Mertice M. Clark

Prenatal influences on reproductive life history strategies

Over the past two decades, evolutionary and behavioural ecologists have become increasingly interested in the adaptive consequences of intraspecific variability in life history and behavioural strategies. Recently, behavioural endocrinologists have begun to uncover surprising relationships between levels of prenatal exposure to gonadal hormones and variation in reproductive behaviour in adulthood. Such relationships may provide a causal explanation for many variations in adult phenotype that are of insterest to behavioural and evolutionary ecologists.

The role of the physical rearing environment in the domestication of the mongolian gerbil (Meriones unguiculatus)

Gerbils reared in tunnel systems responded to a visual stimulus by fleeing, foot-thumping and remaining concealed, whereas many gerbils reared in laboratory cages responded in the same situation by approaching the stimulus. The critical factor in tunnel-rearing was the opportunity to flee to shelter during maturation. Neither isolation from illumination nor isolation from stimuli associated with human handlers produced the observed effect. Gerbils reared in laboratory cages exhibit the pattern of flight and concealment in response to stimulation following 24-hr experience in a tunnel system. The data are discussed in terms of their implications for models of the ontogeny of the behaviour characteristic of domesticated, as compared with wild, strains.

A testosterone-mediated trade-off between parental and sexual effort in male Mongolian gerbils ( Meriones unguiculatus ).

Effects of testosterone (T) on parental behavior of male Mongolian gerbils (Meriones unguiculatus) were examined. After undergoing gonadectomy, castrated males were implanted with empty capsules or capsules containing T; sham-operated males were implanted with empty Silastic capsules. Subsequently, each male was paired with a pregnant female, and after delivery, families were observed 15 min/day for 20 days. Gonadectomized males without T spent more time in contact with, huddling over, and licking pups than did either sham-operated males or gonadectomized males with T. When given a choice between nest sites and displaced pups, females and males with low T preferred pups, whereas intact males and castrated males with T preferred nest sites. The findings are consistent with E. D. Ketterson and V. Nolans (1994) hypothesis implicating T in mediating trade-offs between parental and sexual effort.

Parent-offspring interactions determine time and place of first ingestion of solid food by wild rat pups*

Wild rat pups maintained on ad lib food in a large enclosure with feeding sites not visible from nesting areas ingest their first samples of solid food at the same time and in the same place as adults of their colony. Blind pups maintained under the same conditions do not take initial meals of solid food in the presence of adults.

Concentrations of sex steroid hormones in pregnant and fetal mongolian gerbils

Sex steroid concentrations in the plasma of 24-day pregnant Mongolian gerbils (Meriones unguiculatus) and their male and female fetuses were measured using radioimmunoassays. It was found that, on Day 24 of gestation: (a) androgen levels were higher in those male fetuses developing adjacent to no female fetuses than in those male fetuses developing between two female fetuses and (b) androgen levels were higher in those female fetuses developing between two male fetuses than in those female fetuses with no immediate, male neighbours. Further, plasma taken from 24-day pregnant dams that had exhibited vaginal opening at a relatively early age had significantly lower androgen levels and significantly higher estradiol levels than did plasma taken from 24-day pregnant dams that had exhibited relatively late vaginal opening. The data provide direct evidence of hormonal mediation of previously described differences both in the morphology and reproductive biology of male and female adult gerbils as a function both of their fetal intrauterine locations relative to members of the other sex and of the age at vaginal introitus of their respective dams.

Effects of intrauterine position on the behavior and genital morphology of litter-bearing rodents

We review the literature describing hormonally mediated effects of intrauterine position on the genital morphology and reproductive behaviors of litter‐bearing rodents. We emphasize work carried out in our own laboratory in which male and female Mongolian gerbils served as subjects. The results of the studies we consider indicate that biologically significant aspects of the variance in morphology and reproductive strategy seen in all populations of adult rodents reflect variance in perinatal levels of exposure to gonadal hormones induced by intrauterine position. We conclude that studies of correlations between intrauterine position and adult characteristics provide opportunities to examine the impact of normal variation in perinatal exposure to hormones on adult mammalian phenotypes.

Intrauterine positions and testosterone levels of adult male gerbils are correlated

Those male Mongolian gerbils (Meriones unguiculatus) that developed in intrauterine positions between two male fetuses had significantly higher levels of serum testosterone, as adults, than did those adult male gerbils that developed in intrauterine positions between two female fetuses. The endogenous testosterone levels of adult male gerbils were significantly positively correlated with both the sizes of their ventral scent glands and their frequencies of scent marking. We found no evidence of pulsatile release of testosterone in adult male gerbils.

Fetal uterine position affects copulation and scent marking by adult male gerbils.

Those male Mongolian gerbils (Meriones unguiculatus) that as fetuses resided in uterine locations adjacent to no females, when adult, scent marked more frequently, mounted estrous females with shorter latencies, and ejaculated after fewer intromissions than did those male gerbils that as fetuses resided in uterine locations adjacent to two females. Both the scent-marking frequencies and copulatory patterns of adult males were positively correlated with three indices of their circulating levels of testosterone: ventral gland size, anogenital distance, and relative testes weights. Also, those males that scent marked relatively frequently copulated more reliably than did those males that scent marked relatively infrequently.

Parenting and potency : alternative routes to reproductive success in male Mongolian gerbils

Adult male Mongolian gerbils, Meriones unguiculatusgestated in intrauterine positions between two female fetuses (2F males) are less likely than are adult males gestated between two male fetuses (2M males) to impregnate strange female gerbils with whom they are paired. The reduced copulatory success of 2F males is correlated with both lower circulating levels of and reduced sensitivity to testosterone. We asked whether 2F male gerbils compensated for their reduced copulatory success by increasing their parental effort. 2F male gerbils engaged in less sexual activity with their mates, but were more frequently in contact with pups than were 2M males, huddling over the young when their mates were absent from the nest. Although there were no differences in rates of survival or growth of pups reared by pairs consisting of a female and either a 2M or 2F male, mates of 2F males delivered significantly more pups as a consequence of copulations occurring during postpartum oestrus than did either mates of 2M males or females rearing young alone. We interpreted these results as consistent with Ketterson & Nolans (1992, Am. Nat. (Supplement)140, 533-562) hypothesis of a testosterone-mediated trade-off between investment in sexual and parental behaviours.1997The Association for the Study of Animal Behaviour

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 ditfer 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.