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Departments of Animal Sciences,3 Biomedical Sciences,4 Food Systems and Bioengineering,5 Agriculture Experiment Station-Statistics, University of Missouri-Columbia, Christopher S. Bond Life Sciences Center, Columbia, Missouri 65211
ABSTRACT
Skewing of the sex ratio towards males occurs among pups born to mice fed a very high saturated fat (VHF) diet. In the present study, we tested whether the fat content of the VHF diet rather than the number of calories consumed is responsible for this effect. Eight-week-old NIH Swiss mice were placed on the VHF diet either ad libitum (VHF) or in a restricted manner (VHF-R). The VHF-R mice gained weight at a similar rate to controls fed a standard chow diet. Mice were bred at 15 wk and subsequently at 26 wk and 35 wk of age. Overall, the VHF, VHF-R, and control groups delivered 244, 242, and 274 pups, respectively, with male proportions of 0.60, 0.43, and 0.48, respectively. The pup sex ratios of the VHF group (favoring males) and VHF-R group (favoring females) each differed from 0.5 (P < 0.01). The sex ratios also differed (P < 0.0001) between the VHF and control groups, and between the VHF and VHF-R groups. Within the diet groups, maternal body weight had no effect on sex ratio. Serum leptin concentrations among the dams were similar in the VHF and VHF-R groups but higher than in the control group, while the IGF1 and corticosterone levels were comparable in all three groups. Therefore, the atypical sex ratios of offspring born to dams on the VHF diet seem to be influenced by the amount of fat consumed. Since males fed the VHF diet had neither more Y-sperm nor sired more sons than daughters, the dietary effects are manifested exclusively through the female.
corticosterone, IGF1, leptin, mouse, pregnancy
In polygynous mammalian species, such as deer, a small group of elite males, usually individuals that are larger and aggressive, are reproductively more successful than lower ranking males, who often father no offspring at all [1–9]. In contrast, the majority of females, irrespective of their social rank and body condition, become pregnant through mating to this select group of males. In addition, mothers invariably bear the full burden of rearing the young. Trivers and Willard [1] have predicted in their sex allocation hypothesis that in such species, females in the optimal body condition would tend to give birth to more sons than daughters, since these males would most likely join the elite group of breeders as adults. Conversely, females that are lower in the social structure or in poorer body condition would be anticipated to invest more in female progeny, since their daughters rather than their sons would be likely to have greater lifetime reproductive success. Not only are sons more costly to raise, but their greater variance, both in terms of early mortality and reproductive success, is well established in wild populations of many polygynous species, and some studies have indicated that males born to high ranking/better-fed females may in turn have greater reproductive success than their contemporaries [3, 7–9]. This observation may hold true for mice, which are also a polygynous species, in which larger males are more attractive to females than males of lower body mass [10], and males born to food-deprived mothers are generally smaller as adults than males born to females fed ad libitum, even if such variance is not evident when they are born [11]. Moreover, males born to food-deprived female mice are more likely to lose antagonistic encounters than sons born to control-fed females [12].
Despite these observations, until recently there have been surprisingly few studies aimed directly at testing the Trivers and Willard hypothesis in mice, although there are several reports that are consistent with its applicability to this species. For example, maternal undernutrition may affect litter size and pup viability [11–13]. Female mice on a low fat diet often have abnormally small litters, with a significant sex ratio distortion that favors daughters over sons [13]. On the other hand, dominant females, which can presumably appropriate more food than lower ranking females, tend to produce a greater proportion of male offspring [14]. Finally, mice on a suboptimal diet for 1 wk before mating or that are fed intermittently deliver fewer male pups than control females [12–14].
In earlier experiments, we examined the effects of two defined, ad libitum-fed, nutritionally complete diets, which differed primarily with respect to energy content and amount of fat, on the sex ratios of pups born to NIH Swiss mice [15]. Diet 1 was low in saturated fat (LF), with the majority of calories provided as sugars and complex carbohydrates, while the Diet 2 was very high in saturated fat (VHF), with 54% of the energy content provided as lard. Although there was little effect of these diets on sex ratio distortion in young mice bred at 10 wk of age, sex ratio skewing became obvious at later parities and in virgin mice bred when they were older than 20 wk. Specifically, the VHF diet provided a significantly higher number of male-biased litters and a sex ratio that favored sons over daughters by almost 2:1. In contrast, the LF diet strongly favored the birth of female offspring. Although these results generally supported the sex allocation hypothesis, it was the diet consumed rather than the body condition, i.e., the body weight (BW) of the mother, which was the more important of the two variables in terms of controlling the differences in sex ratio among offspring. What remained unanswered from this previous study was whether it was dietary composition, e.g., the amount or type of fat, or the amount of calories consumed that underpinned the dietary effects.
