Litter sex composition affects first reproduction in female grey mouse lemurs (Microcebus murinus).

Conditions during early development can have long-lasting consequences on behavioural phenotypes, reproduction and survival. We studied the consequences of maternal characteristics and litter sex composition on the reproductive fitness of 849 females during their first reproductive season in a captive population of mouse lemurs. In this primate, litters usually comprise 1 to 3 offspring. For female offspring, body mass at birth or at weaning did not depend on the parity of the mother but was strongly linked to the size of the litter with significant lowest body mass in triplets. Body mass during early growth has no effect on females reproductive success, but a low body mass during oestrus led to more unsuccessful mating, regardless of the litter composition. Successful pregnancy was negatively correlated to the presence of a brother in the litter in which they were born. Moreover, the oestrogen levels at oestrus were significantly reduced by 30% in females born with male littermates, suggesting a potential masculinisation in utero of these female offspring. Finally, a decrease in oestrogen values at oestrus is associated with the production of male-biased litters. These results should help stimulate further debate on the importance of littermates' long-term effects on individual life history traits, especially in the context of manipulation of the offspring sex ratio that is assumed to increase parental fitness.