Litter sex ratios in Richardson’s ground squirrels: long-term data support random sex allocation and homeostasis

When costs of producing male versus female offspring differ, parents may vary allocation of resources between sons and daughters. We tested leading sex-allocation theories using an information-theoretic approach and Bayesian hierarchical models to analyse litter sex ratios (proportion males) at weaning for 1,049 litters over 24 years from a population of Richardson’s ground squirrels (Urocitellus richardsonii), a polygynandrous, annually reproducing mammal in which litter size averages from six to seven offspring and sons are significantly heavier than daughters at birth and weaning. The model representing random Mendelian sex-chromosome assortment fit the data best; a homeostatic model received similar support but other models performed poorly. Embryo resorption was rare, and 5 years of litter data in a second population revealed no differences in litter size or litter sex ratio between birth and weaning, suggesting that litter size and sex ratio are determined in early pregnancy. Sex ratio did not vary with litter size at weaning in any of 29 years, and the observed distribution of sex ratios did not differ significantly from the binomial distribution for any litter size. For 1,580 weaned litters in the two populations, average sex ratio deviated from parity in only 3 of 29 years. Heavier females made a greater reproductive investment than lighter females, weaning larger and heavier litters composed of smaller sons and daughters, but litter sex ratio was positively related to maternal mass in only 2 of 29 years. Such occasional significant patterns emphasize the importance of multi-season studies in distinguishing infrequent events from normal patterns.     

Difference in reproductivity and offspring growth between litter numbers in four strains of mice

As a means to improve mouse production, the differences among litter numbers in reproductivity, post-weaned growth and puberty of offspring were examined in 4 strains of mice, SHN/Mei, SLN/Mei, C3H/HeMei and GRS/AMei. Females were mated to be concurrently pregnant and lactating and reproductivity was recorded from the 1st to the 3rd litters. Some offspring of both sexes were weighed at 30, 40 and 50 days of age and vaginal opening was also checked in females. Pre-weaned growth of offspring differed little between litter numbers in all strains except GRS/A. Meanwhile, post-weaned growth of offspring was higher in the 2nd and 3rd litters than in the 1st litter of SHN and SLN, associated with a decreased rate of still-born pups. In C3H/He, post-weaned growth of offspring was higher in the 1st and 2nd litters and rearing rate decreased for the 3rd litter. In GRS/A, the 1st litter showed the greatest pre- and post-weaning growth rate of offspring. All findings indicate that in SHN and SLN strains the later litters are better for offspring production, while in C3H/He and GRS/A strains it is the earlier litters. The results stress the value of having information on reproductive characteristics of mouse strains for efficient offspring production.     

Food-restricting first generation juvenile female hamsters (Mesocricetus auratus) affects sex ratio and growth of third generation offspring

Female hamsters (Generation 1) were fed ad libitum or were food-restricted to 65-75% of the amount consumed by controls during their first 50 days of life. Subjects were mated at 91 days of age. Their offspring (Generation 2) were fed ad libitum throughout the experiment, and female offspring were also mated at 91 days of age. Generation 3 litters were monitored every fifth day from birth until Day 25 post partum for litter size, sex ratio, and pup weights. Although there were no significant differences in Generation 3 litter sizes at birth, litters descended from food-restricted Generation 1 females (Group R) were significantly smaller on Days 5-25 than litters descended from control Generation 1 females (Group A). Sex ratios remained significantly greater in Group A than in Group R litters from birth to Day 25 but did not vary over time, suggesting similar post-partum mortality rates for both male and female pups. Weights of Generation 3 male and female pups did not vary significantly within treatments at any time. Group A males weighed significantly more than Group R males from birth through Day 25, but weights of Group A and Group R females were always similar. Food restriction early in life may have long-term consequences on sex ratios of subsequent generations in hamsters.     

