Arthur
and environmental gestation length and Penelope
J. Riopelle
ABSTRACT ment and diets
observed.
than
infants
The
females;
conception
4-g
part
produced slightly
Hale
Forty-five pregnancies in rhesus 1 , 2, or 4 g of protein/kilogram
fed about
were
longer
Anne
diet
longer
longer
than
shortened
of difference gestation small
The
breathing Enormous ductive supials,
offspring at birth are capable of and of taking food by mouth. species differences exist in repro-
styles. which
At one extreme are have a very short
the margestation
period but a long postnatal period of oral attachment to the teat. Placental mammals, at the other, have longer gestation periods, and once born they need but to feed at the breast nods.
are
intermittently
Gestation brought
and
only
length, during to their state
semi-independent
living,
similarly
has
is usually
its
brief
pe-
which the young of readiness for
to be primarily genetically crossbreeding experiments genetic component beyond species
for
considered
controlled, demonstrate a doubt.
characteristic
and the Each gesta-
tion length: the rat’s gestation length is 21 days and the human’s is 265 days. Rhesus monkeys (Macaca mulatta) are born after an average gestation period ofabout 165 days (1, 2). Valenio, et al. (2) cite 165 days as average for 600 laboratory-bred rhesus; van Wagenen reports 168.9 ± 6.85 for 225 similar animals and Pickering and Kontaxis (3) report 167 ± 4 days
differences 1 170
for
50 pregnancies.
exist
between
The American
Minor
accounting
laboratories. Journal
ofClinical
Nutrition
monkeys of body
gestation can
than
ones.
Each species of animals from the very first has, within its evolutionary history, developed an effective system of reproduction. In complex animals the fetuses grow within the mother’s body until they reach a state of development compatible with the speciescharacteristic pattern of postnatal maternal care, so that by and large some of the litter survive.
factors in rhesus monkeys”2’3
be
8.5
days.
attributed
did
winter
J.
C/in.
Am.
maintained weight each to
Male their
conception. Nutr.
in a semioutdoor day in otherwise infants
greater Large
28:
1170-1176,
were
environequivalent
carried
birthweight. mothers
6 days Summer
carried
their
l975.
But averages do not tell the whole story for they imply genetic stability rather than physiologic lability. In this paper we report how the gestation lengths of rhesus monkeys, in response to environmental factors, vary as much as 20% of normal term without disaster to either infant or mother. This suggests that gestation involves adjustments to the environment, and the length of gestation in each pregnancy as well as the well-being of the infant is a result of these adjustments. Material
and
methods
The present study involves the mothers of 47 live infants spontaneously delivered. Abortions and stillbirths, though not uniformly distributed among the groups, were excluded. Participants were young adult and fully adult female rhesus monkeys weighing between 3.7 and 9.2 kg and averaging 6.1 kg (standard deviation = 1.53 kg). They were assigned to the study after conditioning to the laboratory routines. The animals were first adapted to a semisynthetic diet containing 15.3% casein (13.4% protein) for at least 3 months prior to mating. We called this diet 4 because if afforded 4 g of high-quality protein/kilogram of body weight daily if the animal ate 30 g (120 kcal) of the food for every kilogram of body weight. Mating was allowed in a 4-day mating period which began I 1 days after the onset of menstruation. All animals were maintained on that diet for one more month. If at that time we concluded on rectal palpation that they were pregnant, they were either continued on From the Department of Psychology, Louisiana University. a Supported by Public Health Service Grant HD-07479. a Reprint requests: Arthur J. Riopelle or Penelope Anne Hale, Department of Psychology, Louisiana State University, Baton Rouge, LA 70803. 1
State
28: OCTOBER
1975,
pp.
1 170-1
176. Printed
in U.S.A.
Downloaded from https://academic.oup.com/ajcn/article-abstract/28/10/1170/4716669 by McMaster University Library user on 31 January 2019
Nutritional affecting
FACTORS the
same
diet
containing protein deficient
they
assigned
to
a similar
diet
(diet 1) of its animals on the were continued on those diets until dclivthey were returned to diet 4. animals to the different diet groups at
diets
whereupon We assigned
random weight.
were
GESTATION
(diet 2) or one-fourth caloric density. The
within roughly stratified levels of maternal body The intent was to maintain a balance of means
and variances of conception groups as new pregnancies replacements were required
weights among the three were encountered and as following fetal loss. The first
dozen infants came from the 4-g and the 2-g diets, for in our ignorance of the severity of the consequences of the deprivation, we were hesitant toassign mothers to the l-g diet for fear that few or no infants would result. When experience showed otherwise, relatively more mothers were assigned to the l-g diet to attain balance. Such balance was eventually achieved, and subsequent assignments were at random. The average conception weights
of the mothers of living infants for the different diet groups were within 0.2 kg of each other. The monkeys lived in individual cages in a roofed compound having screen walls on three sides. The average outdoor temperature of our coldest month is 10 C and that of the highest month is 28 C. We did not regulate the room temperature during the spring, summer, or fall, but we hung a transparent plastic sheet over the wire screen in the winter to keep out the wind and on the coldest nights we provided supplemental heat keeping the room temperature above 15 C on most nights. Gestation technically begins at the instant of fertilization, but practically it is reckoned from some arbitrary point, e.g., the last menstruation in humans or the midpoint of the mating period in monkeys. The second day ofmating was taken as the time ofconception for our animals, and since it is known that sperm can live in the vaginal tract as long as 2 days before fertilizing the ovum (2), there is a potential 4-day error in estimate. That is a (presumably) random variation in gestation length that is beyond our control. It is not likely, however, that this variation systematic.alIy affected the results or was this large.
Results Background
To
put
spective
from note were they gained.
the we
present should
results
in proper
summarize
some
perdata
previous reports of this experiment. We first that when nonpregnant monkeys fed diets 2, 1, or another diet (diet 1/2), lost weight whereas those fed diet 4 In
addition,
they
showed
significant
systematic changes in the total albumin and in the nonessential-to-essential amino acid ratio of the circulating plasma, indicating that protein intake less than 4 g/kg per day was detrimental (4). Comparable data from the pregnant animals whose gestations are described herein have also been reported (5).
