Maternal parity influences the birth sex ratio and birth interval of captive Francois’ langur (Trachypithecus francoisi)

The age and parity of female primates could reflect their physical condition and reproductive experience. Consequently, the individual age-parity dependent condition of mothers and the survival of their offspring represents a fitness tradeoff. This reproductive tradeoff is expressed in the individual birth sex ratio, offspring mortality and interbirth interval (IBI). Maternal antenatal and postnatal investment hypotheses predict that more female offspring are produced by experienced mothers in poor condition (Trivers-Willard hypothesis), and that offspring mortality decreases and IBI shortens with parity (targeted reproductive effort hypothesis). Here we test the latter predictions in captive populations of the endangered Francois’ langur (Trachypithecus francoisi), using long-term demographic and reproductive data from 21 Chinese zoos and three breeding centers. In these captive populations, birth sex ratio changed slightly from male-biased to female-biased as parity increased above five offspring in experienced mothers, consistent with the Trivers-Willard hypothesis. Although mortality of male offspring was greater than female offspring, overall offspring mortality did not vary with maternal parity. There was no significant difference in duration of postnatal care or IBI between male and female offspring. However, IBI for both female and male offspring shortened overall with increasing parity, as predicted by the targeted reproductive effort hypothesis. Females with more reproductive experience that produce more female offspring are critical to captive breeding programs that aim to increase the overall viable population size of this endangered species. For primates with a slow life-history, it is important to understand how mothers age and parity affects their reproductive fitness. We investigated maternal investment strategies of captive Francois’ langurs, specifically the effect of age and parity on the birth sex ratio, offspring mortality and interbirth interval. Mothers relied more on parity (i.e., their experience) to adjust the birth sex ratio, and had more sons in the first few parities. Mothers also spent significantly less time taking care of offspring as their reproductive experience increased and physical condition was likely declining. Thus, Francois’ langurs are able to adjust their investment in reproduction as they become more experienced (greater parity) to optimize reproductive fitness.