BEHAVIORAL BIOLOGY, 14, 175-188 (1975), Abstract No. 4236
Golden Hamster Pups Adapt to Complex Rearing Odors
CATHERINE A. CORNWELL 1
Department of Psychology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138
Hamster pups avoid certain organic odors as early as postnatal day 3 or 4. Continuous exposure to these odors during rearing selectively diminishes their aversiveness for the hamsters as measured in a two-choice situation. This acquired indifference is stimulus-specific and depends upon the integrity of the olfactory bulb. Indifference does not transfer to other normally aversive odors and is not due to any general impairment of odor detection. Although the test situation requires an orientation response and although bilateral lesions of the superficial layers of the superior colliculus have been shown to eliminate visual orientation in adult hamsters (Schneider, 1969), such lesions produce only a transient deficit in orientation to the odors employed in this study. The sense of smell guides critical activities in the lives of marly nonhuman species. The odors involved may arise from various sources in an animal's environment. Most studies of mammalian olfaction to date have focused on the functions of odors produced by members of an animal's own species which may cue its behavior. Some experiments have attempted to examine the influence of other odors. These studies have usually employed the vapors of pure chemicals as stimuli (cf. Carter, 1972) even though odors produced by such sources are seldom encountered in the natural environment. In this investigation, I have examined the extent to which exposure to complex organic odors alters the behavior of neonatal hamsters, Mesocricetus auratus Waterhouse. Newborn animals were selected because manipulation of the sensory environment at an early age usually leads to greater behavioral change than does later intervention (Hess, 1958). Hamsters were used because olfaction seems to guide many facets of behavior throughout their life, 1This research was supported by an NIH predoctoral training grant to the M.I.T. Psychology Department, and by Grant No. MH-07923 from the National Institute of Mental Health to Stephan L. Chorover. The author gratefully acknowledges the many contributions made to this study by S. L. Chorover, M. Devor, D. Frost and G. E. Schneider. Requests for reprints should be sent to Catherine Cornwell, Department of Psychology, Princeton University, Princeton, NJ 08540. 175 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
176
CATHERINE A. CORNWELL
including the early postnatal period. In particular, by day 8, hamster pups show a preference for the odor of bedding taken from their nests (Devor and Schneider, 1974). This preference may be either genetically determined, require exposure in the nest, or both. Taken together, these considerations suggested that neonate hamsters might be good candidates for acquiring environmentally specified odor preferences. The particular idea to be tested was that early exposure to a normally aversive complex organic odor would serve either to neutralize the aversiveness or to induce a preference for that odor.
Subjects and Rearing Subjects were 275 Syrian golden hamster pups from 36 litters born in the laboratory. All animals were reared in plastic cages with floors measuring 40 X 25 cm. The floors were covered with one of four types of bedding: natural cedar, natural Ponderosa pine (Old Mother Hubbard Company), or pine scented with either McCormick's garlic juice or Baker's lemon extract. Both products are sold commercially as food flavoring. The scented shavings were prepared by adding 20 ml undiluted solution to two liters (by volume) of natural Ponderosa pine shavings. The shavings were placed in a clean cage housing a pregnant female four days before an anticipated birth. This bedding was not changed thereafter until the end of the final test session which was usually on postnatal day 12. However, an additional 5 ml of solution was added from a syringe to the bedding daily on postnatal days 3-11, while mother and pups were temporarily removed from the cage. The animals were housed in a common room. Cages containing the various shavings were initially segregated on separate shelves to minimize exposure to non-home odors. However, when I began to rear animals in garlic-scented shavings, the garlic odor could be detected by a human standing anywhere in the room. This raised the possibility that pine-reared "control" pups could also detect the garlic odor. Subsequently, each cage was further isolated from the others by placing it inside a larger plastic container. The latter was completely closed, except for an inlet through which clean air was drawn and an outlet through which the air was passed to a common exhaust line and was thence vented to a duct leading outside the building. Data obtained under this arrangement of olfactory isolation were indistinguishable from those obtained under the original conditions. Although it was concluded that in the initial situation, non-home cage odor diffusion did not affect test results, all subsequent studies were done using the more effective isolation method. The testing apparatus was modified from the design used by Devor and Schneider (1974). It consisted of a 20 × 26 cm Plexiglas frame with a screen floor which sat upon a two-compartment container divided longitudinally. Shavings scented with test odors were placed in one compartment, fresh pine
177
ODOR ADAPTATION IN HAMSTER PUPS Averoge ~
Time Over lest Odor
[
Full Strength
Anise 50[' N=IO
Half Strength
~oo
0 o
0
I
ua i-
7
8
9
I0
II
12
Age (Days) Fig. 1. Odor preferences of hamster pups reared in natural pine. The histograms show the average percent of test time spent over test odors by four different litters. Each pup was tested individually for 3 rain. Full strength test odors were produced by mixing 5 ml of the appropriate extract with 0.6 liter of fresh pine shavings. 2.5 ml extract was used for half strength odors. Unscented natural pine shavings were the source of the comparison odor in all four cases. Garlic juice: McCormick's brand, others, Baker's.
