Litter sex ratio variation in laboratory colonies of two geographically distinct strains of the root vole Microtus oeconomus

Litter sex ratio at birth was studied in two laboratory colonies of root voles Microtus oeconomus originating from northern and southern Norway, respectively Sex ratio was female biased in the northern (57% females), but not in the southern colony Sex seemed to be binomially distributed within litters of both colonies, and did not show consistent relationships with mother weight, litter size, parity or number of generations bred in the laboratory Both colonies originated from populations exhibiting cyclic population dynamics in the wild Thus different demography as such is unable to explain the difference in sex ratio between the two laboratory colonies Social organization has been shown to differ between the two strains of root voles both in the field and in the laboratory, but it is yet unclear whether this factor is related to the sex ratio difference documented in this laboratory study     

Reproductive aspects of American minks (Neovison vison) in central Spain: Testing the effects of prey availability

Some reproductive components of fitness (breeding times, litter size and the proportion of breeding females per year) of two feral populations of American minks in central Spain were investigated by means of direct observation from 2006 to 2010. The effects of prey availability were explored. Mating season was from February to March. Parturition dates did not differ between the two sites and pooled births took place between April (33.3%) and July (2.8%), peaking in May (40.3%). Mean litter size (the number of small cubs following their mother) was 3.43 ± 1.01 cubs (±SD; n = 30) with statistical significant differences between the two study sites. The strong seasonality in births and differences in litter size were related to prey availability (i.e. these are food-limited life-history traits), but in a complex non-linear fashion. A minimum value of prey abundance is necessary to breed. While litter size rises with prey abundance, there is a point where increasing prey did not result in an increase in litter size. Up to 75% of the females in the breeding pool reared cubs each year in both areas, which represents a somewhat high pool of breeding females. Delayed implantation is shorter than a month or does not exist in these populations. Such a short time-span seems to emerge because longer delays would have an important fitness cost to females. The breeding calendar and the litter size were similar to that reported previously from other areas.     

Variation in reproductive investment among and within populations of the scorpion Centruroides vittatus

Although information concerning variation among and within populations is essential to understanding an organism's life history, little is known of such variation in any species of scorpion. We show that reproductive investment by the scorpion Centruroides vittatus varied among three Texas populations during one reproductive season. Females from the Kickapoo population produced smaller offspring and larger litters than females from the Independence Creek or Decatur populations; this pattern remained when adjusting for among population variation in either female mass or total litter mass. Relative clutch mass (RCM) and within-litter variability in offspring mass (V*) did not differ among populations. Among-population variation may result from genetic differences or from phenotypically plastic responses to differing environments. Within populations, the interrelationships among reproductive variables were similar for Decatur and Independence Creek: females investing more in reproduction (measured by total litter mass, TLM) produced larger litters and larger offspring, and V* decreased with increased mean offspring mass (and with decreased litter size at Decatur). At Kickapoo, larger females produced larger litters and had larger TLM; females investing more in reproduction produced larger litters but not larger offspring. Within litter variability in offspring mass was not correlated with any reproductive variables in this latter population. These patterns may be explained by the fractional clutch hypothesis, the inability of females precisely to control investment among offspring or morphological constraints on reproduction.     

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)     

Mink with Divergent Activity Levels have Divergent Reproductive Strategies

Stable individual differences in activity levels within populations have been linked to differences in reproductive rate or parental care in several species, including American mink (Neovison vison). Fur‐farmed mink are good models for studying such effects because they yield large sample sizes and readily allow investigations into maternal behaviour, reproductive success, offspring performance and the relationships between these factors. On farms, very inactive individuals generally have smaller litters, and this held true in our study populations. We tested two competing hypotheses to explain this: (1) inactive individuals are failing to cope with a challenging environment and experiencing chronic stress and/or depression‐like ‘apathy’; this predicts female‐skewed litters, poorer maternal care, higher infant mortality and poorer infant growth and (2) inactive individuals do not have reduced fitness but instead employ an alternative adaptive reproductive strategy, trading off offspring quantity for quality; this predicts enhanced maternal care, reduced infant mortality and enhanced infant growth. Inactive females’ kits, especially their sons, grew faster than active females’, even after statistically controlling for litter size; and by 21 d, inactive and active dams’ litters no longer differed in total biomass, despite the former’s smaller litter sizes. In kit retrieval tests, inactive females were faster than active dams to reach their sons (as well as more likely to contact their sons than their daughters: a bias towards male kits not evident in the active dams). Furthermore, kit growth rates and dam latencies to touch them co‐varied, suggesting the existence of consistent differences in maternal style across inactive and active dams. Hypothesis 2 was thus supported: inactive females favour offspring quality over quantity, investing more resources in fewer kits, particularly males. This potentially boosts their sons’ adult fitness. More broadly for laboratory‐based studies, possible ‘captivity effects’ on the fitness correlates of activity and other personality traits are discussed.     