LENGTH
Although
IN
direct
MONKEYS
1171
comparison,
diet for diet,
has
not been made in any definitive sense, it appears that the pregnant animals are at least as tolerant of the low protein diet as are the nonpregnant animals. Low protein intake during pregnancy nevertheless has deleterious effects on both weight gain and plasmaprotein concentrations. Sizes
of infant
We
were
groups
able
to analyze
the
data
of 47
infants on whom accurate conception and delivery dates were known. They represent the live-born products of 74 pregnancies in 31 ofa total of45 impregnated females. Twenty-
one Ten
infants were male and males and eight females
l-g mothers, five males mothers, and six males 4-g mothers. An analysis
been
reported
analysis
basic
statistical
regression analysis. on a general linear
dependent
value
method
(gestation
factors,
used
was
a
This procedure is based model in which each
to result from a linear of random error and
identifiable
and ten females of 2-g and eight females of of fetal wastage has
(6).
Statistical
The
26 were female. were born of the
length)
is assumed
(additive) combination the effects of certain
some
of which
are
in
discrete classes (e.g., sex, diet) and others are continuous variables (temperature, body weight). The aims of the analysis are twofold: a) to assess the statistical significance of each of the factors in relation to their effect on gestation length and b) to estimate their “true” effects uncontaminated by random or systematic correlation with the other variables. So, if gestations of male infants are longer than those of females, and the diet groups happen to differ by chance or otherwise in the ratio of male to female infants, the
diet means are contaminated by the sex factor, and hence must be adjusted for the sex effects to arrive at the correct diet effect. As an extra benefit, we can tion of any combination
assess the contribuof variables to the
total variance, each independent of the others. If any group of such variables is analyzed simultaneously we can write an equation which is identical in form to the general linear model but in which sample-
Downloaded from https://academic.oup.com/ajcn/article-abstract/28/10/1170/4716669 by McMaster University Library user on 31 January 2019
cry,
or
one-half but of equal
AFFECTING
1172
RIOPELLE
AND
HALE
the roles derived rameters
estimates are of the model. least squares
efficient
forming puters
“packaged”
programs
these analyses with make them routine.
In our initial
studies,
for
high-speed
we included
percom-
paternity
Table
1 shows
the summaries
ses of variance
excludes
and
performed.
the
lower
The
portion
of the analyupper
portion
includes
the
variable birth weight. A formula derived from the data of the upper table can be used to predict the gestation lengths for male and for female infants as soon as the mother is declared to be pregnant and she is assigned to the diet. A formula based on the data of the lower table is perhaps less useful for predicting, but it contributes to our understanding of
TABLE Summary coefficIents
I
Source
of analyses
of variance
and
Partial sum of
df
regression
F
P
squares
Diet Sex Diet x sex CONCWT AMBTEMP Error
2 I 2 I I 39
674.2 228.9 22.8 126.5 319.9
8.33 5.65 0.28 3.13 7.90
0.001 0.022 0.760 0.085 0.008
1578.5
Regression coefficients CONCWT AMBTEMP
Source
1.10 0.52 Partial sum of
df
F
P
9.75 2.15 0.12 1.12 4.00 7.78
0.001 0.151 0.885 0.296 0.053 0.008
squares
Diet Sex Diet x sex CONCWT AMBTEMP BIRTHWT Error
2 I 2 1 I 1 38 Regression coefficients
CONCWT AMBTEMP BIRTHWT
0.63 0.36 0.04
672.6 74.1 8.5 38.7 137.9 268.1 1310.4
factors
in determining
gestation. Overall
gestation
length
The mean gestation length for the 47 deliveries was 165. 1 days. The standard deviation
as one of the factors to be evaluated (see immediately below), but its effects were so insignificant (P > 0.50) that we dropped it from further consideration.
of the various
was
8.3
days.
Paternity
The infants Although there gestation
various fants;
lengths
males longest
differences
and
were were
variations
of
not
were in
infants
(shortest 168 days,
were
probably
sired by differences
time
by
the
157 days, three seven infants),
sired
inthe
significant
due
of conception
(P
in part
correlated
when the effects of diet, conception weight and at
nine males. in average
>
0.50)
to sampling
variables.
Indeed,
infant sex, diet x sex, ambient temperature were
eliminated
from
consideration, the com parable adjusted averages became 164 and 165, considerably different from the corresponding raw values. The apparent paternity effects are likely to be due to other factors. However, the limited data of this experiment cannot rule out the paternal factor as a small but real source of genetic variation
that
factors
discussed
Maternal
perhaps
conception
It was impossible sis of the individual
is independent
of the
below. weight
to make female’s
a similar genetic
analycontribu-
tion because only a few mothers gave birth to more than one infant. As a substitute, we used the mother’s conception weight, which is a crude, composite, quantitative index of her age, her parity, her physical size, and her state of health and nutrition, all of which may relate to gestation in different ways. The overall correlation between conception weight and gestation length was 0.20, suggestive, but not significantly different from zero (P < 0. 16). Heavy mothers tend to have long gestations: for every kilogram increase in maternal weight there was a more than 1-day increase in average gestation length. Protein
intake
The left panel of Fig. 1 (unadjusted) shows the relationship of gestation length to maternal protein intake for male and female infants
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Fortunately,
substituted for the paSuch an equation is an estimate of the model.
FACTORS
AFFECTING
GESTATION
LENGTH
IN
1173
MONKEYS
ITO
I..
0
z
FEMALE
Ui -I
z 0 I-
Ui 0
I.,
151
184 UNADJUSTED
ADJUSTED’
ADJUsTED,
AMST(MP
4
2
FIG. I. Gestation weight of the mother the
infant
I
S
MATERNAL
IETARY
lengths of male and (CONCWT), ambient
female infants temperature
(BIRTHWT).