In the present study, we address this question by comparing the sex ratios of pups from two groups of females that were maintained on the same VHF diet, which was fed either ad libitum (VHF group) or in a restricted manner (VHF-R group). As controls, we examined the sex ratios of a parallel group of mice maintained on a standard chow diet.
IGF1 and leptin serum concentrations were assayed to detect signs of metabolic disturbance among the mice in the dietary groups, while corticosterone was assayed to determine whether the dams on the various diets exhibited differing degrees of anxiety. Stress has previously been reported to influence the sex ratio of rodents [16–18]. Finally, we determined whether the sex ratio skewing effects of the VHF diet could be conferred through the breeder males, who were exposed to the diet while they were temporarily housed with the females during mating.
The 5001 chow diet used to maintain the male mice and the 5015 chow diet fed to females were purchased from LabDiets Purina Mills (PMI Nutrition International LLC, Bentwood, MO). The defined Very High Fat (VHF) and Low Fat (LF) diets were from Research Diets Inc. (New Brunswick, NJ) [15]. The caloric and relative fat contents of these diets are summarized in Table 1.
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NIH Swiss mice (Harlan, Madison, WI) were used in all experiments. All female mice were purchased from Harlan at either 4 wk (experiment 1) or 3 wk (experiment 2) of age. The males used in experiment 2 were either bred in-house (first round of breeding) or purchased at 4 wk of age from Harlan (second round of breeding). All experiments were performed at the University of Missouri-Columbia in the accordance to the NIH Guidelines for the Care and Use of Laboratory Animals and were approved by the University of Missouri Animal Care and Use Committee.
The objective of experiment 1 was to compare the sex ratios of offspring born to dams that were fed a restricted amount of the VHF diet (VHF-R) with those born to dams that were fed the VHF or 5015 control diet ad libitum. First, 4-wk-old female mice (n = 36) were acclimatized to a 12D:12L (i.e., reversed day/night cycle, with darkness from 0600 h to 1800 h) for 4 wk, during which time they were fed the control 5015 diet. At the end of acclimation period, the now 8-wk-old mice were randomly divided into three groups of 12 per diet group, with two mice per cage (dimensions 27 x 16.5 x 13 cm). Mice in the VHF group were allowed ad libitum access to the VHF diet for the remainder of the experiment (see Figure 1). Mice in the VHF-R group had restricted access to the same diet; food was provided to the mice at 0930 h for approximately 7 h, with adjustments of ± 1 h to maintain average body mass close to that of the control group. This protocol was based on that used by Parks et al. [19] to vary the weight gain of laboratory mice. Mice in the control group were fed ad libitum on the Purina Mills 5015 diet. When the mice were 15 wk of age, i.e., after 7 wk on the diet, they were paired with stud males, following the protocol described by Rosenfeld et al. [15]. The mice were bred a second and third time when they were approximately 26 wk and 35 wk of age.
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Those dams that were still receptive were bred a fourth time when they were approximately 44 wk old. When the copulatory plug was evident at the fourth breeding, the dams were killed and nonheparinized blood was withdrawn by cardiac puncture [20]. After cooling on ice for 30 min, the samples were centrifuged to remove the clot. The resulting serum fraction was removed, recentrifuged, and aliquots were stored at –20 °C.
Pregnant females were housed in groups of two mice for the first 2 wk of pregnancy, and thereafter were housed singly until their pups were weaned (21 days postpartum). Each mouse was weighed at least once every 2 days from the time they were on the diets until the end of the study. Pups were sexed on Day 2 by at least two independent observers [15] by measuring the ano-genital distance [21], and the mice were resexed at weaning. Males were maintained ad libitum on the Purina Mills 5001 diet (Table 1), except during the mating period when they were with females (one male per two females), and consumed the same diet as the females. Males had been adapted to the inverted day/night cycle for at least 3.5 wk before they were used as breeders.