Age structure of six populations of old-field mice,Peromyscus polionotus

An accurate age-estimating technique, based on biochemical changes in eye lens protein, was used to study age structures of six populations of the old-field mouse, Peromyscus polionotus. A new mathematical procedure permitted quantitative comparisons of these populations. Four inland populations had essentially the same median ages (75–84 days), maximum ages (248–297 days) and relative production rates (56–58% of the surviving population had been born in the 100 days prior to sampling). Approximately 50% of the females were old enough to have weaned a litter. One inland population had a lower median age (49 days), a lower maximum age (181 days) and a higher relative production rate (73%). Relatively fewer females (38%) could have weaned a litter. The single beach population was also different, with higher median age (180 days), and lower relative production rate (2%). All females were old enough to have weaned a litter. Maximum age was essentially the same as for the four similar, inland populations. Factors which may have contributed to observed differences in these populations are discussed.     

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.     

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.     

Comparison of the gonadal response of wild and laboratory field voles (Microtus agrestis) to different photoperiods

Weanling male and female field voles from laboratory stock and from the F1 generation of wild-caught animals were placed in a long (16L:8D) or short (6L:18D) photoperiod for 28 or 56 days. Both types of field vole showed the well-established effect of photoperiod upon sexual maturation, with animals in the long photoperiod having larger and more active gonads than animals in the short photoperiod. After 28 and 56 days laboratory stock females were more mature, sexually, and had a higher growth rate than did Wild F1 females. There was no difference between the two types of males at 28 days, but by 56 days laboratory stock males were more sexually mature and had a higher growth rate than did Wild F1 males. These differences between the two types occurred in the long and short photoperiods. There was no interaction between photoperiod and type of vole. The use of laboratory stock animals in experiments could lead to an incorrect assessment of the effect of photoperiod in the control of seasonal breeding in wild populations.     

The effects of differential prenatal and postnatal social enviroments on sexual maturation of young prairie deermice (Peromyscus maniculatus bairdii).

We compared the influence of different social enviroments on sexual maturation of young prairie deermice. Between 12 and 48 hr after parturition, one-half of a litter from a population was exchanged with one-half of a litter from a bisexual pair. In each situation the remaining half-litter served as controls for the exchange procedure. The testes and vesicular weights of population-reared males were significantly larger than those of males reared by bisexual pairs, irrespective of prenatal enviroment. Generally, population-reared females also had larger reproductive organs than those reared by bisexual pairs, although these differences were not statistically significant. There were no significant differences in the adrenal weights of males or females, but females reared by bisexual pairs had larger absolute and relative adrenals than females reared in populations.     

Reproductive life history correlates of early and late sexual maturation in female Mongolian gerbils (Meriones unguiculatus)

Age at vaginal introitus is bimodally distributed in female domesticated Mongolian gerbils; some exhibit vaginal perforation before eye-opening (day 16), others after weaning (day 25). We found early- and late-maturing female gerbils to differ significantly in reproductive life history. Early-maturing females first reproduced when younger, had more litters, with more young per litter, and consequently had more than twice as many offspring as late-maturing females. In comparison with late-maturing females, early-maturing females gave birth to and weaned a greater proportion of females per litter and a higher proportion of early-maturing daughters per litter. Further, early-maturing females exhibited reduced maternal behaviour; they spent less time nursing their young and retrieved fewer offspring displaced from the nest than late-maturing females. Even under constant laboratory conditions, there were significant, correlated, circannual rhythms in female fecundity, the proportion of males per litter and the proportion of early-maturing daughters per litter. Review of the field literature suggests that wild Mongolian gerbils may exhibit similar variability in reproductive pattern.     

Reproductive behavior of intersexes of an intertidal amphipod Corophium volutator

Abstract. Intersexes are common in crustaceans. Typically, these intersexes are sterile or function as females, but prior evidence from laboratory experiments suggests that intersexes of a key species of gammaridean amphipod, Corophium volutator , might function as males. We observed that intersexes of C. volutator behaved as males by crawling (mate-searching) on a mudflat during ebb tides and pairing in burrows with female amphipods. In the laboratory, intersexes and males did not differ in aspects of crawling such as movement rate and measures of burrow investigation. I`ntersexuality was costly in that intersexes crawled less often than males on a mudflat, formed fewer pairs with females than males, and remained in tandem less often with receptive females than males. The use of PCR-based identification methods failed to identify the presence of transovarial, feminizing, microsporidian parasites as a major cause of intersexuality in this species in that infected females did not produce broods that contained more intersexes than broods produced by uninfected females. Because intersexes may be mistaken as females, the percentage of functional males in amphipod populations may be underestimated: an important consideration given male limitation in populations of C. volutator. The occurrence of intersexes has significant implications for studies on the evolution and ecology of sex ratios, and the use of crustaceans as indicators of environmental quality.     