Adjustment
is approximately
diets
30 days
of pregnancy.
same
regardless
were
not initiated
The
of whether
until
diet effect the
large or small or whether they the summer or in the winter.
after were
conceived
in
Seasonality
factors such (for
that
might
as those reported which the data
affect
gesta-
for domesare mostly
obtainable), are ordinarily so confounded with differences in nutrition, housing, geographic location, and activity levels that they are difficult to disentangle from the others. But since our animals were fed the same basic diets and they were caged and housed identically throughout the year, our data are free of
4
LEVEL
constant
the
factor
effects and
of conception birth weight
of
identified.
these entanglements. A minor complication is that simians, generally, are seasonal breeders, and rhesus monkeys are no exception. Yet breeding is not restricted to only a few months. Conception dates of 50 pregnancies that resulted in live infants were distributed throughout the year (Fig. 2A). There was a midsummer depression in conceptions, but successful mating could and did take place throughout the year. We noted a tendency for an increased ‘number of fetal fatalities in the protein-deficient groups. Inspection of the conception dates for the gestational mishaps
(Fig.
Seasonality
tion length, tic animals
to holding
ANSTEMP,
S
before and after adjustment for the at time of conception (AMBTEMP)
equivalent
was the
mothers
I
PROTEIN
separately. The diet 1 gestations were about as long as those of the diet 2, but gestations were shortened about 8.5 days in the diet 4 mothers (P < 0.001). The diet effect was the same for both male and female infants. The diet factor exerted its influence only during the last 4.5 months of gestation since the low-protein
4
CONC*T. !NTNWT
‘
2B) show
that
they,
frequently in the months conceptions. The presence neither alters nor biases present report.
The
average
ambient
too,
occurred
most
having the most of mishaps thus the data of the
temperature
of the
conception months correlated 0.40 tation length, which means that conceptions resulted in significantly
with gessummer (P
-I
delivery. CONCEPTION
MONTH
FIG. 2. A: seasonal distribution seasonal distribution of conceptions in a live-born infant.
of conceptions. B: that failed to result
w. U .2 U
0
w
C
I
TEMPERATURE Z,
MONTHS
‘b
3 suggest
conception
determine
also
help
gestation
of
AFTER
in gestation (first
point,
CONCEPTION
length Fig.
(regression
slope
3).
If we ask whether or not physiological events occurring later in pregnancy might even more temperature-related than those and we successive
plot the regression month of gestation,
daily
factor.
photoperiod
Length
or
of daylight
any
other
correlates
very highly with temperature and there is no way to separate daylight length from temperature except by experimental means.
3
FIG. 3. The importance of ambient temperature at various stages of pregnancy for determining length of gestation. Each point denotes the coefticient of regression of gestation length on ambient temperature at successive months after conception.
conception, for each
in Fig.
the other and not some The smooth transition of to negative seems to of each month after
seasonal AMBIENT
0.52)
maxima
the curve from positive imply that the events
length
0 I&I-.4
=
dual
length. More likely, the smoothness is simply a consequence of the fact that the temperatures of adjacent months are more similar to each other than they are to those of more remote months, so that any correlation with the temperature of 1 month implies a similar but reduced correlation for that of adjacent months. Of course, ambient temperature itself may not be the critical variable; it may be the
z4
increase
The
that it is one or midgestation event.
be at
slope the
results are as presented in Fig. 3. The regression slope, which is tantamount to correlation, decreased each month after conception, reaching its greatest negative value at about time of delivery (conception + 6 months).
Sex
of infant
Males were born after 168.5 days and females after 162.3 days, a difference of 6.2 days. Adjusting the mean by partialling out (eliminating) the effects of maternal conception weight and ambient temperature at time of conception reduced the difference to 4.7 days (167.5 vs. 162.8 days) (P < 0.001). The difference
of 4.7
days,
incidentally,
is oppo-
site in direction and considerably greater in magnitude than that (- I day) reported by van Wagenen and Catchpole (I). Valerio et al. (2), were not sufficiently impressed by any difference, if they found one, to report it, despite experience with nearly 600 timed pregnancies.
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That the correlation is at once maximally positive for month of conception and maximally negative for month of delivery is an inevitable consequence of the fact that gestation lasts approximately 6 months, so that animals conceived in the summer are dclivered in the winter and vice versa. Hence, any variable that correlates positively with ambient temperature at conception will correlate negatively with temperature at delivery. Thus, we cannot know from the present data whether the temperature-related event that influences gestation length occurs near the time of conception or near the time of
U,
FACTORS
Birth
AFFECTING
GESTATION
weight
difference
in
birth
weight.
Birth
weight,
of
course, is itself a function of maternal size, season of birth, and perhaps diet. Further regression analysis partialling out the effects of infant birth weight in addition to those of conception weight and conception temperature, yielded an adjusted mean gestation length of 166.4 days for males and 163.5 days for females; the difference being now reduced to 2.9 days, still significant (P < 0.01). This means that for male and female infants weighing the same at birth and conceived by parents of the same size and at the same time of year, the difference in gestation length would be reduced by nearly 2 days. Thus, about 40% of the difference in overall gestation length between the sexes can be attributed to the fetal size difference. Male fetuses, therefore, incubate longer than do females not only because they are males, but also because they are heavier. Thus, there seems to be at least two separate kinds of messages to the mother, one that perhaps is directly or indirectly
and
hormonal
metabolic.
and
Note
also
the
other
that
physical
the
longer
gestation of the heavier infants is contrary in direction to the usually accepted belief that heavy pregnancies are shorter than light preg-
to a standard
the mothers weight.
