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

Abstract The age and parity of female primates reflects their physical condition and reproductive experience. Individual age-parity dependent condition of females 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). Thus, maternal antenatal and postnatal investment hypotheses argue that, dependent on their physical condition determined by available resources, mothers can adjust the birth sex ratio, offspring mortality, and IBI. These 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 (1980 to 2021) demographic and reproductive data from 21 Chinese zoos and three breeding centers. In these food provisioned populations, birth sex ratio changed slightly from male-biased to female-biased as parity increased, consistent with the predictions of the Trivers-Willard hypothesis. Although mortality of male offspring was greater than female offspring, overall offspring mortality did not vary with parity among females. The duration of postnatal care or IBI of male offspring was significantly shorter than for female offspring. However, IBI did shorten significantly, even for male offspring, with increasing parity, as predicted by the targeted reproductive effort hypothesis. Selecting females with more reproductive experience is recommended for captive breeding programs as they produce more female offspring, increasing the overall viable population size of this endangered species.