b
a
b
a
13.40 0.48a
0.83 0.03a
0.76 0.01a
Number of Bouts of Torpor Percent of period of heterothermic spent in torpor
12.33 0.61
0.85 0.03a
10.38 0.42
0.77 0.02a
32.58 6.20
13.90 1.08
Different letters denote significant group differences within each characteristic (P 0.05).
extensively and to maintain a low torpor Tb, thus, reducing expenditure of endogenous energy. Our results, however, indicate that this was not the case. Woodchucks with higher body mass spent less time in torpor and more time in euthermia as a percentage of their total period of heterothermia than did animals with lower body mass. Heavier animals also maintained a higher torpor Tb. Why did they not
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Fig. 1 Relationship of (a) body temperature during torpor (r2 ¼ 0.2202, F ¼ 13.84, P ¼ 0.0005) and (b) percentage of time spent in torpor to body mass (r2 ¼ 0.282, F ¼ 13.39, P ¼ 0.0001) during the period of heterothermia in free-ranging woodchucks over 12 hibernation seasons (1998–2010) at Pieffer Farm, Pennsylvania. Red markers represent females; blue markers males.
conserve as much energy as possible to emerge with greater potential for reproduction and survival, i.e., what is the fitness value of less time in torpor and higher torpor Tb? Physiological and ecological costs of torpor may play a role (French 1975; Humphries et al. 2003b; Munro et al. 2005; Boyles et al. 2007). For example, negative effects of torpor on cellular functions are reversed during arousals (Carey 1993; Grigg and Beard 2000; Humphries et al. 2003b). Torpor inhibits the immune system, whereas euthermy restores its benefits (Burton and Reichman 1999; Prendergast et al. 2002; Luis and Hudson 2006). Other benefits include reduced muscle atrophy and improved development of reproductive organs, especially for males (Millesi et al. 2000; Humphries et al. 2003b; Zervanos 2003; Zervanos and Salsbury 2003). Furthermore, hibernation may have evolved from sleep (Heller and Ruby 2004). Thus, sleep and hibernation may lie on a continuum that provides a mechanism for conservation of energy (Heller and Ruby 2004). As such, animals that can lower their energy costs by sleeping, which includes a reduction in Tb, can lower their energy costs when environmental food supplies are low or non-existent. However, during bouts of deep torpor 5258C, sleep does not occur. Sleep only occurs during arousals (Larkin and Heller 1996). When animals arouse from torpor, they are in a sleep state (Barnes et al. 1993; Larkin and Heller 1996, 1998). Although its function is unknown, sleep is required for normal physiological processes to continue. Thus, getting sufficient amounts of sleep during the period of euthermia prior to re-entering torpor is important. Woodchucks with larger body mass may remain euthermic longer, thereby maximizing the amount of sleep and enhancing its associated physiological functions.
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reflected in our results, bears may be part of a continuum of increasing torpor Tb with increasing mass. Sex and age strategies Duration and number of bouts of torpor during the period of heterothermia for male and female juveniles exhibited similar patterns to those for adult female woodchucks. Adult males exhibited fewer and shorter bouts of torpor. Male and female juvenile woodchucks both had lower torpor Tb and spent a higher percentage of time in torpor than did adults. These relationships appear to be related to the difference in mass between juveniles and adults and to the hibernation strategies of each group (French 1986; 1990). Juveniles have higher mass-specific metabolic rates and higher thermal conductance (French 1990); thus, their hibernation strategy is to conserve energy by spending a higher proportion of their period of heterothermia in torpor and at lower Tb. Although surviving winter is the main objective, about one-third of the juvenile females that are heavier at emergence in this population of woodchucks, reproduce as yearlings (Zervanos, unpublished data). For adult females, the strategy is to survive hibernation with enough energy reserves at emergence to mate, to produce, and then to nurse young, until more sufficient forage becomes available. Although adult females utilize the same proportion of their period of heterothermia in torpor as do males, their longer bouts of torpor bouts and longer periods of heterothermia conserve the energy needed for reproduction (Zervanos and Salsbury 2003). Reproductive strategies of adult males require that they emerge earlier to complete spermatogenesis, defend territories, find females, and mate. Once mating is over, they need to survive a short period until spring forage is available. With shorter bouts of torpor, they utilize more energy, over a shorter period of heterothermia, than do females. However, they are able to still emerge with sufficient reserves of energy to successfully complete reproduction (Zervanos and Salsbury 2003). Possible mechanism Two inherent assumptions of the hibernation-optimization hypothesis are: (1) animals must be able to endogenously and externally estimate the length of their coming hibernation and (2) animals must be able to continuously monitor fat reserves. Over the 12 years of this study the duration of heterothermia both for males and females did not vary significantly, suggesting duration of heterothermia may be genetically fixed (Zervanos et al. 2010). The other