The objective of experiment 2 was to determine whether the short-term consumption of an experimental diet by the breeder males influenced the sex ratio of their offspring after they were mated to females on a control diet. The experiment was performed under a 14L:10D cycle. Twenty-four female mice were placed permanently on the 5015 control diet from the time they were weaned until the end of the experiment. They were first bred when they were 
17 wk old and again when they were 28 wk old. Breeder males, previously maintained on the 5001 chow diet, were divided into two groups and fed ad libitum either the VHF or the LF diet for 1 wk, i.e., a period roughly equivalent to the maximum time that they would spend in a cage with females during a breeding study similar to that of experiment 1. The LF rather than the VHF-R diet was used because it has previously been shown to favor a strong skewing of NIH Swiss dams towards producing daughters [15]. Therefore, any effect was more likely to be noted. After a week on the experimental diets, males (n = 21, 20-wk-old mice bred in-house) were caged overnight with the 17-wk-old females (one male to two females). The following morning, females with plugs were housed singly, and the males were removed from the cage. This procedure was repeated for the second and third nights, each time employing a fresh male from the same dietary group, with the rotation continuing either until both female mice had been successfully bred or for 1 wk, whichever came first. In the second round of breeding, which was performed 5 wk after weaning of the first litters, i.e., when the females were 28 wk old, the design was the same as that in the first round, except that the males were younger (7-wk-old mice; Harlan). In addition, the females that had been bred to the males on the VHF diet in the first round were exposed to males on the LF diet in the second round. Similarly, females that were originally bred to LF males were exposed to VHF males in the second round. Weaning and pup sexing procedures were the same as those described for experiment 1.
The objective of experiment 3 was to test the effects of different diets on the X and Y sperm ratios in the males. Eight stud males (two to three animals per diet, ranging in age from 12 wk to 46 wk) were placed on the VHF, LF or 5015 diet for 14 days, to allow for sperm generated during the time on the diet to reach the epididymis. The males were then killed, and cauda epididymal spermatozoa were collected, spread on poly-L-lysine-coated slides and dried. Dried slides were stored under nitrogen gas at –20°C until processed. X and Y spermatozoa were distinguished by fluorescent in situ hybridization methods [22] with mouse X (directly labeled with FITC) and Y (directly labeled with Cy3) chromosomal probes (Open Biosystems, Huntsville, AL). Sperm smear slides were examined under an Olympus Provis AX-70 inverted microscope with a U-MBC multi-control box (Melville, NY) that was equipped with a cooled, charge-coupled device camera (CoolSnap-ES; Photometrics, Tucson, TX) [22]. At least five different views of each slide were examined, with between 500 and 2000 spermatozoa from each animal being counted and used for the final data analysis.
The serum concentrations of corticosterone, insulin-like growth factor 1 (IGF1), and leptin were determined in duplicate using commercially available enzyme immunoassay kits according to the manufacturers' protocols: corticosterone assay using the OCTEIA kit (Immunodiagnostic Systems, Bolton, UK); IGF1 assay (R&D Systems, Minneapolis, MN); and leptin assay (Crystal Chem, Downers Grove, IL). Absorbance was measured in a Synergy HT multi-detection microplate reader (Bio-Tek, Winooski, VT) at 450 nm (reference wavelengths of 650 nm, 540 nm, and 630 nm for corticosterone, IGF1, and leptin, respectively). The raw data were analyzed using the KC4 ver. 3.3 software (Bio-Tek). The intraassay coefficients of variation were 6.7%, 2.2%, and 2.8% for the corticosterone, IGF1, and leptin assays, respectively. The interassay coefficients of variation for the corticosterone, IGF1, and leptin assays were 5.6%, 0.8%, and 6.3%, respectively.
The data for the pup sex ratios obtained in experiments 1 and 2 were analyzed using the generalized linear model of the Statistical Analysis Systems (SAS) ver. 9.1 (PROC GENMOD procedure). The data were distributed as a binomial and transformed by means of a logit-link function. The model was a repeated measures design, since each dam had repeated parities. For experiment 1, the Generalized Estimating Equation (GEE) contained the effects of weight (BW of dam at the time she was bred), diet (VHF, VHF-R, 5015), parity number, and the interaction of diet and parity (diet*parity). The parameter of maternal BW was used as a covariate to determine whether the BW of the dams influenced the sex ratio of pups born. Chi-square test was used to test deviation from a 1:1 ratio, i.e., a value of 0.5 for the fraction of males born, as well as differences in sex ratio between groups. The antilog of the logit and the antilog of the differences between logit estimates produced the odds and odds ratio, respectively.
Dam BW values were analyzed as a repeated measures, as outlined by Littell et al. [23]. The ANOVA model contained the effects of diet, day (time on the diet), and the interaction of diet with day (diet*day). The dam served as the experimental unit. These data were analyzed by using the PROC MIXED procedure in SAS ver. 9.1 [24]. Differences in maternal BW between diets over time (days) were determined by the Fisher least-significant difference.