shavings in the other. The relative positions of the shavings were reversed from day to day. Pups were tested individually. In each testing session, a pup was placed on the divider and was allowed to crawl on the screen for three minutes. The time spent over each of the test odors was monitored with a stopwatch. The pup's path was observed and drawn on a floor-plan of the testing apparatus.
Series I: Odor Preferences of Pine-Reared Pups In the first series of experiments, the subjects were four litters of hamster pups 7-12 days old, that had been reared on natural pine bedding. The object was to determine if hamsters in this age range would show consistent responses to a series of complex organic odors. Test odors were produced by mixing 5 ml of either anise, garlic, lemon or peppermint extract with 0.6 liters of fresh pine shavings. An equal volume of fresh pine shavings served as the comparison stimulus in all cases. For the first litter (N=10) anise odor was the test stimulus, the second litter (N=10) was tested with garlic, the third (N=IO) with lemon and the fourth (N=7) with peppermint. Concentration sensitivity was tested by reducing the garlic and lemon extracts to 2.5 ml during one testing session each (Fig. 1). Pups tended to prefer anise over fresh pine, that is they consistently averaged more than 50% of test time over anise, were indifferent to peppermint and preferred natural pine to both the garlic and the lemon odors.
178
CATHE~NE A. CORNWELL
CEDAR
GARLIC
i
I
LEMON
Pine Test odor Pine Test odor NATURAL PINE TEST ODOR REARED REARED
Fig. 2. Effects of rearing odor on odor preferences. Results for seven day old pups. Each diagram represents the path of the pup whose score was nearest the average for its rearing group. Paths were observed during the 3-min test period and drawn on a floor plan of the testing apparatus. The figures were traced from original records. Note the tendency of all pine-reared pups to crawl up and down the extreme left edge of the testing apparatus, the area farthest from test odor. For the last two odors, the tracings in the left column o f Fig. 2 (which are from a later experiment) indicate that animals typically turned toward the pine compartment and crawled along the edge of the apparatus farthest removed from the test odor. When they did approach the midline, the animals usually sniffed the garlic or lemon-scented shavings and then veered away back into pine-scented territory. These findings appeared to be concentration dependent: When lemon and garlic concentrations were reduced by half, no strong preference was seen in either case, although some pups tested on weak lemon tended to prefer that odor over unscented pine. It appeared that at the original strength, both lemon and garlic odors were aversive. Extract odors were the only stimuli available to direct differential responses, since equal volumes of pine were in both odor compartments. In summary, the results show that from 7-12 days o f age, hamster pups reared in natural pine shavings can detect a spectrum of odors, show measurably different responses to them and are sensitive to changes in odor concentration,
Series H: Effects of Rearing Exposure on Odor Preferences A second series of experiments was conducted to determine if hamsters reared on bedding scented with a normally aversive odor would thereby
179
ODOR ADAPTATION IN HAMSTER PUPS TABLE 1 Testing Schedule for Series II Test odors
Rearing
Days tested 10 ll
Litter
4
5
6
7
8
9
12
Cedar shavings
1 2 3
x x
x x x
x x
x x
x x
x x x
x x x
x x x
x x
Unscented pine shavings
1 2 3
x
x x x
x x
x x
x x
x x x
x x x
x x x
x x x
Garlic-scented pine shavings
1 2 3
x x x
x x
x x
x x
x x
x x x
x
Unscented pine shavings
1 2 3
x x x
x x
x x
x x
x x
x x x
Lemon-scented pine shavings
1 2 3
x x
x x x
x x
x x
x x
Unscented pine shavings
1 2 3
x x
x x x
x x
13
Cedar
x
x x x
x
Garlic x x
x
x x x
x
x
x
x
x x x
x x x
x
x x x x x x
Lemon x x
x x
x x x
a c q u i r e a p r e f e r e n c e f o r t h a t o d o r o r at l e a s t l o s e t h e i r a v e r s i o n t o it. N i n e e x p e r i m e n t a l l i t t e r s w e r e u s e d . T h r e e w e r e r e a r e d in l e m o n - s c e n t e d a n d t h r e e in g a r l i c - s c e n t e d p i n e shavings. T h e final t h r e e e x p e r i m e n t a l l i t t e r s w e r e r e a r e d on bedding of cedar, which Devor and Schneider had found
t o b e aversive