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By reducing the amount of time spent in torpor and utilizing higher Tb during the hibernation season, woodchucks could also improve their overall fitness. In a previous study of this population, adult male woodchucks utilized euthermia more often and had higher torpor Tb than did adult females, which resulted in a 38% higher energy cost for males (Zervanos and Salsbury 2003). Counteracting those costs is greater physiological ability to defend territories, avoid predation, find mates, and complete spermatogenesis (Zervanos and Salsbury 2003). However, the risks of this behavior are high. If animals miscalculate the length or severity of the hibernation season, fat-storing hibernators cannot easily adjust their utilization of energy (Humphries et al. 2003b; Boyles et al. 2007). A mechanism that can predict the length of the hibernation season and measure the energy reserves would be required (see below). Few woodchucks breed as yearlings (Armitage 2003). Thus, for juveniles, gaining enough mass during the active season to survive hibernation would be more critical than reproductive success after emergence, and we expect that they would utilize more torpor based on mass (French 1990). Their smaller body mass puts them near the minimum energy required for hibernation (Armitage 1998). The observed level of torpor and Tb in our study may represent the minimum levels required to survive hibernation at our study site, particularly for the smallest individuals. Any level of stored fat above this minimum increases the flexibility to: (1) utilize less torpor and remain at a higher Tb, (2) retain fat reserves for the end of the hibernation season, and (3) utilize fat for reproduction. Our study was based on the use of a continuum of body mass from the smallest juveniles to the largest adults, and the inverse relationship of mass to mass-specific metabolic rate is reflected in our results. Larger animals could afford to utilize less torpor and maintain high Tb (French 1986; 1990). Interestingly, bears (Ursus spp.) have relatively high Tb during the hibernation period, but their metabolic rates are greatly depressed (Hellgren 1998; Tøien et al. 2011). The ability of bears to reduce metabolism during hibernation, yet maintain high Tb, results from their large mass of stored fat and their low thermal conductance (Tøien et al. 2011). In addition, bears do not arouse periodically from torpor but instead stay torpid throughout hibernation (Harlow et al. 2004). Arousal may not be necessary at higher Tb because much of the physiological costs of low torpor Tb are avoided. Finally, as
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component of the mechanism requires a way to gauge the amount of fat required to survive the hibernation season. One possible mechanism is to monitor the blood concentration of leptin. Leptin is released from white adipose tissue and is highly correlated with mass of fat (Ahima et al. 2000). Leptin appears to feedback to the hypothalamus and inhibit anabolic pathways, such as food intake in mammals. Previous studies of leptin concentration in woodchucks and yellow-bellied marmots found high levels of the hormone during fall and winter when mass of fat was greatest (Concannon et al. 2001; Florant et al. 2004; Florant and Healy 2012). Leptin decreases dramatically during late winter and early spring, when animals end hibernation and prepare for reproduction. Whether the hypothalamus changes its sensitivity to leptin-signaling during spring is not known for hibernators. However, the overall function of leptin during hibernation is currently an active area of research and may lead to further insights into its role as a mechanism for gauging energy reserves.
Conclusions
Potential effects of climatic change
Funding
Woodchucks show considerable variation in hibernation parameters not only within a population (this study) but also across populations (Zervanos et al. 2010). Based on comparisons of captive (Fenn et al. 2009) and free-living woodchucks (Zervanos et al. 2010), hibernation characteristics, such as duration of bouts of torpor and torpor Tb, exhibit a degree of phenotypic plasticity, whereas a characteristic such as the length of the period of heterothermia is more fixed and perhaps genetically based. Phenotypic plasticity has broader ecological and evolutionary implications, e.g., the potential for rapid adjustments in the face of climatic change (Boyles et al. 2011; Sheriff et al. 2011; Armitage 2013). For example, the plasticity in duration of bouts of torpor is inversely related to ambient temperatures in woodchucks (Zervanos et al. 2010). As temperature increases, duration of bouts of torpor would be expected to decrease, with animals spending a lower percentage of time in torpor. In contrast, the lengths of periods of heterothermia may require several generations to adapt to changing climate, through microevolution (Davis 2005; Lane 2012) and/or entrainment of the circannual cycle (Davis and Finnie 1975; Lane 2012). As woodchucks are widely distributed, they may not be threatened by climatic change immediately, although their overall range may constrict (Armitage 2013).
The Pennsylvania State University/Berks Faculty Research and Development Fund (to S.M.Z.); Lipid Foundation grant #645848 (to G.L.F.).
Using data from a 12-year study of woodchucks in Pennsylvania, we found support for the hibernationoptimization hypothesis, based on the significant correlation of mass to torpor Tb and percentage of time spent in torpor. These results further support the hypothesis by adding another fat-storing hibernating species, measured under field conditions, to our understanding of this concept. Given the broad geographic range of this species, future studies made under different, and changing, climatic conditions, can provide additional tests of the hypothesis.
We would like to thank the many students who have assisted with this study over the years. A special thank you to June K. Brown, research assistant to S.M.Z., for collecting and organizing much of the field data used in this study and to Mary Ann Mengel for her technical assistance. All animal procedures were approved by the Pennsylvania State University Animal Care and Use Committee (#20562).
