Further evidence of sex-biased maternal investment by Mongolian gerbil dams

Abstract

There is theoretical reason to expect that ill--PDlygynous species, as a consequence of competition between males for access to females, (I) a given increment in parental investment (Trivers 1972) will have a greater effect on the fitness of males than of females and (2) parents will make greater investment in individual sons than in individual daughters (Trivers & Willard 1973;Clutton-Brock & Albon 1982;Clutton-Brock & lason 1986). Unfortunately, as Clutton-Brock & Albon (1982), Clutton-Brock & lason (1986) and Hrdy (1987) each have made clear, in their respective reviews of relevant literature, predictions about sexbiased parental investment are difficult to test empirically because of the lack of data describing either the fitness costs to parents of rearing young of each sex or the relative effects of a given amount of parental investment on the survival and reproduction of offspring of each sex. In a recent paper (Clark et al. 1990),we presented evidence that female Mongolian gerbils, Meriones unguiculatus, exhibit a greater loss in reproductive potential after rearing sons than after rearing daughters. Here, we examine effects of reductions in maternal investment on rates of growth of nursing male and female Mongolian gerbil pups. On theoretical grounds (Clutton-Brock & lason 1986), one would expect male pups to be more vulnerable to parental deprivation than their sisters. Our subjects were 41 litters of gerbils, born to primiparous females descended from breeding stock acquired from Tumblebrook Farms (Brookfield, Massachusetts). On the day of birth, each litter was culled to two male and two female pups and each pup was toe clipped for individual recognition. Because of the demands of our experimental design, we used only those 30 litters that, on day 5 postpartum, had all four pups both alive and (to exclude runts) within a range of 0,5 g in weight. Three female and two male pups, one in each of five litters, died between days I and 5 postpartum. On day 5 postpartum, we randomly assigned each litter to one of three groups (10 litters/group) that experienced either 0, 2 or 4 h/day of separation