Friday, April 18, 2008

Dying for Sex

One of the projects I am working on currently is an analysis, using data from primates, of what life history factors are correlated with sex-biased longevity. To put that plainly, I want to know why in some species the females live longer, in some the males live longer, and in some they live equally long. One of the dozens of hypotheses out there explaining why females live longer, in species where they do, is the 'risky male behavior' hypothesis' which says that males don't live as long because they take risks while out looking for sexual partners.

There has been limited support for this hypothesis, and most of the others, because so many hypotheses make the same predictions that one can rarely conclude that a particular factor is at play unless one ignores all the other possibilities (which seems to be the standard practice.)

This paper from Proc.Roy.Soc.B. takes an interesting new tack, looking not at whether males that are shorter lived than their mates are taking more risks, but rather at whether their short-livedness can be explained by increased mortality during the season of risk taking.

Here is the abstract:
Abstract

Male excess mortality is widespread among mammals and frequently interpreted as a cost of sexually selected traits that enhance male reproductive success. Sex differences in the propensity to engage in risky behaviours are often invoked to explain the sex gap in survival. Here, we aim to isolate and quantify the survival consequences of two potentially risky male behavioural strategies in a small sexually monomorphic primate, the grey mouse lemur Microcebus murinus: (i) most females hibernate during a large part of the austral winter, whereas most males remain active and (ii) during the brief annual mating season males roam widely in search of receptive females. Using a 10-year capture–mark–recapture dataset from a population of M. murinus in Kirindy Forest, western Madagascar, we statistically modelled sex-specific seasonal survival probabilities. Surprisingly, we did not find any evidence for direct survival benefits of hibernation—winter survival did not differ between males and females. By contrast, during the breeding season males survived less well than females (sex gap: 16%). Consistent with the ‘risky male behaviour’ hypothesis, the period for lowered male survival was restricted to the short mating season. Thus, sex differences in survival in a promiscuous mammal can be substantial even in the absence of sexual dimorphism.

2 comments:

jte said...

What's that mean at the end, "even in the absence of sexual dimorphism"?

My hypothesis about attracting mates is this: males (for example) don't attract females directly. Instead, they attract them indirectly by gaining status among other males. That's what it seems like in humans and helps explain why (or is consistent with how) a "hard to get" strategy can be so enticing to a potential partner. This surely isn't very different from other hypotheses, if at all. Can you think of any experiments that might distinguish between this strategy and others?

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