Independence
of variables
nancies
or
that
carry
their
infants
IN
MONKEYS
and perhaps
diet.
considered
“independent”
Thus,
by no means
upshot
of this,
then,
is that
conceives
in
winter,
is
on
independent;
ception
instead
weight,
their
it is a function
ambient
temperature,
of con-
sex,
a
high
plane
of
protein intake, and she bears a small female infant, she will shorten her gestation by more than 2 weeks. She is thus exquisitely sensitive to both internal and external environmental change. Such effects have not been reported for animals raised in the usual laboratory setting, although they are well known for domestic animals. We conceive of the pregnant primate, perhaps more than the pregnant, litter-bearing rodent or domestic animal, as adopting a strategy of pregnancy that involves assessment of her environment, her nutrition, and the feedback from her single fetus.
regarding
a
These data show that the protein concentration of the pregnant monkey’s diet, her weight at conception, the sex of her infant, and the season of her pregnancy significantly and collectively affect the length of her gestation, if she is maintained in an environment that exposes her partly to the elements. If she
She
may
interrupt
the fetus
the
can survive
ity of the environmental-gestational
other
diet
despite
Discussion
before
length. As the’similarity of in the several panels of general pattern of dietary effects appears whether or (adjust for) the effects of ambient temperature, or on the other hand, is not
a high-protein
during pregnancy shortens gestation wide variety of other conditions.
independent
each
Al-
though, as we have already seen, it accounts for some of the difference in gestation length that exists for male and female infants, it does not change the dietary effect, for the latter emerges regardless of the inclusion of birth weight (Fig. 1) as an isolated variable. The
it
of
can it be
logically.
Diet, maternal conception weight, and ambient temperature are “independent” variables in the ordinary meaning of the term. Infant sex is also independent in the sense that it is not determined by the previous three. Interestingly, these variables seem also to be effect on gestation the graphs portrayed Fig. 1 shows, the and of infant sex not we eliminate conception weight, both. Birth weight,
1175
dies
however,
in
utero.
will
Her
most
infant
likely
experience the mother’s result in a physiologically
Clegg’s domestic
review animals
pregnancy
early,
or perhaps,
after
if born
alive,
survive, adjustments competent
for in our tend to infant.
(7) of gestation length in emphasizes the particularrelations
for different species. Cows respond differently than horses in altering their gestation lengths to environmental stimulation and even strains of one species may differ remarkably among themselves. Factors controlling human gestation length are likewise complex. The difficulty of analysis is compounded when gestation is counted from last menstruation
rather
than
from
ovulation
or fertil-
Downloaded from https://academic.oup.com/ajcn/article-abstract/28/10/1170/4716669 by McMaster University Library user on 31 January 2019
Since males at birth were heavier than females (460 g vs. 4 1 3 g), their longer gestation length might be attributable to the
LENGTH
RIOPELLE
1176
ization. monkey, ments
As inconvenient its willingness involving
AND
as it is to study the to serve in experi-
environmental
parameters
ing
its
time
of conception
make
it a fruitful
animal for study, and the relations discovered with it might have application to humans. When environmental uniformity prevails, the genetic potential expresses itself. The observations clic, perhaps
reported here suggest even taxing, environmental
that
cyfluc-
tuation sensitizes the mother to external and internal stimulation and assists her to respond to the progress of her pregnancy and to cope with
it.
Recapitulation
The results of this investigation can be summarized by two empirical, approximate formulas relating estimated gestation length in monkeys to environmental factors within the ranges of the variables studied here. Estimated gest 168.26 + 2.35 5 (P-l)2 + 1.1 (W-6.06) + 0.52 (T-l5.5), where S denotes infant sex and takes the value + 1 for males and - 1 for females, P is =
the number of grams of protein ingested daily for each kilogram of body weight, W is the conception weight of the mother in kilograms, degrees
and T is the average Celsius for the month
temperature of conception.
in
The constants within the parentheses represent the mean conception weight and the mean ambient temperature of the conception months. In use, the formula would predict two scores, one for males and one for females;
they would differ by 4.7 days. The predicted gestations correlate 0.69 with the actual. If birth weight is added to the equation, the formula becomes: estimated gest 169.57 + 1 .42 S - 1 .87 P - 0.06 P2 + 0.63 (W- 6.06) + 0.36 (T - 15.5) + 0.04 (B - 434.04). Gestations predicted by this formula correlate 0.75 with actual gestations. Of more interest are the revised regression coefficients, which reflect the altered interrelations among the factors. =
References 1. VAN WAGENEN, G., AND H. R. CATCHPOLE, with the assistance of J. Negri and D. Butzko. Growth of the fetus and placenta of the monkey (Macaca muiatta). Am. J. Phys. Anthropol. 2J: 23-4, 1965. 2. VALERIO, D. A., R. L. MiLLER, J. R. M. INNES, K. D. COURTNEY, A. J. PALLOTTA AND R. M. GurrMACHER. Macaca mulatta. Management of a laboratory breeding colony. New York: Academic, 1969, p. xii + 140. 3. PICKERING, D. E., AND N. E. KONTAXIS. Thyroid function in the foetus of the macaque monkey (Macaca mulatta). II. Chemical and morphological characteristics of the foetal thyroid gland. J. Endocrinol. 23: 267-275, 1961. 4. RIOPELLE, A. J., C. W. HILL, S-C Li, R. H. WOLF, H. R. SEIBOLD AND J. L. SMITH. Protein deprivation in primates. I. Nonpregnant adult rhesus monkeys. Am. J. Clin. Nutr. 27: 13-21, 1974. 5. RIOPELLE, A. J., C. W. HiLL, S-C. Li, R. H. WOLF, H. R. SEIBOLD AND J. L. SMITH. Protein deprivation in primates: IV. Pregnant rhesus monkeys. Am. J. Clin. Nutr. 28: 20-28, 1975. 6. RIOPELLE, A. J., C. W. Hiii, AND S-C. Li. Protein deprivation in primates. V. Fetal mortality and neonatal status of infant monkeys born of deprived mothers. Am. J. Clin. Nutr. 28: 989-993, 1975. 7. CLEGG, M. T. Factors affecting gestation length and parturition. In: Reproduction in Domestic Animals, edited by H. H. Cole, and P. T. Cupps. New York: Academic, 1959, p. 509-538.
