Influence of social factors on sex ratio at birth, maternal investment and young survival in a prosimian primate

Abstract

In order to determine whether social factors influence sex ratio at birth in lesser mouse lemurs, experiments were conducted during 5 successive breeding periods on 51 females. At the beginning of the breeding season, females were either isolated (I) or grouped (G) in heterosexual groups with an increasing number of females (2, 3 or 4). To ensure mating, I females were introduced in a group only during the oestrous period. After mating, both I and G females were isolated during pregnancy and lactation. Reproductive capacities of females in terms of oestrus occurrences (n = 324), impregnations (n=210), pregnancies (n= 136) or abortions (n =38) or litter sizes (1-3 young) were affected neither by age and parity of females nor by group housing prior to conception. G females produced significantly more sons than daughters (67% males for 189 newborn) while females living alone except during the mating period demonstrated a significant inverse tendency (39.6% males for 96 newborn). Distribution of sexes in litters was statistically different from random and varied according to the shift of sex ratio at birth. In G females, the shift in the sex ratio towards males was consistent across the different groups, independent of the number of females living together, suggesting that the presence of only 1 female is sufficient to induce a bias in the sex ratio. No correlation was found between infant survival at weaning and age, parity or group housing of the mother. The maternal investment allocated to male or female newborn was similar provided the litter contained at least I male. In litters without males, growth and survival of female infants were significantly less. These results on sex ratio bias in captive female mouse lemurs agree with directions of bias predicted by the local resource competition model for facultative sex ratio adjustment (Clark 1978). Nevertheless, the pattern observed in mouse lemurs appears to be independent of the nutritional state of the female and of differential maternal investment.

Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception

“…Perret (1990) reported a similar result after mating female mouse lemurs that were either previously isolated or grouped with other females. Perret (1986) had noted that, in that species, hormone levels throughout the ovarian cycle are strongly modified by the presence of other females.…”

Environmental and Social Determinants of Sexual Function in the Male Lesser Mouse Lemur (Microcebus murinus)

“…Moreover, data ob tained with grouped females in captivity strongly suggest a decrease in the reproduc tive capacities of wild females exposed to overcrowding [82]: increased duration of oes trous cycles, higher frequency of abortive pregnancies, disturbed maternal behaviours leading to low infant survival or reduced in fant growth. We also found that female mouse lemurs exposed to 'social stress' produced a significant excess of male newborn young [28], Similar biases in the sex ratio have been reported for other prosimians [83], Among gregarious primate species, deviations of the sex ratio of offspring towards one sex or the other observed in subordinate females (i.e. under social stress) are thought to be depen dent on complex factors including parental ability to invest, food competition or inheri tance of maternal rank [84][85][86], Several ex amples support the model that, under re source competition, females would show an increasing tendency to bias the sex of their offspring towards the sex that emigrates on reaching adulthood for successful breeding [87,88],…”

“…The female lesser mouse lemur shows oes trus 2-3 times a year, and gestation lasts 61 ± 0.2 days (n = 136) [28]. At birth, infants weigh 5-6 g; they grow up rapidly to reach 30-40 g when weaned 7 weeks after birth.…”

Experimental alteration of litter sex ratios in a mammal

“…For example, social stress leads to the production of more sons than daughters (Krackow & Hoeck 1989;Perret 1990), which does not appear to have an adaptive explanation. An interaction between glucose levels and early development may also provide a link to other hypotheses (e.g.…”