Hormonal manipulation of offspring number: maternal effort and reproductive costs

We used exogenous gonadotropin hormones to physiologically enlarge litter size in the bank vole (Clethrionomys glareolus). This method allowed the study design to include possible production costs of reproduction and a trade-off between offspring number and body size at birth. Furthermore, progeny rearing and survival and postpartum survival of the females took place in outdoor enclosures to capture salient naturalistic effects that might be present during the fall and early winter. The aim of the study was to assess the effects of the manipulation on the growth and survival of the offspring and on the reproductive effort, survival, and future fecundity of the mothers. Mean offspring body size was smaller in enlarged litters compared to control litters at weaning, but the differences disappeared by the winter. Differences in litter sizes disappeared before weaning age due to higher mortality in enlarged litters. In addition to the effects of the litter size, offspring performance was probably also influenced by the ability of the mother to support the litter. Experimental females had higher reproductive effort at birth, and they also tended to have higher mortality during nursing. Combined effects of high reproductive effort at birth and high investment in nursing the litter entailed costs for the experimental females in terms of decreased probability of producing a second litter and a decreased body mass gain. Thus, enlarged litter size had both survival and fecundity costs for the mothers. Our results suggest that the evolution of litter size and reproductive effort is determined by reproductive costs for the mothers as well as by a trade-off between offspring number and quality.     

Litter size and fetal sex ratio adjustment in a highly polytocous species: the wild boar

For species in which reproductive success is more variable inone sex than the other, the Trivers and Willard model (TWM)predicts that females are able to adjust their offspring sexratio. High-quality mothers should provide greater investmentto one sex than the other. Previous tests of the TWM have beeninconsistent, and whether the TWM applies to species with severaloffspring per litter is unclear due to possible trade-offs betweensize, number, and sex of the offspring. Williams' model (WM)accounts for confounding effects of these trade-offs on sexratio variation. Lastly, the "extrinsic modification hypothesis"predicts changes in offspring sex ratio in relation to climaticconditions and population density. Using wild boar as a model,we tested 1) whether the WM fitted observed sex ratio variationand 2) whether sex ratio variations were related to maternalattributes (test of the TWM) and/or to resource availability(test of the extrinsic modification hypothesis). Females adjustedtheir litter size rather than their litter composition, so thatthe WM was not supported. Likewise, changes in resource availabilitydid not influence the fetal sex ratio, so that the extrinsicmodification hypothesis was not supported. The fetal sex ratiowas negatively related to increasing litter size, providingsome support for the TWM. Sex ratio was male biased for littersizes up to 6 and then became female biased in larger litters.Our results provide the first case study showing marked changesin sex ratio in relation to litter size in a large mammal.     

Reproductive lifespan and reproductive performance in SPF C3H mice: the onset of reproductive life and production efficiency (author's transl)]

To improve the production system, the onset and the termination of reproductive life of C3Hf/HeMsNrs mice mated immediately after weaning and reared for 400 days of life, were studied. From weaning females mated with a full grown male (group A), the first litter was obtained at a mean age of 47 days, suggesting the first copulation at 26 days of age. The age of males at the first copulation was estimated to be at 44 days of age from the age giving the first litters in weanling males mated with weanling (group B) and full grown (group C) females. The sex ratio of litters delivered by young dams tended to be excess in males. The reproductive performance of dams in later life was not affected by the parturition in earlier age. The production efficiency with weaned youngs per pair during the first 200 days after mating was the highest in group A. It was found from these results that the C3H females attained their sexual maturity at 5 to 6 days after weaning, being available for breeding without any deletion in reproductive performance.     

Optimal allocation of reproductive effort: manipulation of offspring number and size in the bank vole

The number of offspring attaining reproductive age is an important measure of an individual's fitness. However, reproductive success is generally constrained by a trade-off between offspring number and quality. We conducted a factorial experiment in order to study the effects of an artificial enlargement of offspring number and size on the reproductive success of female bank voles (Clethrionomys glareolus). We also studied the effects of the manipulations on growth, survival and reproductive success of the offspring. Potentially confounding effects of varying maternal quality were avoided by cross-fostering. Our results showed that the number of offspring alive in the next breeding season was higher in offspring number manipulation groups, despite their smaller body size at weaning. Offspring size manipulation had no effect on offspring growth or survival. Further, the first litter size of female offspring did not differ between treatments. In conclusion, females may be able to increase the number of offspring reaching reproductive age by producing larger litters, whereas increasing offspring size benefits neither the mother nor the offspring.     

