Sex differences in longevity

Nobody knows for certain how long mandrills might live in the wild. Records show that some captive mandrills live to be 30–40 years of age in Zoological Gardens (Hill, 1970).This is most unlikely to be the case in the wild, where predators, parasites, diseases, and the struggle to find sufficient food and to reproduce must exert a heavy toll. Even in the semi-free ranging mandrill groups at the CIRMF, where the animals enjoy a comparatively sheltered existence, females typically live until they reach their early twenties, while the median lifespan for males is 17 years (Setchell et al., 2005a). These authors recorded a significant sex difference in mortality, with more males than females ‘disappearing’ (and presumed dead) in the large rainforested enclosures from the age of six years onwards. The oldest (post-reproductive) female in the CIRMF colony was still alive at 37.3 years, whereas the oldest surviving male was 21.2 years of age (Charpentier et al., 2013).

The only estimates of longevity in free-ranging mandrills are those provided by Abernethy and White (2013), based upon their observations of radio-collared animals (15 males: three females). These authors reported that wild mandrills might have a lifespan of 12–14 years, but as the ages of the mandrills they radio-collared were unknown at the outset, these estimates are approximate at best. Under completely natural conditions, sex differences in mandrill mortality are pronounced. I say this because, although the percentage of males:females at birth is approximately 50:50, wild mandrill supergroups contain six times more adult females than adult/sub-mature males during the annual mating season (Abernethy et al., 2002). Even allowing for the fact that some males probably remain on the fringes of groups during the annual mating period, and were thus not counted by Abernethy et al., the mandrill clearly has a very high socionomic sex ratio (i.e. the ratio of adult females:adult males in the population).

Effects of predation

Male mandrills probably have a higher risk of mortality than females for a variety of reasons. Adolescent males emigrate from their natal groups, and it is likely that a significant proportion of these inexperienced young males might perish before they reach maturity. Predation by leopards may be a significant factor, as examination of leopard scats collected in the Lopé National Park has shown that mandrills form part of their diet (Henschel et al., 2005).