Downloaded from https://academic.oup.com/ajcn/article-abstract/28/10/1170/4716669 by McMaster University Library user on 31 January 2019
beyond those allowable for humans and the reasonable precision attainable in determin-
HALE
and environmental gestation length and Penelope
J. Riopelle
ABSTRACT ment and diets
observed.
than
infants
The
females;
conception
4-g
part
produced slightly
Hale
Forty-five pregnancies in rhesus 1 , 2, or 4 g of protein/kilogram
fed about
were
longer
Anne
diet
longer
longer
than
shortened
of difference gestation small
The
breathing Enormous ductive supials,
offspring at birth are capable of and of taking food by mouth. species differences exist in repro-
styles. which
At one extreme are have a very short
the margestation
period but a long postnatal period of oral attachment to the teat. Placental mammals, at the other, have longer gestation periods, and once born they need but to feed at the breast nods.
are
intermittently
Gestation brought
and
only
length, during to their state
semi-independent
living,
similarly
has
is usually
its
brief
pe-
which the young of readiness for
to be primarily genetically crossbreeding experiments genetic component beyond species
for
considered
controlled, demonstrate a doubt.
characteristic
and the Each gesta-
tion length: the rat’s gestation length is 21 days and the human’s is 265 days. Rhesus monkeys (Macaca mulatta) are born after an average gestation period ofabout 165 days (1, 2). Valenio, et al. (2) cite 165 days as average for 600 laboratory-bred rhesus; van Wagenen reports 168.9 ± 6.85 for 225 similar animals and Pickering and Kontaxis (3) report 167 ± 4 days
differences 1 170
for
50 pregnancies.
exist
between
The American
Minor
accounting
laboratories. Journal
ofClinical
Nutrition
monkeys of body
gestation can
than
ones.
Each species of animals from the very first has, within its evolutionary history, developed an effective system of reproduction. In complex animals the fetuses grow within the mother’s body until they reach a state of development compatible with the speciescharacteristic pattern of postnatal maternal care, so that by and large some of the litter survive.
factors in rhesus monkeys”2’3
be
8.5
days.
attributed
did
winter
J.
C/in.
Am.
maintained weight each to
Male their
conception. Nutr.
in a semioutdoor day in otherwise infants
greater Large
28:
1170-1176,
were
environequivalent
carried
birthweight. mothers
6 days Summer
carried
their
l975.
But averages do not tell the whole story for they imply genetic stability rather than physiologic lability. In this paper we report how the gestation lengths of rhesus monkeys, in response to environmental factors, vary as much as 20% of normal term without disaster to either infant or mother. This suggests that gestation involves adjustments to the environment, and the length of gestation in each pregnancy as well as the well-being of the infant is a result of these adjustments. Material
and
methods
The present study involves the mothers of 47 live infants spontaneously delivered. Abortions and stillbirths, though not uniformly distributed among the groups, were excluded. Participants were young adult and fully adult female rhesus monkeys weighing between 3.7 and 9.2 kg and averaging 6.1 kg (standard deviation = 1.53 kg). They were assigned to the study after conditioning to the laboratory routines. The animals were first adapted to a semisynthetic diet containing 15.3% casein (13.4% protein) for at least 3 months prior to mating. We called this diet 4 because if afforded 4 g of high-quality protein/kilogram of body weight daily if the animal ate 30 g (120 kcal) of the food for every kilogram of body weight. Mating was allowed in a 4-day mating period which began I 1 days after the onset of menstruation. All animals were maintained on that diet for one more month. If at that time we concluded on rectal palpation that they were pregnant, they were either continued on From the Department of Psychology, Louisiana University. a Supported by Public Health Service Grant HD-07479. a Reprint requests: Arthur J. Riopelle or Penelope Anne Hale, Department of Psychology, Louisiana State University, Baton Rouge, LA 70803. 1
State
28: OCTOBER
1975,
pp.
1 170-1
176. Printed
in U.S.A.
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Nutritional affecting
FACTORS the
same
diet
containing protein deficient
they
assigned
to
a similar
diet
(diet 1) of its animals on the were continued on those diets until dclivthey were returned to diet 4. animals to the different diet groups at
diets
whereupon We assigned
random weight.
were
GESTATION
(diet 2) or one-fourth caloric density. The
within roughly stratified levels of maternal body The intent was to maintain a balance of means
and variances of conception groups as new pregnancies replacements were required
weights among the three were encountered and as following fetal loss. The first
dozen infants came from the 4-g and the 2-g diets, for in our ignorance of the severity of the consequences of the deprivation, we were hesitant toassign mothers to the l-g diet for fear that few or no infants would result. When experience showed otherwise, relatively more mothers were assigned to the l-g diet to attain balance. Such balance was eventually achieved, and subsequent assignments were at random. The average conception weights
of the mothers of living infants for the different diet groups were within 0.2 kg of each other. The monkeys lived in individual cages in a roofed compound having screen walls on three sides. The average outdoor temperature of our coldest month is 10 C and that of the highest month is 28 C. We did not regulate the room temperature during the spring, summer, or fall, but we hung a transparent plastic sheet over the wire screen in the winter to keep out the wind and on the coldest nights we provided supplemental heat keeping the room temperature above 15 C on most nights. Gestation technically begins at the instant of fertilization, but practically it is reckoned from some arbitrary point, e.g., the last menstruation in humans or the midpoint of the mating period in monkeys. The second day ofmating was taken as the time ofconception for our animals, and since it is known that sperm can live in the vaginal tract as long as 2 days before fertilizing the ovum (2), there is a potential 4-day error in estimate. That is a (presumably) random variation in gestation length that is beyond our control. It is not likely, however, that this variation systematic.alIy affected the results or was this large.