Some data on the fertility of bank voles (Clethrionomys glareolus britannicus) in the laboratory supporting the hypothesis of induced ovulation

83 (40%) litters with an average of 3.42 pups and unbiased sex ratio were born of 207 couples. Fertility and litter average were higher in summer and autumn. Most females were fertilized on the 1st day spent with a male, which supports the mating-induced ovulation hypothesis. Infantile mortality was highest at birth and during the 1st 5 days of life. 26% of the pups died before weaning (day 25), i.e. an average of 1 per litter.     

Food restricting young hamsters (Mesocricetus auratus) affects sex ratio and growth of subsequent offspring

Trivers and Willard (1973) predicted that stressed adult female mammals may enhance their fitness by skewing offspring sex ratios and maternal investment to favor daughters. The present study investigated whether stressing young hamsters might also influence sex ratio and growth of subsequent offspring. Control females received food ad libitum (A) on Days 1-50 postpartum (AA). Experimental females were food-restricted (R) either on Days 1-25 (RA), Days 26-50 (AR), or Days 1-50 (RR) postpartum. Subjects were mated when 91-95 days old. Litter sizes and survivorship (= % litters within a treatment that contained at least one pup), sex ratio (= % males), and pup weights in the next generation were recorded every fifth day from parturition until Day 25 postpartum. Control litters contained significantly more offspring at birth than did RR litters. Sex ratio was significantly higher at birth for AA litters than for the other treatments. Postpartum sex ratio within each group remained similar to that recorded at birth. RR litters contained significantly fewer pups compared to the other three treatments from Days 5-25. RR female pups weighed significantly more at birth than their counterparts in the other treatments. Weights of males at birth were similar in all treatments. By Day 25, both male and female RR pups weighed significantly less than control, AR, and RA pups. Food restriction early in life may have long-term consequences on sex ratio and pup growth in golden hamsters.     

Reproductive effort during gestation and lactation by Richardson's ground squirrels

Summary Reproductive effort by yearling and older female Richardson's ground squirrels was studied over a 4-year period in southern Alberta by obtaining serial weight records from marked individuals to compare the mother's mass at critical points in the annual cycle (emergence from hibernation, estrus, parturition, and litter emergence) with her litter's mass at birth and weaning. Yearlings weighed only 80% of older adults at emergence from hibernation, but they mated at the same time as older females, attained adult mass coincident with pregnancy, and weaned litters that were not significantly smaller in size or mass than those of older females. Age and maternal mass were weak predictors of litter size and litter mass. Of the net increase in mass of the combined mother-litter unit during gestation, over half (60% of 139 g for yearlings; 52% of 127 g for older females) was attributable to an increase in the mother's own mass, whereas during lactation almost all of the net increase (93% of 545 g for yearlings; 96% of 567 g for older females) was attributable to an increase in the litter's mass. On a daily basis, deposition of mass in the litter was 6 times greater during lactation than gestation. On average, neonates weighed 2.3% (6.5 g) of maternal mass at birth and 23.1% (81 g) at emergence from the natal burrow; offspring masses at birth and at emergence were significantly negatively correlated with litter size. On average, litters weighed 16.3% (48 g) of maternal mass at birth and 157.5% (578 g) at emergence from the natal burrow. Compared with other hibernating sciurids, Richardson's ground squirrels have a similar offspring mass relative to maternal mass both at birth and at emergence from the natal burrow. However, because of the large litter size (typically 6–8), absolute reproductive effort, measured either as litter mass at birth or at natal emergence, is large for the body size of the species.     