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Heller HC, Ruby NF. 2004. Sleep and circadian rhythms in mammalian torpor. Annu Rev Physiol 66:275–89. Hellgren EC. 1998. Physiology of hibernation in bears. Ursus 10:467–77. Humphries MM, Kramer DL, Thomas DW. 2003a. The role of energy availability in mammalian hibernation: An experimental test in free-ranging eastern chipmunks. Physiol Biochem Zool 76:180–86. Humphries MM, Thomas DW, Kramer D. 2003b. The role of energy availability in mammalian hibernation: A costbenefit approach. Physiol Biochem Zool 76:165–79. Jonasson KA, Willis CKR. 2011. Changes in body condition of hibernating bats support the thrifty female hypothesis and predict consequences for populations with white-nose ssyndrome. PLoS One 6:e21061. Landry-Cuerrier M, Munro D, Thomas DW, Humphries MM. 2008. Climate and resource determinants of fundamental and realized metabolic niches of hibernating chipmunks. Ecology 89:3306–16. Lane JE. 2012. Evolutionary ecology of mammalian hibernation phenology. In: Ruf T, Bieber C, Arnold W, Millesi E, editors. Living in a seasonal world. Berlin: Springer. p. 51–61. Larkin JE, Heller HC. 1996. Temperature sensitivity of sleep homeostasis during hibernation in the golden-mantled ground squirrel. Am J Physiol 270:R777–84. Larkin JE, Heller HC. 1998. The disappearing slow wave activity of hibernators. Sleep Res Online 1:96–101. Levy O, Dayan T, Kronfeld-Schor N. 2011. Adaptive thermoregulation in golden spiny mice: The influence of season and food availability on body temperature. Physiol Biochem Zool 84:175–84. Luis AD, Hudson PJ. 2006. Hibernation patterns in mammals: a role for bacterial growth? Funct Ecol 20:471–7. Millesi ES, Huber S, Walzl M, Dittami JP. 2000. Follicular development and hibernation in European ground squirrels. In: Heldmaier G, Klingenspor M, editors. Life in the cold. Berlin: Springer. p. 285–92. Munro D, Thomas DW, Humphries MM. 2005. Torpor patterns of hibernating eastern chipmunks Tamias striatus vary in response to size and fatty acid composition of food hoards. J Animal Ecol 74:692–700. Prendergast BJ, Freeman DA, Zucker I, Nelson RJ. 2002. Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels. Am J Physiol 282:R1054–62. Sheriff MJ, Kenagy GJ, Richter M, Lee T, Tøien Ø, Kohl F, Buck CL, Barnes BM. 2011. Phenological variation in annual timing of hibernation and breeding in nearby populations of arctic ground squirrels. Proc Royal Soc B 278:2369–75. Snyder RL, Davis DE, Christian JJ. 1961. Seasonal changes in weights of woodchucks. J Mammal 42:297–312. Tøien Ø, Blake J, Edgar DM, Grahn DA, Heller HC, Barnes BM. 2011. Hibernation in black bears: independence of metabolic suppression from body temperature. Science 331:906–9. Wabmer T, Wollnik F. 1997. Timing of torpor bouts during hibernation in European hamsters (Cricetus cricetus L). J Comp Physiol B 167:270–9.
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Horwitz B, editors. Life in the cold: ecological, physiological, and molecular mechanisms. Westview: Bolder, Co. p. 24–35. Concannon P, Levac K, Rawson R, Tennant B, Bensadoun A. 2001. Seasonal changes in serum leptin, food intake, and body weight in photoentrained woodchucks. Am J Physiol 281:R951–9. Davis DE, Finnie EP. 1975. Entrainment of circannual rhythm in weight of woodchucks. J Mammal 56:199–203. Davis EB. 2005. Comparision of climate space and phylogeny of Marmota (Mammalia: Rodentia) indicates a connection between evolutionary history and climate preference. Proc Royal Soc B 272:519–26. Fenn AM, Zervanos SM, Florant GL. 2009. Energetic relationships between field and laboratory woodchucks (Marmota monax) along a latitudinal gradient. Ethol Ecol Evol 21:299–315. Florant GL, Ameednuddin HS, Rintoul DA. 1993. The effect of a low essential fatty acid diet on hibernation in marmots. Am J Physiol 264:R747–53. Florant GL, Healy JE. 2012. The regulation of food intake in mammalian hibernators: a review. Comp Physiol B 182:451–67. Florant GL, Porst H, Peiffer A, Hudachek SF, Pittman C, Summers SA, Rajala MW, Scherer PE. 2004. Fat-cell mass, serum leptin and adiponectin changes during weight gain and loss in yellow-bellied marmots (Marmota flaviventris). J Comp Physiol B 174:633–9. Frank CL. 1992. The influence of dietary fatty acids on hibernation by golden-mantled ground squirrels (Spermophilus lateralis). Physiol Zool 65:906–20. French AR. 1975. Selection of high temperatures for hibernation by the pocket mouse Perognathus longimembris: Ecological advantages and energetic consequences. Ecology 57:185–91. French AR. 1986. Patterns of thermoregulation during hibernation. In: Heller CH, Wang LCH, editors. Living in the cold: physiological and biochemical adaptations. New York, NY: Elsevier. p. 393–402. French AR. 1990. Age-class differences in the pattern of hibernation in yellow-bellied marmots, Marmota flaviventris. Oecologia 82:93–6. French AR. 2000. Interdependency of stored food and changes in body temperature during hibernation of eastern chipmunk Tamias striatus. J Mammal 81:979–85. Geiser F, Kenagy GJ. 1987. Polyunsaturated lipid diet lengthens torpor and reduces body temperature in a hibernator. Am J Physiol 252: R897–901. Geiser F, McAllen BM, Kenagy GJ. 1994. The degree of dietary fatty acid unsaturation affects torpor pattern and lipid composition of a hibernator. J Comp Physiol, B 164:299–305. Grigg G, Beard L. 2000. Hibernation by echidnas in mild climates: hints about the evolution of endothermy. In: Heldmaier G, Klingenspor M, editors. Berlin, Springer. p. 5–19. Harlow HJ, Lohuis T, Anderson-Sprecher RC, Beck TDI. 2004. Body surface temperature of hibernating black bears may be related to periodic muscle activity. J Mammal 85:414–9.
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Hibernation strategies of woodchucks Wojciechowski MS, Jefimow M, Tegowska E. 2007. Environmental conditions, rather than season, determine torpor use and temperature selection in large mouse-eared bats (Myotis myotis). Comp Biochem Physiol A 147:828–40. Zervanos SM. 2003. Timing of hibernation immergence and emergence in woodchucks (Marmota monax). In: Ramousse R, Allaine D, LeBerre M, editors. Adaptive strategies and diversity in marmots. Proceedings of the 4th International Conference on the Genus Marmota, Montreux, Switzerland. P. 157–64. International Network on Marmots (http://www.cons-:dev.org/marm/MARM/ EMARM/framarm/framarm.html).