An analysis of mouse pup survival after birth to weaning was accomplished by means of the PROC LIFETEST protocol in the SAS ver. 9.1 software [24]. Life testing procedures allow the testing of differences between survival curves when the data are censored, i.e., when not all of the animals die. Testing the homogeneity of survival curves was carried out by the log-rank test.
The relationship between the BW of the dam at breeding and the survival of pups preweaning was analyzed by logistic regression methodology with the SAS 9.1 PROC LOGISTIC procedure. Differences between logistic regression coefficients were determined by the Wald chi-square statistic [24].
The data for serum IGF1, leptin, and corticosterone concentrations were analyzed for normality using the Wilk-Shapiro test [24]. The corticosterone data were logarithmically transformed to approach a normal distribution. All dependent variables were analyzed using the general linear model (GLM) procedure of the SAS software, with diet as the main effect [24]. Differences in serum hormone concentrations among diet groups were determined by the Fisher least-significant difference.
The CATMOD procedure of the SAS program [24] was used to determine the effects of diet on the X and Y sperm ratios in the males in experiment 3.
Effects of Diet on Body Weights of Dams, Litter Size, Gestation Length, and Postnatal Mortality
The parities for the mice on the three diets were well-synchronized and marked by a rapid increase in BW corresponding to each gestation period and a sharp drop following the birth of pups. Although the VHF and control mice showed an immediate rebound in BW as they began to lactate, the VHF-R dams did not demonstrate a comparable weight gain during the immediate postpartum period (P < 0.05) (Fig. 1). Overall, however, the VHF-R and control mice did not differ in BW, except during lactation. As anticipated, the mice on the VHF diet gained weight faster when first introduced to the diet than either the controls or the mice on the VHF-R diet and maintained high BW over the 40 wk of the study (Fig. 1). However, there was considerable variability in mouse BWs within each diet group throughout the study. At the time of breeding, the dam BW values for the VHF group ranged from 25.2 to 51.7 g, 30.5 to 53.8 g, and 34.7 to 57.1 g at the first, second, and third parities, respectively. For the VHF-R group, the corresponding values were 24.5 to 39.0 g, 28.0 to 42.8 g, and 32.6 to 44.3 g, while for the controls they were 25.8 to 41.3 g, 29.3 to 41.1 g, and 33.5 to 48.9 g (Table 2).
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Each group of mice produced three litters of pups. Although the mice on the VHF and VHF-R diets had a tendency to breed less readily than control mice, neither litter size nor length of gestation differed with diet or parity (Tables 2 and 3). Overall, 90 litters and 674 live pups were born for the three groups. Although the pups were not weighed individually in the present study, the drop in BW of the dams following parturition (Fig. 1) provided an estimate of litter weight, which when divided by the number of pups born provided an estimate of average pup BW. These estimated pup BW values did not differ across the diet groups, and there was no significant effect of parity on pup BW (Table 3). These data suggest that mouse fertility is not influenced by the diets consumed by the dams.
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Postnatal mortality rates were significantly higher among dams fed the VHF and VHF-R diets compared to the controls, and this was most pronounced in the VHF-R group (Fig. 2; Table 3). Most deaths occurred within the first 2 days after birth and few occurred after Day 4 (Fig. 2). Nevertheless, the genders of the majority of the dead pups were successfully determined. Importantly, the sex ratio of pups that survived within the different dietary groups did not change significantly between birth and weaning, indicating that pups of one sex were not more likely to die than those of the other sex during the immediate postpartum period (Table 3).
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Effects of Dam Diet on Sex Ratio of Offspring and Distribution of Male and Female Pups among Litters
The sex ratio of pups born in experiment 1 was strongly influenced by the diets consumed by the dams (Tables 3 and 4). Dams on the VHF diet produced more sons than daughters at each parity (Table 3), in the overall proportion of 0.60 to 0.40, which deviated significantly from that anticipated by chance, i.e., 0.50 (Table 5) and from those of the other dietary groups (P < 0.0001). The tendency for sex skewing of pups towards males was noted to increase in the VHF group of dams with either advanced age or parity of the mice (Tables 3 and 5). For example, the fraction of males at parity 3, when the dams had been on the diets for almost 30 wk and were 38 wk of age, was 0.70, whereas at parity 1, when they were 18 wk old, it was 0.54. Of the 27 litters born to dams on the VHF diet fed ad libitum, 18 litters were biased towards sons, with 7 litters biased towards daughters and 2 litters evenly balanced (Table 3). Since there was considerable variability in the BW values of dams within each treatment group (Table 2), it was necessary to consider dam BW as a covariate in the statistical model. The likelihood of dams within any of the three diet groups producing more sons was not influenced by maternal BW at the time the female mice were bred (data not shown). Instead, the increase in sex ratio observed in the VHF group appeared to be a function of the diet consumed (P < 0.01) (Table 4).
