Sex Ratio Bias in Managed Populations of Hylobatids

Abstract

A number of explanations have been proposed for the occurrence of sex ratio bias in primates, including the Trivers-Willard hypothesis on differential investment, local resource competition or enhancement as a result of sex-biased dispersal, dominance-related advantages conferred on one sex but not the other, and the fragile male hypothesis. However, none of these theories was thought to be applicable to monogamous primate species. Here, we examine data on zoo-housed populations of three hylobatid species to test the null hypothesis of equal sex ratio. We analyzed over 40 years of demographic data on Nomascus leucogenys, Hylobates lar and Symphalangus syndactylus. We identified a strong male-biased birth sex ratio in N. leucogenys. Male infant mortality was consistently higher than female infant mortality for N. leucogenys, but this difference was not significant. We found that prime-age N. leucogenys females (aged 13–20) produced significantly more male offspring than female offspring, and young S. syndactylus females (<13 years) produced significantly more female offspring. Recent field evidence of more flexible mating systems in H. lar and S. syndactylus may also be occurring in N. leucogenys. However, N. leucogenys has not been well studied in the wild. Ecological differences among species may further contribute to these observed patterns.

Reproductive males are effective at managing conflict in captive Sulawesi crested macaques (Macaca nigra)

“…Female competition, on the other hand, is strongly associated with the availability of resources required for reproduction (Sterck et al, 1997), and unsurprisingly, female competition for mates increases in female-biased populations (Cheney et al, 2012). Birth-sex ratios in captivity are predominantly male-skewed (Faust & Thompson, 2000) and managing a surplus number of males is a primary challenge for many breeding program coordinators (Faust & Thompson, 2000;Margulis et al, 2011;Vermeer & Devreese, 2015). In an attempt to artificially mitigate sexually driven aggression, animal managers increasingly use surgical and hormonal contraception to suppress the production of testosterone (Asa & Porton, 2005;de Nys et al, 2010;Muller, 2017;Penfold et al, 2005).…”