9 Zervanos SM, Maher CR, Waldvogel JA, Florant GL. 2010. Latitudinal differences in the hibernation characteristics of woodchucks (Marmota monax). Physiol Biochem Zool 83:135–41. Zervanos SM, Salsbury CM. 2001. Telemetry system for the continuous monitoring of temperature, location and activity of free-ranging animals. Bull Ecol Soc Am 82:189–91. Zervanos SM, Salsbury CM. 2003. Seasonal body temperature fluctuations and energetic strategies in free-ranging eastern woodchucks (Marmota monax). J Mammal 84:299–310.
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Integrative and Comparative Biology Integrative and Comparative Biology, pp. 1–9 doi:10.1093/icb/ict100
Society for Integrative and Comparative Biology
SYMPOSIUM
Effect of Body Mass on Hibernation Strategies of Woodchucks (Marmota monax) Stam M. Zervanos,1,* Christine R. Maher† and Gregory L. Florant‡
1
E-mail: [email protected]
Synopsis The benefits of mammalian hibernation have been well documented. However, the physiological and ecological costs of torpor have been emphasized only recently as part of a hibernation-optimization hypothesis. This hypothesis predicts that hibernators with greater availability of energy minimize costs of torpor by less frequent utilization of torpor and by maintaining higher body temperatures (Tb) during torpor. In order to further examine the relationship between body mass and other parameters of hibernation, we present data, collected over a 12-year period, on the hibernation patterns of free-living woodchucks (Marmota monax) in southeastern Pennsylvania. Body mass was positively correlated with Tb and negatively correlated with percentage of the heterothermic period spent in torpor. Thus, woodchucks with greater mass exhibited less time in torpor as a proportion of their heterothermic period and at higher Tb than those with lesser mass. This strategy potentially enhances the physiological and physical ability of woodchucks to defend territories, avoid predation, find mates, and complete the reproductive cycle upon emergence from hibernation. Our results further support the hibernation-optimization hypothesis by demonstrating the relationship between body mass and characteristics of torpor and contributing toward a fuller understanding of this concept.
Introduction The ability to enter hibernation in winter enhances survival during shortages of food and water. Nonetheless, the adaptive value of undergoing torpor does not come without physiological and ecological costs, which include reduced sensory and motor function leading to increased potential for predation, reduced protein synthesis, sleep deprivation, and decreased immune response (Humphries et al. 2003b; Munro et al. 2005; Boyles et al. 2007). Mammalian hibernators balance negative aspects of torpor with energy-saving benefits by optimizing utilization of energy during the hibernation season (Boyles et al. 2007). This cost-benefit hypothesis or ‘‘hibernation-optimization hypothesis’’ (Boyles et al. 2007) suggests that to reduce the negative effects of torpor, hibernators with sufficient energy stores spend less time in torpor and more time at euthermic (378C) body temperature (Tb) (Humphries et al. 2003b).
Evidence supporting the hibernation-optimization hypothesis comes both from food-hoarding and fat-storing hibernators. Eastern chipmunks (Tamias striatus), which are food-hoarding hibernators, when provided with supplementary food before entering hibernation spent more time in euthermia and maintained higher Tb during torpor (French 2000; Humphries et al. 2003a). In another study, time spent in torpor was drastically reduced when the natural food supply for free-ranging chipmunks was high (Landry-Cuerrier et al. 2008). Similarly, among little pocket mice (Perognathus longimembris), time spent in torpor was inversely related to the quantity of food (French 1975). Composition of the food supply may also play a role in the expression of torpor (Geiser et al. 1994; Munro et al. 2005). When the diet of free-ranging chipmunks was supplemented with polyunsaturated fatty acid (PUFA), they spent significantly less time in torpor and exhibited higher Tb during torpor than did control
ß The Author 2013. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: [email protected].
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*Department of Biology, Pennsylvania State University/Berks, PO Box 7009, Reading, PA 19610-6009, USA; †Department of Biological Sciences, University of Southern Maine, Portland, ME 04104, USA; ‡Department of Biology, Colorado State University, Ft. Collins, CO 80523, USA
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hibernation during winter and to commence reproduction in spring (Zervanos et al. 2010). We tested the hibernation-optimization hypothesis by analyzing the relationship between body mass and other parameters of hibernation in a free-ranging population of woodchucks in southeastern Pennsylvania over a 12-year period. Juvenile woodchucks in this population enter hibernation with a body mass of 2–3 kg, whereas adults enter hibernation with body mass of 3–5 kg, thus providing a continuous spectrum of body mass from 2 to 5 kg. We recognize that we have not measured fat mass directly; however, previous work suggests body mass and fat mass are correlated in woodchucks, (Snyder et al. 1961). We expect woodchucks with greater mass to spend a lower percentage of their heterothermic period in torpor and at higher Tb during torpor than those with lesser mass.
Materials and methods Study site The study was conducted over 12 hibernation seasons from October 1998 to March 2010 on the Pieffer Farm of the Pennsylvania State University’s Berks Campus located in southeastern Pennsylvania (75822’ W; 40822’ N). An estimated population of 55 woodchucks inhabits the farm, primarily along the ecotones of woodlots and fields (Zervanos and Salsbury 2003). Collection of data For the first five hibernation seasons (1998–2003), surgically implanted intraperitoneal temperature transmitters were used to monitor hourly Tb from free-ranging woodchucks (Zervanos and Salsbury 2001). During the 2003–2004 to 2009–2010 hibernation seasons, surgically implanted iButton temperature data loggers (MAXIM Inc., TX) were used to measure Tb hourly. All transmitters provided at least 0.28C resolution. iButtons provided 0.18C resolution and were recovered from recaptured animals in April of each year. Success of recovering loggers was 63%. During each hibernation season, three to six temperature transmitters or iButtons were placed 2–3 m into the burrows of our study animals to monitor burrow temperatures. A weather station (Model 900ET, Spectrum Tech., IL) located on the studysite monitored climatic conditions during the study. Over the 12-year period, 37 adult males, 6 juvenile males, 48 adult females, and 7 juvenile females were monitored. Times of immergence and emergence of study animals were documented by placing infrared motion-triggered cameras at entrances to burrows.
