Male mammals typically die sooner than females. Why is that? Researchers in Madagascar are asking this very question. Recently published in the Journal of Behavioral Ecology a team of researchers studying Milne Edwards sifaka may have discovered why males of this species die so much younger than females.
Milne-Edwards’ sifaka are a large dark lemur that lives in the tropical wet forests of eastern Madagascar. This large lemur spends much of its day consuming leaves and resting. However, they are no slouch when it comes to moving. These species employ a fantastic form of locomotion called vertical clinging and leaping. To move through the forest they spring off a large tree with powerful and especially long legs. As they fly through the air they turn to land on the next tree. As the make contact their body compresses and they shoot off that tree to the next - like a pinball they bounce through the forest. Some sifaka species can breach horizontal gaps between trees as large as 8m (26 feet)!
There are many hypothesis as to why males tend to die off quicker than females. Tecot et al. (2013) describe a few which include the "high risk, high gain" hypothesis which states that males engage in more risky behaviours such as competition for mates or dispersal which increase mortality than females. Imagine lemurs in a bar fight! There is the "fragile male" hypothesis which suggests that males and females have different developmental strategies with males having for example faster growth rates leading to higher mortality as juveniles - especially in times of resource scarcity. This evokes images of male teenagers growing like sky scrapers all while trying to keep up through incredible consumption of food. While females may employ their own strategies like the "live slow, die old" hypothesis which predicts that females actually slow down development in response to increase resource unpredictability and in some cases females may not even reproduce during times of resource scarcity.
Male and female Milne-Edwards' sifaka are roughly the same size and weight, which is particularly unusual for group living primates - but not so for lemurs as many lemur species show little to no sexual dimorphism (differences in size between sexes). Like many lemur species females are dominant. Both sexes leave their natal group (disperse). However, males in this species continue to secondarily disperse long after maturity. Growth rates between the sexes of this species are very similar as are levels of testosterone. Based on their life history one might expect that males and females have the same mortality based on the above hypotheses.
Tecot et al. (2013), determined that male and female mortality were virtually the same until 18 years of age. At this point males became very likely to die while females of this age were more likely to live into their 30's. They suggest that because males engage in "risky" behaviour through adulthood by continuing to disperse while females remained within their group resulted in higher males mortality. It seems that for this species the most likely reason for increased mortality is that males perform more risky behaviour through adulthood than females.
Research like this is fundamental to our understanding of many fascinating aspects of evolution and opens doors to more questions such as why do males disperse more during adulthood than females?