Offspring growth, survival and reproductive success in the bank vole: a litter size manipulation experiment

To estimate the optimality of brood size, it is essential to study the effects of brood size manipulation on offspring survival and reproductive success. Moreover, testing the generality of the hypothesis of reproductive costs requires experimental data from a diversity of organisms. Here I present data on the growth, survival and reproductive success of bank vole Clethrionomys glareolus individuals from manipulated litters. Furthermore, the survival of mothers whose litter size was manipulated was studied. At weaning, the mean weight of pups from enlarged litters was lower and from reduced litters higher compared to control litters. After winter, at the start of the breeding season, individuals from enlarged litters, especially males, were still lighter than individuals from the other two treatments. Litter enlargements did not increase the number of reproducing female offspring per mother, nor did the litter sizes of female offspring differ between treatments. There were no differences between treatments in winter survival of offspring after weaning, but among female offspring, weaning weight explained the survival probabilities over winter. A higher weight of females at winter determined the probability of starting to reproduce in spring. The survival of mothers did not seem to be influenced by litter manipulation performed the previous year. According to the results, mothers nursing enlarged or reduced litters do not gain any fitness benefits in terms of number of offspring surviving to breeding. The results are consistent with the majority of experiments conducted in birds, which have found costs of enlarged brood appearing as offspring trade-offs rather than parent trade-offs. Received: 14 December 1997 / Accepted: 1 March 1998     

Breeding success in cooperative meerkats: effects of helper number and maternal state

Studies of cooperatively breeding birds and mammals generallyconcentrate on the effects that helpers have on the number ofreproductive attempts females have per year or on the numberand size of offspring that survive from hatching/weaning toindependence. However, helpers may also influence breeding successbefore hatching or weaning. In the present study, we used anultrasound imager to determine litter sizes close to birth,and multivariate statistics to investigate whether helpers influencefemale fecundity, offspring survival to weaning, and offspringsize at weaning in cooperative meerkats, Suricata suricatta.We found that the number of helpers in a group was correlatedwith the number of litters that females delivered each year,probably because females in large groups gave birth earlierand had shorter interbirth intervals. In addition, althoughpup survival between birth and weaning was primarily influencedby maternal dominance status, helper number may also have asignificant positive effect. By contrast, we found no evidenceto suggest that helpers have a direct effect on either littersizes at birth or pup weights at weaning, which were both significantlyinfluenced by maternal weight at conception. However, becausedifferences in maternal weight were associated with differencesin helper number, helpers have the potential to influence maternalfecundity and offspring size within reproductive attempts indirectly.These results suggest that future studies may need to considerdirect and indirect helper effects on female fecundity and investmentbefore assessing helper effects on reproductive success in societiesof cooperatively breeding vertebrates.     

Sex ratio in silver foxes: effects of domestication and the star gene

The course of changes in secondary sex ratio (proportion of males at birth) in silver foxes bred at the fur farm of this Institute was analyzed. Data collected over several years of breeding of a domesticated (experimental) population selected for amenability to domestication and of a commercial (control) were compared. A tendency to increase in secondary sex ratio was demonstrated in both populations. However, the proportion of males at birth was higher in domestic foxes. This proportion, calculated from the combined data for 1978–1993, was 0.538±0.005 and 0.511±0.007 in the selected and commercial populations, respectively. The minimal departure of the observed sex ratio from 0.5 was demonstrated for litters with five pups, which is close to the average litter size in fox populations. The proportion of males increases with both increasing and decreasing litter size. An analysis of secondary sex ratio with respect to maternal age revealed a minimal departure of sex ratio from the expected in offspring from foxes of optimal reproductive age (2–4 years). An effect of the autosomal semidominant coat color mutation star on male excess at birth was also found: secondary sex ratio was higher (0.583±0.015) in offspring of mothers heterozygous for the star mutation than from standard types of the domesticated population. The increase in secondary sex ratio in the analyzed fox populations is viewed as a correlated response to selection for domestication. The hormonal mechanisms mediating the effects of both this selection and the star mutation on sex ratio at birth are discussed.     