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animals that had access only to natural food supplies (Munro et al. 2005). However, some fat-storing hibernators showed the opposite pattern (Geiser and Kenagy 1987; Frank 1992; Florant et al. 1993). Golden-mantled ground squirrels (Spermophilus lateralis) and yellow-bellied marmots (Marmota flaviventris) had longer and deeper bouts of torpor when fed a high PUFA diet compared with a diet of saturated fatty acid (Geiser and Kenagy 1987; Frank 1992; Florant et al. 1993). In some fat-storing hibernators, if available stored fat, as measured by mass, is high, then torpor is reduced and a higher level of torpor Tb is maintained. For example, large mouse-eared bats (Myotis myotis) that were deprived of food spent more time in torpor than did those given food ad libitum (Wojciechowski et al. 2007). In another study, little brown bats hibernating in the warmer portions of caves had higher body mass than did those hibernating in cooler portions (Boyles et al. 2007; Jonasson and Willis 2011). Researchers similarly found that big brown bats (Eptesicus fuscus) with experimentally reduced availability of fatty acid chose significantly lower temperatures for hibernation than did controls (Boyles et al. 2007). These studies suggest that bats minimize the negative effects of torpor by selecting microclimates that allow them to hibernate at higher temperatures based on their level of stored fat. The hibernation-optimization hypothesis is also supported by a study on a non-hibernator, the golden spiny mouse (Acomys russatus). These desert-dwellers use torpor to survive shortages of food and water. When food is supplemented in their natural environment, golden spiny mice spend less time torpid and with fewer bouts of torpor. Therefore, they regulate the amount of torpor based on availability of energy (Levy et al. 2011). Thus, several studies lend support to the hibernation-optimization hypothesis. However, most of these studies have been conducted on either foodhoarders (e.g., chipmunks) or small, fat-storing species (e.g., bats). Woodchucks (Marmota monax) represent some of the largest fat-storing hibernators (Armitage 2003). Woodchucks distributed along a latitudinal gradient from Maine to South Carolina exhibit a direct relationship between latitude and total time spent in torpor (Zervanos et al. 2010). Animals in Maine spend 68% more time in torpor compared to South Carolinean animals. However, total time spent in euthermia does not differ among the three populations. Woodchucks in each population maximize time spent in euthermia, utilizing torpor only at the level needed to survive
S. M. Zervanos et al.
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Hibernation strategies of woodchucks
Body mass at immergence was measured each year during October and November. Only data from animals whose body mass was measured over a 3-week period prior to the start of hibernation were used in this study. Analysis of data
Results Ambient and burrow temperatures Mean ambient temperatures during the entire 4month hibernation season did not differ significantly among the 12 years of the study (P ¼ 0.084; Table 1). Mean burrow temperatures also did not differ significantly among the 12 years (P ¼ 0.363; Table 1). Furthermore, yearly ambient and burrow temperatures were not correlated (r ¼ 0.252, P ¼ 0.174). Torpor Mean length of the heterothermic period for adult males and for adult females did not differ among years (P40.05). Over the 12 years, adult males had significantly shorter periods of heterothermy than did adult females (t ¼ 2.69; d.f. ¼ 78; P ¼ 0.017; Table 1). Juveniles did not differ from their adult counterparts. Juvenile males had mean (SE) duration of heterothermia of 108.3 9.4 days (adults ¼ 101.1 4.3 days; N ¼ 6 juveniles and 37 adults; P ¼ 0.271), whereas juvenile females averaged 124.4 5.9 days (adults ¼ 119.3 4.0 days; N ¼ 7 juveniles and 48 adults; P ¼ 0.211). Juvenile males and females did not differ in duration of heterothermia (P ¼ 0.187), but sample sizes were small (N ¼ 6 and 7, respectively). A similar pattern was observed for the dates of first torpor. Males entered their heterothermic period significantly later than did females, on mean ( SE) days-of-year 320.6 5.6 and 305.8 3.6, respectively (t ¼ –2.94; d.f. ¼ 78; P ¼ 0.009). Again, juveniles did not differ from adults. Male juveniles had mean first torpor dates of 326.7 16.5 (N ¼ 6; P ¼ 0.304; compared to adult males), whereas juvenile females averaged 315.8 11.4 (N ¼ 7; P ¼ 0.158; compared to adult females). However, juvenile males
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Data on body temperature were analyzed via an Excel (Microsoft Corporation, Redmond, WA) spreadsheet program. Only data from multi-day bouts of torpor 424 h were used. We defined deep torpor as the period of low stable Tb5158C. To measure duration of deep torpor, we adopted criteria used by Wabmer and Wollnik (1997). Time of entry into deep torpor (end of euthermia) began when Tb fell to 308C. Time of arousal from deep torpor (beginning of euthermia) began when Tb rose to 308C. Duration of torpor was measured from the time of entry into torpor to the time of arousal from torpor. Duration of the interval between bouts of euthermia was the time measured between the end of one bout of torpor to the beginning of the next bout. Mean duration of bouts for each animal was calculated from all durations of bouts during the heterothermic periods. The percentage of the heterothermic period spent in torpor for each animal was determined by summing the total duration of bouts of torpor and the total duration of bouts of euthermia and calculating the percentage based on the total duration of the heterothermic period. We defined ‘‘hibernation season’’ as the period between fall immergence and spring emergence and ‘‘heterothermic period’’ as the interval between the date of the first bout of deep torpor in autumn to the date of arousal from the final bout of deep torpor in spring. First torpor was defined as the date and time when Tb reached 308C during entrance into the first bout of deep torpor. Final arousal was defined as the date and time when Tb reached 308C during arousal from the last bout of deep torpor. Torpor Tb was the mean of the lowest Tb reached during each bout of the heterothermic period. Mean duration of bouts of torpor, mean duration of bouts of euthermia, mean torpor Tb, and mean number of bouts of torpor were calculated for each animal during its period of heterothermy. These four dependent variables were analyzed in separate threeway analyses of variance (ANOVA) where sex, year, and body mass were designated as main effects in a general linear model. The same four dependent variables were used in a two-way ANOVA for age and sex effects. In addition, mean burrow temperatures from each burrow during each hibernation season
were used in a one-way ANOVA to determine year effects. Significance was assigned based on 50.05. Significant results were examined in post hoc tests using the Tukey–Kramer multiple comparisons method. All ANOVA and post hoc tests were performed using Minitab statistical software (Release 13 for Windows, Minitab Incorporated, State College, PA). We compared sexes and age classes using two-sample t-tests. We examined relationships of body mass to bout lengths of torpor, torpor Tb, number of bouts of torpor, and percentage of the heterothermic period spent in torpor by calculating Pearson correlation coefficients and least squares linear regressions (Statistics 9, Analytical Software, Tallahassee, FL). Significance was assigned based on 50.05.