relative t o f r e s h p i n e f o r h a m s t e r p u p s r e a r e d o n b e d d i n g o f n a t u r a l p i n e . T w o o f t h e l i t t e r s i n e a c h r e a r i n g o d o r c o n d i t i o n w e r e given a single d a i l y trial o n at
least
seven
occasions
between
postnatal
days
4
and
12. P u p s
in the
r e m a i n i n g l i t t e r o f e a c h c o n d i t i o n w e r e t e s t e d o n c e o n d a y 4 o r 5, a n d again o n d a y 9. T h i s t e s t i n g s c h e d u l e w a s u s e d as a c o n t r o l t o see i f daily p r e f e r e n c e c h a n g e s w e r e d u e t o f r e q u e n t l y r e p e a t e d t e s t i n g . A p i n e - r e a r e d l i t t e r s e r v e d as control for each experimental litter (Table the entire experiment.
1). T h u s , t h e r e w e r e 18 l i t t e r s in
A s in D e v o r a n d S c h n e i d e r ' s s t u d y , p i n e - r e a r e d p u p s t e n d e d t o a v o i d c e d a r shavings. B y c o n t r a s t , c e d a r - r e a r e d p u p s a d o p t e d a n e s s e n t i a l l y n e u t r a l attitude
toward
cedar during the test sessions. On most
their time on the screen almost equally between
days, they divided
t h e sides c o n t a i n i n g c e d a r
a n d p i n e shavings. W h e n c o m p a r e d w i t h t h e p i n e - r e a r e d p u p s , t h e y a v e r a g e d
CATHERINE A. CORNWELL
180 ....
Reared in Test-scented Shavings
o ~
Reared in Natural Pine Shavings
P < .01 11)11P
Golden Hamster Pups Adapt to Complex Rearing Odors
CATHERINE A. CORNWELL 1
Department of Psychology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138
Hamster pups avoid certain organic odors as early as postnatal day 3 or 4. Continuous exposure to these odors during rearing selectively diminishes their aversiveness for the hamsters as measured in a two-choice situation. This acquired indifference is stimulus-specific and depends upon the integrity of the olfactory bulb. Indifference does not transfer to other normally aversive odors and is not due to any general impairment of odor detection. Although the test situation requires an orientation response and although bilateral lesions of the superficial layers of the superior colliculus have been shown to eliminate visual orientation in adult hamsters (Schneider, 1969), such lesions produce only a transient deficit in orientation to the odors employed in this study. The sense of smell guides critical activities in the lives of marly nonhuman species. The odors involved may arise from various sources in an animal's environment. Most studies of mammalian olfaction to date have focused on the functions of odors produced by members of an animal's own species which may cue its behavior. Some experiments have attempted to examine the influence of other odors. These studies have usually employed the vapors of pure chemicals as stimuli (cf. Carter, 1972) even though odors produced by such sources are seldom encountered in the natural environment. In this investigation, I have examined the extent to which exposure to complex organic odors alters the behavior of neonatal hamsters, Mesocricetus auratus Waterhouse. Newborn animals were selected because manipulation of the sensory environment at an early age usually leads to greater behavioral change than does later intervention (Hess, 1958). Hamsters were used because olfaction seems to guide many facets of behavior throughout their life, 1This research was supported by an NIH predoctoral training grant to the M.I.T. Psychology Department, and by Grant No. MH-07923 from the National Institute of Mental Health to Stephan L. Chorover. The author gratefully acknowledges the many contributions made to this study by S. L. Chorover, M. Devor, D. Frost and G. E. Schneider. Requests for reprints should be sent to Catherine Cornwell, Department of Psychology, Princeton University, Princeton, NJ 08540. 175 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
176
CATHERINE A. CORNWELL
including the early postnatal period. In particular, by day 8, hamster pups show a preference for the odor of bedding taken from their nests (Devor and Schneider, 1974). This preference may be either genetically determined, require exposure in the nest, or both. Taken together, these considerations suggested that neonate hamsters might be good candidates for acquiring environmentally specified odor preferences. The particular idea to be tested was that early exposure to a normally aversive complex organic odor would serve either to neutralize the aversiveness or to induce a preference for that odor.