Do Laboratory Mouse Females that Lose Their Litters Behave Differently around Parturition?

Efficiency in laboratory mouse breeding is hampered by poor reproductive performance, including the loss of entire litters shortly after birth. However, the underlying mechanisms are not yet fully understood and establishing the cause of death in laboratory mouse pups can be complicated. Newborn mouse pups are generally hidden in nests, dead pups are often eaten by the female, and the widespread practice of leaving periparturient females undisturbed complicates inspection, which may delay the discovery of pup loss. In order to efficiently prevent problems with litter loss, it is important to find key factors for survival. We investigated differences in periparturient behavior between female laboratory mice whose pups survived until weaning and females whose entire litters were lost. Video recordings of 82 primiparous females of the C57BL/6 strain or knockouts with C57BL/6 background were used. The mice were observed from 24 h before until 24 h after parturition and female behaviors coded using a pre-established ethogram. The relationship between behavior and survival was analyzed using logistic models, where litter survival was regressed on the proportion of 30-s observations with at least one occurrence of the behavior. We found that females with surviving litters performed more nest building behavior during the last 24 h before parturition (p = 0.004) and spent less time outside the nest during the entire observation period (p = 0.001). Increased litter survival was also associated with more passive maternal behaviors and the female ignoring still pups less. Females that lost their litters performed more parturition-related behaviors, suggesting prolonged labor. The results indicate that maternal behavior plays a significant role in laboratory mouse pup survival. Complications at parturition also contribute to litter mortality.     

Hypoxia and life‐history traits in a eurytopic African cichlid

This study quantified variation in key life‐history traits of the widespread African mouth‐brooding cichlid Pseudocrenilabrus multicolor victoriae. Egg size, number, batch reproductive effort, size at maturity and brooding efficiency were compared among field populations across a wide range of dissolved oxygen (DO) concentrations from extreme hypoxia to normoxia. In the laboratory, a similar suite of characters was quantified in F1 of low‐ and high‐DO origin reared under low or high DO. In general, females from low‐DO habitats and females reared under low DO were characterized by a smaller size at maturity and no difference in batch reproductive effort when compared with females from high‐DO habitats or females reared under normoxia. A trade‐off between egg size and number was evident in the field and in the laboratory‐rearing experiment, but the direction of the trade‐off differed. Egg size was negatively correlated with egg number across field populations; females collected from low‐DO sites generally had more, smaller eggs relative to females from high‐DO sites. In the laboratory‐rearing experiment, F1 females of high‐DO origin produced larger, fewer eggs than F1 females of low‐DO origin, lending support to the field results and suggesting a heritable component to these traits. There was also an element of developmental plasticity, F1 females raised under low DO produced larger, fewer eggs compared with F1 females raised under high DO (regardless of population) suggesting that DO may interact with other variables to determine egg size in the field.     

A comparison of fertility in vitro and in vivo after exposure of male mice to high environmental pressure

Male mice of the TO strain were exposed to 50 ATA helium (0.5 ATA oxygen) over 5 weeks and then assessed for fertility both in vitro and in vivo. Significantly lower values for acrosome loss and fertilization of zona intact eggs in vitro were obtained with spermatozoa from the pressure-treated males. These results, coupled with the significant decrease in incidence and degree of polyspermy in zona-free eggs, indicated that fewer competent cells were present after preincubation under capacitating conditions. Fewer motile cells were observed in most samples, along with a reduction in the incidence of hyperactivated motility in some. Testis weight was also significantly lower. Results of fertilization in vivo, after mating with untreated females, showed a consistent trend toward subfertility, with a lower pregnancy rate and a smaller litter size. These differences did not reach significance, however, unlike those obtained in earlier in vivo experiments with males of the BALB/c strain. The discrepancy between in vitro and in vivo results with TO males probably represents differences between unselected and selected sperm populations, while that observed in vivo between the two strains probably reflects the superior reproductive capacity of the TO strain.     

Variation of reproductive effort between parthenogenetic and sexual populations of the scorpion Tityus columbianus

Abstract. Geographic differences in reproductive effort were examined between sexual and parthenogenetic populations of the Colombian scorpion Tityus columbianus , from Iza (sexual) and from Mosquera (unisexual). The sexual females from Iza were significantly larger and had significantly greater relative litter mass (RLM) than the parthenogenetic ones from Mosquera. In both populations litter size increased significantly with female body size.
Litter size and RLM in the sexuals averaged 12.7 and 0.49 respectively, and in the unisexuals 8 and 0.36. We suggest that the higher values are related to greater primary productivity at the Iza site.     