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S. M. Zervanos et al.
Table 1 Mean SE ambient temperatures, burrow temperatures, and lengths of periods of heterothermia for adult woodchucks over a 12-year period at Pieffer Farm, Pennsylvania.
Year
Mean ambient temperature (8C)
Mean burrow temperature (8C) (N)
Duration, in days, of heterothermia in males (N)
Duration, in days, of heterothermia in females (N)
1998–1999
3.4 0.3
7.3 0.2 (4)
109 7.1 (2)
118 1.6 (7)
1999–2000
2.8 0.5
7.7 0.2 (4)
103 8.2 (3)
121 1.8 (7)
2000–2001
4.2 0.4
7.0 0.5 (4)
101 6.6 (3)
113 3.4 (4)
2001–2002
4.3 0.5
7.4 0.6 (3)
103 5.2 (4)
117 2.2 (3)
2002–2003
4.5 0.9
6.7 0.7 (3)
100 7.4 (3)
119 2.8 (4)
2003–2004
3.6 0.7
7.1 0.6 (4)
102 5.8 (4)
121 4.5 (4)
2.6 0.6
7.2 0.4 (6)
99 2.3 (4)
122 8.9 (3)
3.3 0.8
7.1 0.6 (6)
101 4.6 (3)
119 6.5 (4)
2006–2007
3.2 0.6
7.5 0.3 (5)
98 4.9 (3)
128 7.3 (3)
2007–2008
2.8 0.9
6.9 0.5 (4)
109 8.0 (2)
116 6.4 (3)
2008–2009
2.3 0.6
6.3 0.7 (4)
94 7.3 (3)
114 7.6 (3)
2009–2010
6.1 1.1
7.7 0.6 (3)
98 6.6 (3)
117 7.2 (3)
Mean
3.5 1.1
7.1 0.47
101 4.3
119 4.0
Data for mean temperatures were collected from November 1 to February 28 of each year. Sample sizes refer to number of burrows or number of woodchucks.
and juvenile females did not differ significantly from each other (P ¼ 0.102). The dates of last arousal were not significantly different among the four groups (P ¼ 0.630). Adult males ended their period of heterothermia on Day 56.9 4.9 and adult females on 59.1 5.5, whereas juvenile males ended on 62.3 9.9 and juvenile females on 61.3 11.6. During periods of heterothermia, bouts of torpor and euthermia followed the pattern described by Zervanos and Salsbury (2003). We found interactions between mean burrow temperature and the mean durations of bouts of euthermia and of torpor. By using only data from burrow temperatures between 6.58C and 7.58C, interactions became non-significant for both durations (P ¼ 0.196 and P ¼ 0.439, respectively). Sample sizes were reduced by removing animals whose body mass was measured earlier than 3 weeks prior to hibernation. We used these reduced data for all subsequent analyses (16 adult males, 5 juvenile males, 22 adult females, and 7 juvenile females). No significant interactions of sex and year were found for body mass, duration of euthermia, Tb, or percent time spent in torpor (P40.05). Thus, data from all 12 years were combined for further analysis (Table 2). Adult males had shorter duration of bouts of torpor and fewer number of bouts than did adult females (duration of bouts: t ¼ 2.28, d.f. ¼ 35, P ¼ 0.030; number of bouts: t ¼ 4.38, d.f. ¼ 35, P ¼ 0.001; Table 2). Juvenile duration of bouts of
torpor and number of bouts of torpor were not significantly different from those of adult females (P40.05; Table 2). Duration of euthermia bouts did not differ according to age or sex (P40.05; Table 2). However, adults exhibited higher torpor Tb than did juveniles (t ¼ 2.10, d.f. ¼ 18; P ¼ 0.047), and adults tended to spend less percent time in torpor than did juveniles, although the difference was not statistically significant (t ¼ –3.14, d.f. ¼ 28, P ¼ 0.064; Table 2). Body mass prior to immergence and mean duration of bouts of torpor were inversely correlated in males (r ¼ –0.483, N ¼ 21, P ¼ 0.036), but not in females (r ¼ –0.046, N ¼ 29, P ¼ 0.811). In addition, body mass was not correlated with number of bouts of torpor in females (r ¼ –0.146, N ¼ 29, P ¼ 0.441), whereas in males, heavier animals had fewer bouts of torpor (r ¼ –0.465, N ¼ 21, P ¼ 0.045). For both sexes combined, body mass was positively correlated with duration of bouts of euthermia (r ¼ 0.399, N ¼ 50, P ¼ 0.006) and with torpor Tb (r ¼ 0.48, N ¼ 50, P ¼ 0.001; Fig. 1). Finally, body mass was negatively correlated with percentage of the period of heterothermic spent in torpor (r ¼ –0.535, N ¼ 50, P ¼ 0.001; Fig. 1).