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In contrast to the VHF group, the dams on the VHF-R diet produced more daughters than sons (126 vs. 97; Table 3), with the overall fraction of males (0.43) differing from the anticipated value of 0.50 (P < 0.01; Table 4) and differing significantly (P < 0.0001) from that of the VHF group (Table 5).
Effect of Feeding the VHF Diet to Breeder Males
In experiment 2, males fed the VHF diet and bred to control females generated 16 litters of 119 pups (seven male-biased, six female-biased, and three litters with equal numbers of males and female pups). The fraction of male pups was 0.48. For the LF diet, there were 15 litters (six male-biased, five female-biased and three litters with equal numbers of each sex), and 84 pups were born, with the fraction of male pups at 0.54. In no case was there a significant deviation from the anticipated sex ratio of 0.5.
In experiment 3, mature males were fed the VHF (n = 3), LF (n = 3) or control 5015 (n = 2) diet for 2 wk before they were killed, and the ratios of X- and Y-epididymal sperm were assessed by a highly specific FISH procedure [22]. The Y-sperm ratios were 0.51, 0.50, and 0.51 for the LF, VHF, and control diet groups, respectively. These values did not deviate significantly from 0.50.
Serum Hormone Levels of the Dams on the VHF, VHF-R, and Control Diets
Serum samples were collected from dams after they had been bred for a fourth time. The serum IGF1 levels did not differ between the dams on the VHF, VHF-R, and control diets, whereas the leptin concentrations were significantly lower in the controls than in the VHF and VHF-R groups (P < 0.05), which did not differ from each other (Table 6). The corticosterone concentrations were highly variable, particularly in the control group, and the mean values did not differ between the mice on the three diets (Table 6).
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The main goal of the present study was to test the hypothesis that the production of an excess of sons and male-biased litters that occurs when female NIH Swiss mice are maintained continuously on a diet with a high saturated fat content would also be evident when the diet was supplied in a restricted manner but in amounts sufficient for normal weight gain and fertility. This hypothesis proved incorrect. The pups of dams on the restricted (VHF-R) diet did not exhibit a significant skewing of their sex ratio towards male offspring (as observed for the VHF mothers) and produced significantly more daughters than sons. We conclude that it is not the proportion of fat, and particularly the content of unsaturated fat in the form of lard, in the diet that causes sex ratio skewing towards males. Rather, it appears that when the amount of calories consumed exceeds the norm, a greater likelihood exists that a murine mother will produce more sons than daughters. On the other hand, the high BW mice within any of the dietary groups had no greater tendency to produce male offspring than the lower BW mothers. Although these results generally support the sex allocation hypothesis [1], they also suggest that it is ready access to food that is high in calories, i.e., fat, and not body condition per se that prompts the change in sex ratio in favor of males. Conceivably, in wild populations, those females that are best able to compete for food are the ones that are most likely to give birth to litters that are biased towards sons.
Mice have previously been observed to produce a predominance of daughters when calorie intake is reduced by limiting access to food [11–13]. Women who conceive at a time of severe food shortage may [25] or may not [26] have more daughters than sons. In the case of mice, food restriction leads to small, female-biased litters. Although the VHF-R diet did not appear to reduce dam fertility and provided normal length pregnancies, litter sizes, and pup BW, its consumption undoubtedly resulted in a greater postnatal loss of pups. This pup mortality was associated with a failure of the lactating dams to gain weight in the immediate postpartum period (Fig. 1), and we speculate that pup death was the outcome of insufficient milk production to meet the full needs of the litter. Postnatal pup mortality significantly higher than that for the controls was also observed for the VHF group of dams (Table 3). These results are consistent with those of Aoka et al. [27], who showed that lactating female mice fed high amounts of fat produce milk with reduced fat and energy contents. The combination of fatty diet and restricted access to food most likely prevented these mothers from providing sufficient milk to all their pups. Consistent with this conclusion is the observation that in the VHF-R group, the heavier the mother, the better the chance her pups had of surviving after birth (data not shown), which is consistent with the observations made in female hamsters by Schneider and Wade [28].