Sex-specific fitness returns are too weak to select for non-random patterns of sex allocation in a viviparous snake

When environmental conditions exert sex-specific selection on offspring, mothers should benefit from biasing their sex allocation towards the sex with the highest fitness in a given environment. Yet, studies show mixed support for such adaptive strategies in vertebrates, which may be due to mechanistic constraints and/or weak selection on facultative sex allocation. In an attempt to disentangle these alternatives, we quantified sex-specific fitness returns and sex allocation (sex ratio and sex-specific mass at birth) according to maternal factors (body size, age, birth date, and litter size), habitat, and year in a viviparous snake with genotypic sex determination. We used data on 106 litters from 19 years of field survey in two nearby habitats occupied by the meadow viper Vipera ursinii ursinii in south-eastern France. Maternal reproductive investment and habitat quality had no differential effects on the growth and survival of sons and daughters. Sex ratio at birth was balanced despite a slight female-biased mortality before birth. No sexual mass dimorphism between offspring was evident. Sex allocation was almost random apart for a trend towards more male-biased litters as females grew older, which could be explained by an inbreeding avoidance strategy. Thus, a weak selection for facultative sex allocation seems sufficient to explain the almost equal sex allocation in the meadow viper.     

Relationship of cryptorchidism with sex ratios and litter sizes in 12 dog breeds

The aim of this study was to identify the influence of genetic carriership for cryptorchidism on litter sizes and sex ratios in the offspring. Weaning data of 11,230 litters in 12 purebred dog breeds were evaluated. Parents were classified as cryptorchidism 'carriers' (C) when at least one of their offspring was found cryptorchid. Subsequently the effects of 'carrier' and 'non-carrier' (NC) parents on their litters were studied. In litters from C x C parents we found an increased number of males per litter in all breeds, a reduced number of females per litter in 8 breeds and an increased litter size in 11 breeds in comparison with litters from NC x NC parents. Over all breeds the effects on litter size, on number of males per litter and on sex ratio were highly significant. Mixed litters from C x NC and NC x C did not show these effects and were not significantly different from the NC x NC offspring. Our results suggest a general mechanism in the dog species which causes cryptorchidism as well as increased male/female ratios and increased litter sizes. A consequence of such a mechanism is that selection in favor of increasing reproduction output frustrates selective efforts to eliminate cryptorchidism.     

Determinants of reproductive success in female adders,Vipera berus

Summary Female lifetime reproductive success in a small population of individually-marked adders in southern Sweden was studied over a period of seven years. Reproductive characteristics varied little from year to year and were consistent through time in individual females. Most females mature at four years of age and reproduce every two years. The total number of offspring produced by a female depends on her adult body size (and thus, litter size) and longevity (and thus, number of litters per lifetime). Adult body size in females is influenced mainly by subadult growth rates. Offspring size depends on maternal body size and a tradeoff between offspring size and offspring number. Maternal age does not affect litter sizes and offspring sizes except through ontogenetic changes in maternal body size.Survival of females after parturition is low because of the high energy costs of reproduction, compounded by low feeding rates of gravid females because of their sedentary behaviour at this time. About one-half of females produce only a single litter during their lifetimes, although some females live to produce four or five litters. On a proximate basis, rates of energy accumulation for growth (in subadults) and reproduction (in adults) may be the most important determinants of fitness in female adders.     

Long-term effects of accelerated or delayed sexual maturation on reproductive output in wild female house mice (Mus musculus)