Genetic architecture of population differences in oviposition behaviour of the seed beetle Callosobruchus maculatus

Few studies have examined the genetic architecture of population differences in behaviour and its implications for population differentiation and adaptation. Even fewer have examined whether differences in genetic architecture depend on the environment in which organisms are reared or tested. We examined the genetic basis of differences in oviposition preference and egg dispersion between Asian (SI) and African (BF) populations of the seed beetle, Callosobruchus maculatus. We reared and tested females on each of two host legumes (cowpea and mung bean). The two populations differed in mean oviposition preference (BF females preferred cowpea seeds more strongly than did SI females) and egg dispersion (SI females distributed eggs more uniformly among seeds than did BF females). Observations of hybrid and backcross individuals indicated that only the population difference in oviposition preference could be explained by complete additivity, whereas substantial dominance and epistasis contributed to the differences in egg dispersion. Both rearing host and test host affected the relative magnitude of population differences in egg dispersion and the composite genetic effects. Our results thus demonstrate that the relative influence of epistasis and dominance on the behaviour of hybrids depends on the behaviour measured and that different aspects of insect oviposition are under different genetic control. In addition, the observed effect of rearing host and oviposition host on the relative importance of dominance and epistasis indicates that the genetic basis of population differences depends on the environment in which genes are expressed.     

Phylogeographic structure without pre-mating barriers: Do habitat fragmentation and low mobility preserve song and chorus diversity in a European bushcricket?

When animal species have a strong phylogeographic structure questions arise on the origin, maintenance and future evolutionary trajectory of that structure. One prediction is that phenotypic differences among populations serve as pre-mating barriers should secondary contact occur. Post-mating barriers may also function and ensure further separation of the populations. We tested these predictions in an acoustic insect, the European bushcricket Ephippiger diurnus, that is geographically distributed in separated, genetically isolated populations distinguished by pronounced differences in male songs and chorusing and female preferences for song. We staged mixed-population choruses to examine how males sing when in the company of other populations, and we released females in these choruses to assess their preference for or aversion toward males of their own population versus a different one. We also paired males and females of the same and different populations to test mating success, spermatophore transfer, and oviposition in the various pairings. In most cases males sang as effectively when accompanied by males from another population as when in a single-population chorus, and females overall expressed little aversion toward males from a different population. Inter-population pairs did not mate less frequently, transfer smaller spermatophores, or deposit fewer or smaller eggs than pairs of males and females from the same population. We infer that pre-mating barriers play little role in maintaining phylogeographic structure despite the acoustic differences between populations. Rather, the structure probably reflects the fragmented distribution of suitable habitat and the low mobility of both juveniles and adults. Thus, if secondary contact does occur, and we predict that it will in several key locations owing to ongoing environmental change, coalescence of populations and reduction of genetic and acoustic diversity may follow.     

Age at puberty and first litter size in early and late paired rats

The rate of sexual maturation among female Sprague-Dawley rats was measured in a variety of intraspecific social environments. It was found that females of this strain differ from at least one other strain of laboratory rat in that neither age at vaginal perforation nor age at first estrus was affected in Sprague-Dawley females by the presence or absence of male, regardless of his age or breeding history. Sizes of first litter among females who mated at their first estrus were compared with those among females who were first inseminated at older ages. On average, females bred at first estrus produced litters that contained more than 3 fewer pups than females mated at older ages. This observation suggests that female Sprague-Dawley rats do not attain full reproductive competence until sometime after the onset of puberty.     

Growth rate during early life affects sexual differentiation in roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis>)

Many environmental factors have been shown to influence sex differentiation in fish, resulting in sex-biased populations, but the effects of growth rate have received limited attention. We conducted a controlled laboratory experiment in which growth rate and population density were manipulated in roach (Rutilus rutilus) during early development, and the subsequent effects on sex ratio determined. Significant differences in growth rate between fish populations were induced through provision of three different ration levels. In the slowest growing population there were fewer females compared within the fastest growing population (19% compared to 36% females), suggesting that in roach it may be more advantageous to become a small male than a small female when growth potential is limited. This may result from the fact that fecundity is limited by body size in female roach and that male roach are able to reproduce at a significantly smaller body size than females. In contrast, where roach were kept at different stocking densities, and there were no differences in growth rate, the subsequent proportion of females did not vary. Our data highlight the importance of controlling for growth rate in research on sexual differentiation in this species, notably when assessing for the effects of endocrine disrupting chemicals and other environmental factors, and have implications for fisheries management and aquaculture. The underlying mechanism for the influence of growth rate on sex differentiation has yet to be determined but is likely to have a strong endocrinological basis.