Discussion Support of hypothesis To conserve the greatest amount of energy, we would expect a mammalian hibernator to utilize torpor
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2004–2005 2005–2006
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Hibernation strategies of woodchucks
Table 2 Comparison of characteristics (mean SE) of male and female woodchucks during periods of heterothermia, 1998–2010, at Pieffer Farm, Pennsylvania. Males Body mass (kg) Duration of bout of torpor (h)
Females
Juveniles (N ¼ 5)
Adults (N ¼ 16)
Juveniles (N ¼ 7)
Adults (N ¼ 22)
2.15 0.16a
4.18 0.10b
2.23 0.13a
3.90 0.12b
128.40 6.65b
154.04 9.94a
152.97 7.53a
a
a
42.86 4.67a
152.32 15.37a a
Duration of bout of euthermia (h)
32.32 6.19
40.55 5.18
Torpor Tb (8C)
10.23 0.72a
12.32 0.47b
9.91 0.76a
11.27 0.35b
a
b
a
13.40 0.48a
0.83 0.03a
0.76 0.01a
Number of Bouts of Torpor Percent of period of heterothermic spent in torpor
12.33 0.61
0.85 0.03a
10.38 0.42
0.77 0.02a
32.58 6.20
13.90 1.08
Different letters denote significant group differences within each characteristic (P 0.05).
extensively and to maintain a low torpor Tb, thus, reducing expenditure of endogenous energy. Our results, however, indicate that this was not the case. Woodchucks with higher body mass spent less time in torpor and more time in euthermia as a percentage of their total period of heterothermia than did animals with lower body mass. Heavier animals also maintained a higher torpor Tb. Why did they not
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Fig. 1 Relationship of (a) body temperature during torpor (r2 ¼ 0.2202, F ¼ 13.84, P ¼ 0.0005) and (b) percentage of time spent in torpor to body mass (r2 ¼ 0.282, F ¼ 13.39, P ¼ 0.0001) during the period of heterothermia in free-ranging woodchucks over 12 hibernation seasons (1998–2010) at Pieffer Farm, Pennsylvania. Red markers represent females; blue markers males.
conserve as much energy as possible to emerge with greater potential for reproduction and survival, i.e., what is the fitness value of less time in torpor and higher torpor Tb? Physiological and ecological costs of torpor may play a role (French 1975; Humphries et al. 2003b; Munro et al. 2005; Boyles et al. 2007). For example, negative effects of torpor on cellular functions are reversed during arousals (Carey 1993; Grigg and Beard 2000; Humphries et al. 2003b). Torpor inhibits the immune system, whereas euthermy restores its benefits (Burton and Reichman 1999; Prendergast et al. 2002; Luis and Hudson 2006). Other benefits include reduced muscle atrophy and improved development of reproductive organs, especially for males (Millesi et al. 2000; Humphries et al. 2003b; Zervanos 2003; Zervanos and Salsbury 2003). Furthermore, hibernation may have evolved from sleep (Heller and Ruby 2004). Thus, sleep and hibernation may lie on a continuum that provides a mechanism for conservation of energy (Heller and Ruby 2004). As such, animals that can lower their energy costs by sleeping, which includes a reduction in Tb, can lower their energy costs when environmental food supplies are low or non-existent. However, during bouts of deep torpor 5258C, sleep does not occur. Sleep only occurs during arousals (Larkin and Heller 1996). When animals arouse from torpor, they are in a sleep state (Barnes et al. 1993; Larkin and Heller 1996, 1998). Although its function is unknown, sleep is required for normal physiological processes to continue. Thus, getting sufficient amounts of sleep during the period of euthermia prior to re-entering torpor is important. Woodchucks with larger body mass may remain euthermic longer, thereby maximizing the amount of sleep and enhancing its associated physiological functions.