Subjects and Rearing Subjects were 275 Syrian golden hamster pups from 36 litters born in the laboratory. All animals were reared in plastic cages with floors measuring 40 X 25 cm. The floors were covered with one of four types of bedding: natural cedar, natural Ponderosa pine (Old Mother Hubbard Company), or pine scented with either McCormick's garlic juice or Baker's lemon extract. Both products are sold commercially as food flavoring. The scented shavings were prepared by adding 20 ml undiluted solution to two liters (by volume) of natural Ponderosa pine shavings. The shavings were placed in a clean cage housing a pregnant female four days before an anticipated birth. This bedding was not changed thereafter until the end of the final test session which was usually on postnatal day 12. However, an additional 5 ml of solution was added from a syringe to the bedding daily on postnatal days 3-11, while mother and pups were temporarily removed from the cage. The animals were housed in a common room. Cages containing the various shavings were initially segregated on separate shelves to minimize exposure to non-home odors. However, when I began to rear animals in garlic-scented shavings, the garlic odor could be detected by a human standing anywhere in the room. This raised the possibility that pine-reared "control" pups could also detect the garlic odor. Subsequently, each cage was further isolated from the others by placing it inside a larger plastic container. The latter was completely closed, except for an inlet through which clean air was drawn and an outlet through which the air was passed to a common exhaust line and was thence vented to a duct leading outside the building. Data obtained under this arrangement of olfactory isolation were indistinguishable from those obtained under the original conditions. Although it was concluded that in the initial situation, non-home cage odor diffusion did not affect test results, all subsequent studies were done using the more effective isolation method. The testing apparatus was modified from the design used by Devor and Schneider (1974). It consisted of a 20 × 26 cm Plexiglas frame with a screen floor which sat upon a two-compartment container divided longitudinally. Shavings scented with test odors were placed in one compartment, fresh pine
177
ODOR ADAPTATION IN HAMSTER PUPS Averoge ~
Time Over lest Odor
[
Full Strength
Anise 50[' N=IO
Half Strength
~oo
0 o
0
I
ua i-
7
8
9
I0
II
12
Age (Days) Fig. 1. Odor preferences of hamster pups reared in natural pine. The histograms show the average percent of test time spent over test odors by four different litters. Each pup was tested individually for 3 rain. Full strength test odors were produced by mixing 5 ml of the appropriate extract with 0.6 liter of fresh pine shavings. 2.5 ml extract was used for half strength odors. Unscented natural pine shavings were the source of the comparison odor in all four cases. Garlic juice: McCormick's brand, others, Baker's.
shavings in the other. The relative positions of the shavings were reversed from day to day. Pups were tested individually. In each testing session, a pup was placed on the divider and was allowed to crawl on the screen for three minutes. The time spent over each of the test odors was monitored with a stopwatch. The pup's path was observed and drawn on a floor-plan of the testing apparatus.
Series I: Odor Preferences of Pine-Reared Pups In the first series of experiments, the subjects were four litters of hamster pups 7-12 days old, that had been reared on natural pine bedding. The object was to determine if hamsters in this age range would show consistent responses to a series of complex organic odors. Test odors were produced by mixing 5 ml of either anise, garlic, lemon or peppermint extract with 0.6 liters of fresh pine shavings. An equal volume of fresh pine shavings served as the comparison stimulus in all cases. For the first litter (N=10) anise odor was the test stimulus, the second litter (N=10) was tested with garlic, the third (N=IO) with lemon and the fourth (N=7) with peppermint. Concentration sensitivity was tested by reducing the garlic and lemon extracts to 2.5 ml during one testing session each (Fig. 1). Pups tended to prefer anise over fresh pine, that is they consistently averaged more than 50% of test time over anise, were indifferent to peppermint and preferred natural pine to both the garlic and the lemon odors.