There appeared to be little effect of paternal diet on XY sperm counts and the resulting offspring sex ratio. In our previous experiments, the males had been randomized before being used for breeding but we could not rule out that the effects observed were due to the limited time the males had access to the experimental diets when housed with the females. The experiments reported in the present study strongly rule out this possibility, since males fed either the VHF or LF diet and mated to mature females on the control diet sired approximately equal numbers of sons and daughters and had equal X:Y sperm ratios.
We have previously shown that serum insulin and glucose concentrations are not elevated in mice fed the VHF diet relative to mice on a control or LF diet (unpublished results). Therefore, the finding in the present experiments that the IGF1 concentrations were similarly unaffected by diet was not unexpected. As these ageing mice had been consuming the fatty diets for over 40 wk and many were obese (see footnote to Table 2), it is perhaps surprising that they did not show signs of metabolic disturbance and energy imbalance. Instead, it would appear that mice adapt well over time to a high fat diet, whether provided ad libitum or in restricted amounts, and they retain normal fecundity relative to dams on the control diet. Parks et al. [19] noted a similar ability of mice to adapt to such diets.
Leptin is essential for implantation in mice [29], is present in the oocyte [30], and likely influences development of the trophoblast [31]. Interestingly, the leptin levels were elevated in the groups that consumed the lard-rich VHF diet either in a restricted or ad libitum manner (Table 6). Since only the VHF mothers produced more sons than daughters, it seems unlikely that leptin plays a role in skewing the sex ratio in this case. Importantly, the mean BW values of the mice in the VHF-R group were comparable to those of the control mice (Fig. 1). Therefore, the experiment indicates that leptin concentrations are influenced by the diet consumed and not just by the amount of adipose tissue mass associated with an animal.
Corticosterone is frequently measured to assess whether a mouse is under stress [32]. Since high anxiety levels are believed to lower the sex ratio of pups and since unbalanced diets may elevate corticosteroid levels in pregnant women [33], the mice on the food-restricted diet, who tended to produce more daughters than sons, might be expected to have higher circulating corticosterone levels than the VHF ad libitum-fed females who were not food-restricted and produced more sons than daughters. Such a difference was not observed (Table 6). Although the values obtained were within the ranges obtained by others who have used this particular assay kit (references obtained from the manufacturer) and were consistent within replicates, the serum corticosterone values were exceedingly variable, especially in the control group. Indeed, if the four highest outlier values from the control group (>150 ng/ml) are removed from consideration in the analysis of the control data, the adjusted mean concentration of corticosterone for the controls becomes 9.1 ng/ml, a value that is significantly lower than that observed among dams on either the VHF or VHF-R diet (see footnote to Table 6). Whichever interpretation is correct, no obvious correlation exists between the mean corticosterone concentrations of the different diet groups and the sex ratio of their pups.
In summary, these experiments confirm that when mature female mice are maintained continuously on a diet that is very high in fat, they shift the sex ratios of their pups strongly towards males. Heavier, presumably obese, mothers have no greater tendency to produce sons than lighter mothers in the same group. When consumption of this diet is reduced by providing food in a restricted manner, the sex ratio falls and favors daughters over sons. These data are consistent with the predictions of the Trivers and Willard sex allocation hypothesis [1] and suggest that unrestricted access of females to a calorie-rich diet tends to favor the production of sons over daughters. While the causative mechanism appears not to be due to alterations in the circulating levels of metabolic hormones, dietary-induced adjustments in steroid hormone concentrations may contribute to this shift in offspring sex ratio [20].
ACKNOWLEDGMENTS
We thank Dr. Kevin Fritsche (Division of Animal Sciences) for helpful input into the effects of dietary alterations on mice, and the undergraduate assistants Stacey Schlanker and Emily Fountain for assistance with animal husbandry. We are grateful to Mr. Andy Cardin for his editorial assistance.
FOOTNOTES
1Supported by NIH grant HD44042 and the University of Missouri Food for the 21st Century Program. 
Correspondence: 2R. Michael Roberts, 240B Christopher S. Bond Life Sciences Center, Columbia, MO 65211-7310. FAX: 573 884 9676; e-mail: robertsrm{at}missouri.edu
Received: 11 April 2007.
First decision: 2 May 2007.
Accepted: 18 May 2007.
REFERENCES
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  W. H James Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception J. Endocrinol., July 1, 2008; 198(1): 3 - 15. [Abstract] [Full Text] [PDF]
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