Results Background
To
put
spective
from note were they gained.
the we
present should
results
in proper
summarize
some
perdata
previous reports of this experiment. We first that when nonpregnant monkeys fed diets 2, 1, or another diet (diet 1/2), lost weight whereas those fed diet 4 In
addition,
they
showed
significant
systematic changes in the total albumin and in the nonessential-to-essential amino acid ratio of the circulating plasma, indicating that protein intake less than 4 g/kg per day was detrimental (4). Comparable data from the pregnant animals whose gestations are described herein have also been reported (5).
LENGTH
Although
IN
direct
MONKEYS
1171
comparison,
diet for diet,
has
not been made in any definitive sense, it appears that the pregnant animals are at least as tolerant of the low protein diet as are the nonpregnant animals. Low protein intake during pregnancy nevertheless has deleterious effects on both weight gain and plasmaprotein concentrations. Sizes
of infant
We
were
groups
able
to analyze
the
data
of 47
infants on whom accurate conception and delivery dates were known. They represent the live-born products of 74 pregnancies in 31 ofa total of45 impregnated females. Twenty-
one Ten
infants were male and males and eight females
l-g mothers, five males mothers, and six males 4-g mothers. An analysis
been
reported
analysis
basic
statistical
regression analysis. on a general linear
dependent
value
method
(gestation
factors,
used
was
a
This procedure is based model in which each
to result from a linear of random error and
identifiable
and ten females of 2-g and eight females of of fetal wastage has
(6).
Statistical
The
26 were female. were born of the
length)
is assumed
(additive) combination the effects of certain
some
of which
are
in
discrete classes (e.g., sex, diet) and others are continuous variables (temperature, body weight). The aims of the analysis are twofold: a) to assess the statistical significance of each of the factors in relation to their effect on gestation length and b) to estimate their “true” effects uncontaminated by random or systematic correlation with the other variables. So, if gestations of male infants are longer than those of females, and the diet groups happen to differ by chance or otherwise in the ratio of male to female infants, the
diet means are contaminated by the sex factor, and hence must be adjusted for the sex effects to arrive at the correct diet effect. As an extra benefit, we can tion of any combination
assess the contribuof variables to the
total variance, each independent of the others. If any group of such variables is analyzed simultaneously we can write an equation which is identical in form to the general linear model but in which sample-
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cry,
or
one-half but of equal
AFFECTING
1172
RIOPELLE
AND
HALE
the roles derived rameters
estimates are of the model. least squares
efficient
forming puters
“packaged”
programs
these analyses with make them routine.
In our initial
studies,
for
high-speed
we included
percom-
paternity
Table
1 shows
the summaries
ses of variance
excludes
and
performed.
the
lower
The
portion
of the analyupper
portion
includes
the
variable birth weight. A formula derived from the data of the upper table can be used to predict the gestation lengths for male and for female infants as soon as the mother is declared to be pregnant and she is assigned to the diet. A formula based on the data of the lower table is perhaps less useful for predicting, but it contributes to our understanding of
TABLE Summary coefficIents
I
Source
of analyses
of variance
and
Partial sum of
df
regression
F
P
squares
Diet Sex Diet x sex CONCWT AMBTEMP Error
2 I 2 I I 39
674.2 228.9 22.8 126.5 319.9
8.33 5.65 0.28 3.13 7.90
0.001 0.022 0.760 0.085 0.008
1578.5
Regression coefficients CONCWT AMBTEMP
Source
1.10 0.52 Partial sum of
df
F
P
9.75 2.15 0.12 1.12 4.00 7.78
0.001 0.151 0.885 0.296 0.053 0.008
squares
Diet Sex Diet x sex CONCWT AMBTEMP BIRTHWT Error
2 I 2 1 I 1 38 Regression coefficients
CONCWT AMBTEMP BIRTHWT
0.63 0.36 0.04
672.6 74.1 8.5 38.7 137.9 268.1 1310.4
factors
in determining
gestation. Overall
gestation
length
The mean gestation length for the 47 deliveries was 165. 1 days. The standard deviation
as one of the factors to be evaluated (see immediately below), but its effects were so insignificant (P > 0.50) that we dropped it from further consideration.
of the various
was
8.3
days.
Paternity
The infants Although there gestation
various fants;
lengths
males longest
differences
and
were were
variations
of
not
were in
infants
(shortest 168 days,
were
probably
sired by differences
time
by
the
157 days, three seven infants),
sired
inthe
significant
due
of conception
(P
in part
correlated
when the effects of diet, conception weight and at
nine males. in average
>
0.50)
to sampling
variables.
Indeed,
infant sex, diet x sex, ambient temperature were
eliminated
from
consideration, the com parable adjusted averages became 164 and 165, considerably different from the corresponding raw values. The apparent paternity effects are likely to be due to other factors. However, the limited data of this experiment cannot rule out the paternal factor as a small but real source of genetic variation
that
factors
discussed
Maternal
perhaps
conception
It was impossible sis of the individual
is independent
of the
below. weight
to make female’s
a similar genetic
analycontribu-
tion because only a few mothers gave birth to more than one infant. As a substitute, we used the mother’s conception weight, which is a crude, composite, quantitative index of her age, her parity, her physical size, and her state of health and nutrition, all of which may relate to gestation in different ways. The overall correlation between conception weight and gestation length was 0.20, suggestive, but not significantly different from zero (P < 0. 16). Heavy mothers tend to have long gestations: for every kilogram increase in maternal weight there was a more than 1-day increase in average gestation length. Protein
intake
The left panel of Fig. 1 (unadjusted) shows the relationship of gestation length to maternal protein intake for male and female infants
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Fortunately,
substituted for the paSuch an equation is an estimate of the model.
FACTORS
AFFECTING
GESTATION
LENGTH
IN
1173
MONKEYS
ITO
I..
0
z
FEMALE
Ui -I
z 0 I-
Ui 0
I.,
151
184 UNADJUSTED
ADJUSTED’
ADJUsTED,
AMST(MP
4
2
FIG. I. Gestation weight of the mother the
infant
I
S
MATERNAL
IETARY
lengths of male and (CONCWT), ambient
female infants temperature
(BIRTHWT).