178
CATHE~NE A. CORNWELL
CEDAR
GARLIC
i
I
LEMON
Pine Test odor Pine Test odor NATURAL PINE TEST ODOR REARED REARED
Fig. 2. Effects of rearing odor on odor preferences. Results for seven day old pups. Each diagram represents the path of the pup whose score was nearest the average for its rearing group. Paths were observed during the 3-min test period and drawn on a floor plan of the testing apparatus. The figures were traced from original records. Note the tendency of all pine-reared pups to crawl up and down the extreme left edge of the testing apparatus, the area farthest from test odor. For the last two odors, the tracings in the left column o f Fig. 2 (which are from a later experiment) indicate that animals typically turned toward the pine compartment and crawled along the edge of the apparatus farthest removed from the test odor. When they did approach the midline, the animals usually sniffed the garlic or lemon-scented shavings and then veered away back into pine-scented territory. These findings appeared to be concentration dependent: When lemon and garlic concentrations were reduced by half, no strong preference was seen in either case, although some pups tested on weak lemon tended to prefer that odor over unscented pine. It appeared that at the original strength, both lemon and garlic odors were aversive. Extract odors were the only stimuli available to direct differential responses, since equal volumes of pine were in both odor compartments. In summary, the results show that from 7-12 days o f age, hamster pups reared in natural pine shavings can detect a spectrum of odors, show measurably different responses to them and are sensitive to changes in odor concentration,
Series H: Effects of Rearing Exposure on Odor Preferences A second series of experiments was conducted to determine if hamsters reared on bedding scented with a normally aversive odor would thereby
179
ODOR ADAPTATION IN HAMSTER PUPS TABLE 1 Testing Schedule for Series II Test odors
Rearing
Days tested 10 ll
Litter
4
5
6
7
8
9
12
Cedar shavings
1 2 3
x x
x x x
x x
x x
x x
x x x
x x x
x x x
x x
Unscented pine shavings
1 2 3
x
x x x
x x
x x
x x
x x x
x x x
x x x
x x x
Garlic-scented pine shavings
1 2 3
x x x
x x
x x
x x
x x
x x x
x
Unscented pine shavings
1 2 3
x x x
x x
x x
x x
x x
x x x
Lemon-scented pine shavings
1 2 3
x x
x x x
x x
x x
x x
Unscented pine shavings
1 2 3
x x
x x x
x x
13
Cedar
x
x x x
x
Garlic x x
x
x x x
x
x
x
x
x x x
x x x
x
x x x x x x
Lemon x x
x x
x x x
a c q u i r e a p r e f e r e n c e f o r t h a t o d o r o r at l e a s t l o s e t h e i r a v e r s i o n t o it. N i n e e x p e r i m e n t a l l i t t e r s w e r e u s e d . T h r e e w e r e r e a r e d in l e m o n - s c e n t e d a n d t h r e e in g a r l i c - s c e n t e d p i n e shavings. T h e final t h r e e e x p e r i m e n t a l l i t t e r s w e r e r e a r e d on bedding of cedar, which Devor and Schneider had found
t o b e aversive
relative t o f r e s h p i n e f o r h a m s t e r p u p s r e a r e d o n b e d d i n g o f n a t u r a l p i n e . T w o o f t h e l i t t e r s i n e a c h r e a r i n g o d o r c o n d i t i o n w e r e given a single d a i l y trial o n at
least
seven
occasions
between
postnatal
days
4
and
12. P u p s
in the
r e m a i n i n g l i t t e r o f e a c h c o n d i t i o n w e r e t e s t e d o n c e o n d a y 4 o r 5, a n d again o n d a y 9. T h i s t e s t i n g s c h e d u l e w a s u s e d as a c o n t r o l t o see i f daily p r e f e r e n c e c h a n g e s w e r e d u e t o f r e q u e n t l y r e p e a t e d t e s t i n g . A p i n e - r e a r e d l i t t e r s e r v e d as control for each experimental litter (Table the entire experiment.
1). T h u s , t h e r e w e r e 18 l i t t e r s in
A s in D e v o r a n d S c h n e i d e r ' s s t u d y , p i n e - r e a r e d p u p s t e n d e d t o a v o i d c e d a r shavings. B y c o n t r a s t , c e d a r - r e a r e d p u p s a d o p t e d a n e s s e n t i a l l y n e u t r a l attitude
toward
cedar during the test sessions. On most
their time on the screen almost equally between
days, they divided
t h e sides c o n t a i n i n g c e d a r
a n d p i n e shavings. W h e n c o m p a r e d w i t h t h e p i n e - r e a r e d p u p s , t h e y a v e r a g e d
CATHERINE A. CORNWELL
180 ....
Reared in Test-scented Shavings
o ~
Reared in Natural Pine Shavings
P < .01 11)11P