The effects of acceleration and delay of puberty in female house mice on survival and reproduction were tested using 6 experimental groups: (1) control females mated at the time of first oestrus, (2) females mated at weaning, (3) females treated with male urine starting at weaning and mated at first oestrus, (4) females housed in groups and mated at first oestrus, (5) females housed alone, treated with urine from grouped females and mated at first oestrus, and (6) females housed alone and mated at 68 days of age. Females caged with males at weaning or treated with male urine and mated at puberty had lower rates of survival to 180 days of age, but did not differ in rates of fertility from mice in the other four treatments. Those females that were housed with males from weaning or treated with male urine also had smaller total numbers of litters, fewer total young, and smaller average litter sizes than did females for which the age of mating was delayed, by grouping or treatment with urine from grouped females, or by being held until age 68 days before mating. Control females mated at first oestrus generally were intermediate or did not differ from the male treatments on these dependent variables. There were no differences in the average number of female young/litter across the 6 treatments. However, females that were delayed in age of first mating had significantly more male young/litter than did females that were accelerated in their sexual development or control females.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lifespan ontogeny of breeding and reproductive success in Mongolian gerbils

An analysis was made of demographic data collected from a colony of Mongolian gerbils (Meriones unguiculatus). The data on births of over 3800 gerbils provide evidence on several aspects of development and fecundity. Approximately 4% more females than males survived to weaning. Mean number of pups per dam per year was estimated to be 48.3, greater reproductive success than reported previously. The shortest period of gestation resulting in viable pups was 25 days. Implantation was delayed by the presence of suckling pups. When gestation was prolonged, there was a direct relationship between the number of pups being suckled concurrently and the length of the delay. Length of gestation did not affect the size of the subsequent litter. Female gerbils gave birth as early as 72 days of age. Gerbils continued to reproduce until after 2 years of age in some cases. There were fewer pups per litter born to older females, fewer pups per litter with increasing parity, and longer intervals between litters with increasing age.     

Experimental manipulation of breeding density and litter size: effects on reproductive success in the bank vole

1. Reproductive success of individual females may be determined by density-dependent effects, especially in species where territory provides the resources for a reproducing female and territory size is inversely density-dependent.
2. We manipulated simultaneously the reproductive effort (litter size manipulation: ± 0 and + 2 pups) and breeding density (low and high) of nursing female bank voles Clethrionomys glareolus in outdoor enclosures. We studied whether the reproductive success (number and quality of offspring) of individual females is density-dependent, and whether females can compensate for increased reproductive effort when not limited by saturated breeding density.
3. The females nursing their young in the low density weaned significantly more offspring than females in the high density, independent of litter manipulation.
4. Litter enlargements did not increase the number of weanlings per female, but offspring from enlarged litters had lower weight than control litters.
5. In the reduced density females increased the size of their home range, but litter manipulation had no significant effect on spacing behaviour of females. Increased home range size did not result in heavier weanlings.
6. Mother's failure to successfully wean any offspring was more common in the high density treatment, whereas litter manipulation or mother's weight did not affect weaning success.
7. We conclude that reproductive success of bank vole females is negatively density-dependent in terms of number, but not in the quality of weanlings.
8. The nursing effort of females (i.e. the ability to provide enough food for pups) seems not to be limited by density-dependent factors.     

Delayed motherhood decreases life expectancy of mouse offspring

This study analyzes the long-term effects of delayed motherhood on reproductive fitness and life expectancy of offspring in the mouse. Hybrid (C57BL/6JIco x CBA/JIco) first-generation (F1) females, either at the age of 10 or 51 wk, were individually housed with a randomly selected 12- to 14-wk-old hybrid male following a breeding pen system until females reached the end of their reproductive life. Reproductive fitness of second-generation (F2) females was tested from the age of 25 wk until the end of their reproductive life. In F2 males, the testing period ranged from the age of 52 wk until their natural death. Delayed motherhood of hybrid F1 female mice was associated with a decreased percentage of male F3 offspring at birth and lower life expectancy and body weight during adulthood of F2 offspring. There was, however, no evident negative effect of delayed motherhood on several reproductive fitness variables in either male or female F2 offspring. This included between-parturition interval, litter size at birth and at weaning, body weight at weaning and preweaning mortality of F3 pups, percentage of F3 litters with at least one pup cannibalized, and time at which female and male F2 offspring ceased their reproductive life. These data clearly show that delayed motherhood in the mouse is associated with negative long-term effects on offspring survival.