Adjustment
is approximately
diets
30 days
of pregnancy.
same
regardless
were
not initiated
The
of whether
until
diet effect the
large or small or whether they the summer or in the winter.
after were
conceived
in
Seasonality
factors such (for
that
might
as those reported which the data
affect
gesta-
for domesare mostly
obtainable), are ordinarily so confounded with differences in nutrition, housing, geographic location, and activity levels that they are difficult to disentangle from the others. But since our animals were fed the same basic diets and they were caged and housed identically throughout the year, our data are free of
4
LEVEL
constant
the
factor
effects and
of conception birth weight
of
identified.
these entanglements. A minor complication is that simians, generally, are seasonal breeders, and rhesus monkeys are no exception. Yet breeding is not restricted to only a few months. Conception dates of 50 pregnancies that resulted in live infants were distributed throughout the year (Fig. 2A). There was a midsummer depression in conceptions, but successful mating could and did take place throughout the year. We noted a tendency for an increased ‘number of fetal fatalities in the protein-deficient groups. Inspection of the conception dates for the gestational mishaps
(Fig.
Seasonality
tion length, tic animals
to holding
ANSTEMP,
S
before and after adjustment for the at time of conception (AMBTEMP)
equivalent
was the
mothers
I
PROTEIN
separately. The diet 1 gestations were about as long as those of the diet 2, but gestations were shortened about 8.5 days in the diet 4 mothers (P < 0.001). The diet effect was the same for both male and female infants. The diet factor exerted its influence only during the last 4.5 months of gestation since the low-protein
4
CONC*T. !NTNWT
‘
2B) show
that
they,
frequently in the months conceptions. The presence neither alters nor biases present report.
The
average
ambient
too,
occurred
most
having the most of mishaps thus the data of the
temperature
of the
conception months correlated 0.40 tation length, which means that conceptions resulted in significantly
with gessummer (P
-I
delivery. CONCEPTION
MONTH
FIG. 2. A: seasonal distribution seasonal distribution of conceptions in a live-born infant.
of conceptions. B: that failed to result
w. U .2 U
0
w
C
I
TEMPERATURE Z,
MONTHS
‘b
3 suggest
conception
determine
also
help
gestation
of
AFTER
in gestation (first
point,
CONCEPTION
length Fig.
(regression
slope
3).
If we ask whether or not physiological events occurring later in pregnancy might even more temperature-related than those and we successive
plot the regression month of gestation,
daily
factor.
photoperiod
Length
or
of daylight
any
other
correlates
very highly with temperature and there is no way to separate daylight length from temperature except by experimental means.
3
FIG. 3. The importance of ambient temperature at various stages of pregnancy for determining length of gestation. Each point denotes the coefticient of regression of gestation length on ambient temperature at successive months after conception.
conception, for each
in Fig.
the other and not some The smooth transition of to negative seems to of each month after
seasonal AMBIENT
0.52)
maxima
the curve from positive imply that the events
length
0 I&I-.4
=
dual
length. More likely, the smoothness is simply a consequence of the fact that the temperatures of adjacent months are more similar to each other than they are to those of more remote months, so that any correlation with the temperature of 1 month implies a similar but reduced correlation for that of adjacent months. Of course, ambient temperature itself may not be the critical variable; it may be the
z4
increase
The
that it is one or midgestation event.
be at
slope the
results are as presented in Fig. 3. The regression slope, which is tantamount to correlation, decreased each month after conception, reaching its greatest negative value at about time of delivery (conception + 6 months).
Sex
of infant
Males were born after 168.5 days and females after 162.3 days, a difference of 6.2 days. Adjusting the mean by partialling out (eliminating) the effects of maternal conception weight and ambient temperature at time of conception reduced the difference to 4.7 days (167.5 vs. 162.8 days) (P < 0.001). The difference
of 4.7
days,
incidentally,
is oppo-
site in direction and considerably greater in magnitude than that (- I day) reported by van Wagenen and Catchpole (I). Valerio et al. (2), were not sufficiently impressed by any difference, if they found one, to report it, despite experience with nearly 600 timed pregnancies.
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That the correlation is at once maximally positive for month of conception and maximally negative for month of delivery is an inevitable consequence of the fact that gestation lasts approximately 6 months, so that animals conceived in the summer are dclivered in the winter and vice versa. Hence, any variable that correlates positively with ambient temperature at conception will correlate negatively with temperature at delivery. Thus, we cannot know from the present data whether the temperature-related event that influences gestation length occurs near the time of conception or near the time of
U,
FACTORS
Birth
AFFECTING
GESTATION
weight
difference
in
birth
weight.
Birth
weight,
of
course, is itself a function of maternal size, season of birth, and perhaps diet. Further regression analysis partialling out the effects of infant birth weight in addition to those of conception weight and conception temperature, yielded an adjusted mean gestation length of 166.4 days for males and 163.5 days for females; the difference being now reduced to 2.9 days, still significant (P < 0.01). This means that for male and female infants weighing the same at birth and conceived by parents of the same size and at the same time of year, the difference in gestation length would be reduced by nearly 2 days. Thus, about 40% of the difference in overall gestation length between the sexes can be attributed to the fetal size difference. Male fetuses, therefore, incubate longer than do females not only because they are males, but also because they are heavier. Thus, there seems to be at least two separate kinds of messages to the mother, one that perhaps is directly or indirectly
and
hormonal
metabolic.
and
Note
also
the
other
that
physical
the
longer
gestation of the heavier infants is contrary in direction to the usually accepted belief that heavy pregnancies are shorter than light preg-
to a standard
the mothers weight.