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reflected in our results, bears may be part of a continuum of increasing torpor Tb with increasing mass. Sex and age strategies Duration and number of bouts of torpor during the period of heterothermia for male and female juveniles exhibited similar patterns to those for adult female woodchucks. Adult males exhibited fewer and shorter bouts of torpor. Male and female juvenile woodchucks both had lower torpor Tb and spent a higher percentage of time in torpor than did adults. These relationships appear to be related to the difference in mass between juveniles and adults and to the hibernation strategies of each group (French 1986; 1990). Juveniles have higher mass-specific metabolic rates and higher thermal conductance (French 1990); thus, their hibernation strategy is to conserve energy by spending a higher proportion of their period of heterothermia in torpor and at lower Tb. Although surviving winter is the main objective, about one-third of the juvenile females that are heavier at emergence in this population of woodchucks, reproduce as yearlings (Zervanos, unpublished data). For adult females, the strategy is to survive hibernation with enough energy reserves at emergence to mate, to produce, and then to nurse young, until more sufficient forage becomes available. Although adult females utilize the same proportion of their period of heterothermia in torpor as do males, their longer bouts of torpor bouts and longer periods of heterothermia conserve the energy needed for reproduction (Zervanos and Salsbury 2003). Reproductive strategies of adult males require that they emerge earlier to complete spermatogenesis, defend territories, find females, and mate. Once mating is over, they need to survive a short period until spring forage is available. With shorter bouts of torpor, they utilize more energy, over a shorter period of heterothermia, than do females. However, they are able to still emerge with sufficient reserves of energy to successfully complete reproduction (Zervanos and Salsbury 2003). Possible mechanism Two inherent assumptions of the hibernation-optimization hypothesis are: (1) animals must be able to endogenously and externally estimate the length of their coming hibernation and (2) animals must be able to continuously monitor fat reserves. Over the 12 years of this study the duration of heterothermia both for males and females did not vary significantly, suggesting duration of heterothermia may be genetically fixed (Zervanos et al. 2010). The other
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By reducing the amount of time spent in torpor and utilizing higher Tb during the hibernation season, woodchucks could also improve their overall fitness. In a previous study of this population, adult male woodchucks utilized euthermia more often and had higher torpor Tb than did adult females, which resulted in a 38% higher energy cost for males (Zervanos and Salsbury 2003). Counteracting those costs is greater physiological ability to defend territories, avoid predation, find mates, and complete spermatogenesis (Zervanos and Salsbury 2003). However, the risks of this behavior are high. If animals miscalculate the length or severity of the hibernation season, fat-storing hibernators cannot easily adjust their utilization of energy (Humphries et al. 2003b; Boyles et al. 2007). A mechanism that can predict the length of the hibernation season and measure the energy reserves would be required (see below). Few woodchucks breed as yearlings (Armitage 2003). Thus, for juveniles, gaining enough mass during the active season to survive hibernation would be more critical than reproductive success after emergence, and we expect that they would utilize more torpor based on mass (French 1990). Their smaller body mass puts them near the minimum energy required for hibernation (Armitage 1998). The observed level of torpor and Tb in our study may represent the minimum levels required to survive hibernation at our study site, particularly for the smallest individuals. Any level of stored fat above this minimum increases the flexibility to: (1) utilize less torpor and remain at a higher Tb, (2) retain fat reserves for the end of the hibernation season, and (3) utilize fat for reproduction. Our study was based on the use of a continuum of body mass from the smallest juveniles to the largest adults, and the inverse relationship of mass to mass-specific metabolic rate is reflected in our results. Larger animals could afford to utilize less torpor and maintain high Tb (French 1986; 1990). Interestingly, bears (Ursus spp.) have relatively high Tb during the hibernation period, but their metabolic rates are greatly depressed (Hellgren 1998; Tøien et al. 2011). The ability of bears to reduce metabolism during hibernation, yet maintain high Tb, results from their large mass of stored fat and their low thermal conductance (Tøien et al. 2011). In addition, bears do not arouse periodically from torpor but instead stay torpid throughout hibernation (Harlow et al. 2004). Arousal may not be necessary at higher Tb because much of the physiological costs of low torpor Tb are avoided. Finally, as
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Hibernation strategies of woodchucks
component of the mechanism requires a way to gauge the amount of fat required to survive the hibernation season. One possible mechanism is to monitor the blood concentration of leptin. Leptin is released from white adipose tissue and is highly correlated with mass of fat (Ahima et al. 2000). Leptin appears to feedback to the hypothalamus and inhibit anabolic pathways, such as food intake in mammals. Previous studies of leptin concentration in woodchucks and yellow-bellied marmots found high levels of the hormone during fall and winter when mass of fat was greatest (Concannon et al. 2001; Florant et al. 2004; Florant and Healy 2012). Leptin decreases dramatically during late winter and early spring, when animals end hibernation and prepare for reproduction. Whether the hypothalamus changes its sensitivity to leptin-signaling during spring is not known for hibernators. However, the overall function of leptin during hibernation is currently an active area of research and may lead to further insights into its role as a mechanism for gauging energy reserves.
Conclusions
Potential effects of climatic change
Funding
Woodchucks show considerable variation in hibernation parameters not only within a population (this study) but also across populations (Zervanos et al. 2010). Based on comparisons of captive (Fenn et al. 2009) and free-living woodchucks (Zervanos et al. 2010), hibernation characteristics, such as duration of bouts of torpor and torpor Tb, exhibit a degree of phenotypic plasticity, whereas a characteristic such as the length of the period of heterothermia is more fixed and perhaps genetically based. Phenotypic plasticity has broader ecological and evolutionary implications, e.g., the potential for rapid adjustments in the face of climatic change (Boyles et al. 2011; Sheriff et al. 2011; Armitage 2013). For example, the plasticity in duration of bouts of torpor is inversely related to ambient temperatures in woodchucks (Zervanos et al. 2010). As temperature increases, duration of bouts of torpor would be expected to decrease, with animals spending a lower percentage of time in torpor. In contrast, the lengths of periods of heterothermia may require several generations to adapt to changing climate, through microevolution (Davis 2005; Lane 2012) and/or entrainment of the circannual cycle (Davis and Finnie 1975; Lane 2012). As woodchucks are widely distributed, they may not be threatened by climatic change immediately, although their overall range may constrict (Armitage 2013).
The Pennsylvania State University/Berks Faculty Research and Development Fund (to S.M.Z.); Lipid Foundation grant #645848 (to G.L.F.).
Using data from a 12-year study of woodchucks in Pennsylvania, we found support for the hibernationoptimization hypothesis, based on the significant correlation of mass to torpor Tb and percentage of time spent in torpor. These results further support the hypothesis by adding another fat-storing hibernating species, measured under field conditions, to our understanding of this concept. Given the broad geographic range of this species, future studies made under different, and changing, climatic conditions, can provide additional tests of the hypothesis.
We would like to thank the many students who have assisted with this study over the years. A special thank you to June K. Brown, research assistant to S.M.Z., for collecting and organizing much of the field data used in this study and to Mary Ann Mengel for her technical assistance. All animal procedures were approved by the Pennsylvania State University Animal Care and Use Committee (#20562).
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