Independence
of variables
nancies
or
that
carry
their
infants
IN
MONKEYS
and perhaps
diet.
considered
“independent”
Thus,
by no means
upshot
of this,
then,
is that
conceives
in
winter,
is
on
independent;
ception
instead
weight,
their
it is a function
ambient
temperature,
of con-
sex,
a
high
plane
of
protein intake, and she bears a small female infant, she will shorten her gestation by more than 2 weeks. She is thus exquisitely sensitive to both internal and external environmental change. Such effects have not been reported for animals raised in the usual laboratory setting, although they are well known for domestic animals. We conceive of the pregnant primate, perhaps more than the pregnant, litter-bearing rodent or domestic animal, as adopting a strategy of pregnancy that involves assessment of her environment, her nutrition, and the feedback from her single fetus.
regarding
a
These data show that the protein concentration of the pregnant monkey’s diet, her weight at conception, the sex of her infant, and the season of her pregnancy significantly and collectively affect the length of her gestation, if she is maintained in an environment that exposes her partly to the elements. If she
She
may
interrupt
the fetus
the
can survive
ity of the environmental-gestational
other
diet
despite
Discussion
before
length. As the’similarity of in the several panels of general pattern of dietary effects appears whether or (adjust for) the effects of ambient temperature, or on the other hand, is not
a high-protein
during pregnancy shortens gestation wide variety of other conditions.
independent
each
Al-
though, as we have already seen, it accounts for some of the difference in gestation length that exists for male and female infants, it does not change the dietary effect, for the latter emerges regardless of the inclusion of birth weight (Fig. 1) as an isolated variable. The
it
of
can it be
logically.
Diet, maternal conception weight, and ambient temperature are “independent” variables in the ordinary meaning of the term. Infant sex is also independent in the sense that it is not determined by the previous three. Interestingly, these variables seem also to be effect on gestation the graphs portrayed Fig. 1 shows, the and of infant sex not we eliminate conception weight, both. Birth weight,
1175
dies
however,
in
utero.
will
Her
most
infant
likely
experience the mother’s result in a physiologically
Clegg’s domestic
review animals
pregnancy
early,
or perhaps,
after
if born
alive,
survive, adjustments competent
for in our tend to infant.
(7) of gestation length in emphasizes the particularrelations
for different species. Cows respond differently than horses in altering their gestation lengths to environmental stimulation and even strains of one species may differ remarkably among themselves. Factors controlling human gestation length are likewise complex. The difficulty of analysis is compounded when gestation is counted from last menstruation
rather
than
from
ovulation
or fertil-
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Since males at birth were heavier than females (460 g vs. 4 1 3 g), their longer gestation length might be attributable to the
LENGTH
RIOPELLE
1176
ization. monkey, ments
As inconvenient its willingness involving
AND
as it is to study the to serve in experi-
environmental
parameters
ing
its
time
of conception
make
it a fruitful
animal for study, and the relations discovered with it might have application to humans. When environmental uniformity prevails, the genetic potential expresses itself. The observations clic, perhaps
reported here suggest even taxing, environmental
that
cyfluc-
tuation sensitizes the mother to external and internal stimulation and assists her to respond to the progress of her pregnancy and to cope with
it.
Recapitulation
The results of this investigation can be summarized by two empirical, approximate formulas relating estimated gestation length in monkeys to environmental factors within the ranges of the variables studied here. Estimated gest 168.26 + 2.35 5 (P-l)2 + 1.1 (W-6.06) + 0.52 (T-l5.5), where S denotes infant sex and takes the value + 1 for males and - 1 for females, P is =
the number of grams of protein ingested daily for each kilogram of body weight, W is the conception weight of the mother in kilograms, degrees
and T is the average Celsius for the month
temperature of conception.
in
The constants within the parentheses represent the mean conception weight and the mean ambient temperature of the conception months. In use, the formula would predict two scores, one for males and one for females;
they would differ by 4.7 days. The predicted gestations correlate 0.69 with the actual. If birth weight is added to the equation, the formula becomes: estimated gest 169.57 + 1 .42 S - 1 .87 P - 0.06 P2 + 0.63 (W- 6.06) + 0.36 (T - 15.5) + 0.04 (B - 434.04). Gestations predicted by this formula correlate 0.75 with actual gestations. Of more interest are the revised regression coefficients, which reflect the altered interrelations among the factors. =
References 1. VAN WAGENEN, G., AND H. R. CATCHPOLE, with the assistance of J. Negri and D. Butzko. Growth of the fetus and placenta of the monkey (Macaca muiatta). Am. J. Phys. Anthropol. 2J: 23-4, 1965. 2. VALERIO, D. A., R. L. MiLLER, J. R. M. INNES, K. D. COURTNEY, A. J. PALLOTTA AND R. M. GurrMACHER. Macaca mulatta. Management of a laboratory breeding colony. New York: Academic, 1969, p. xii + 140. 3. PICKERING, D. E., AND N. E. KONTAXIS. Thyroid function in the foetus of the macaque monkey (Macaca mulatta). II. Chemical and morphological characteristics of the foetal thyroid gland. J. Endocrinol. 23: 267-275, 1961. 4. RIOPELLE, A. J., C. W. HILL, S-C Li, R. H. WOLF, H. R. SEIBOLD AND J. L. SMITH. Protein deprivation in primates. I. Nonpregnant adult rhesus monkeys. Am. J. Clin. Nutr. 27: 13-21, 1974. 5. RIOPELLE, A. J., C. W. HiLL, S-C. Li, R. H. WOLF, H. R. SEIBOLD AND J. L. SMITH. Protein deprivation in primates: IV. Pregnant rhesus monkeys. Am. J. Clin. Nutr. 28: 20-28, 1975. 6. RIOPELLE, A. J., C. W. Hiii, AND S-C. Li. Protein deprivation in primates. V. Fetal mortality and neonatal status of infant monkeys born of deprived mothers. Am. J. Clin. Nutr. 28: 989-993, 1975. 7. CLEGG, M. T. Factors affecting gestation length and parturition. In: Reproduction in Domestic Animals, edited by H. H. Cole, and P. T. Cupps. New York: Academic, 1959, p. 509-538.
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beyond those allowable for humans and the reasonable precision attainable in determin-
HALE
