Friday, April 10, 2009

Sex-biased longevity

Ive spent much of the last couple of weeks working on a paper on sex-biased longevity in primates.

Here is a draft of a portion of an introduction to said paper. Any similarities between this and the final published paper are purely coincidental:

The difference in longevity between the sexes of a population depends upon the selective forces each sex experiences, as well as the degree to which common genetic material limits independent demographic evolution. Sex-biased longevity has been proposed to arise from difference between the sexes in selective forces as diverse as reproductive physiology, care of offspring, parasite risk, mortality associated with reproduction, genomic stability and late-life support from kin. But measurements of sex-biased longevity have been made for relatively few species, and we have little sense of the degree to which sex-biased longevity is constrained by shared genetics or phylogenetic conservatism.

Most organisms, and likely all mammals, experience an evolved rapid increase in mortality and decrease in fertility at advanced ages, limiting longevity. The force of selection against mortality at a given age depends upon the likelihood of surviving to that age, and the mean remaining reproduction of individuals who do survive that long. Reproduction and survival are expected to drop to zero at similar ages, but reproduction can be direct (fertility) or indirect (care of offspring and kin effects).

Captive populations, the source of most demographic data on non-humans, will reflect sex-differences in evolved capacity for longevity more so than do wild populations, which tend to die younger (there are exceptions). For understanding the influence of experienced mortality patterns on evolved capacity for longevity, it is useful to make the distinction between extrinsic and intrinsic mortality. Extrinsic mortality is generally said to be that caused by the environment, while intrinsic death is driven by a failure of the organism's internal processes. In practice this distinction is difficult to make, as all environmental risk is influenced by the organism's characteristics and behaviors, and the timing and risk of intrinsic failure is inevitably influenced by an organism's history and environment. None-the less, much of our theory on the evolution of longevity is based on this intrinsic/extrinsic distinction. In general higher extrinsic mortality will lead to increased intrinsic mortality. However increased extrinsic risk at age x will influence evolved mortality at many ages, and not necessarily lead to a spike in intrinsic mortality centered at age x. This leaves overall sex-biased longevity as our best measure of sex-biased mortality when employing data from captive populations. Sex-biased longevity in captivity is highly correlated with sex-biased longevity in the wild even though the causes and timing of mortality may vary between species.

Much of the discussion of sex-biased longevity has focused on the idea that if one sex provides extended care to descendants, that sex should gain greater selective benefit from increased longevity. Allman et al. suggest that in primates, males who are the primary caregivers tend to live as long or longer than their females, while in species with little paternal care females tend to live longer. The Grandmother Hypothesis and its derivatives use indirect reproduction by post-menopausal women to explain their post-reproductive lifespan and therefore their tendency to live longer than men despite earlier decline in fertility. The Patriarch hypothesis instead argues that women's post-fertile survival is explicable based on late-life reproduction in males and the non-independence of male and female longevity.

Sex-biased longevity can also result from differences in mortality risk between the sexes. Trivers proposed that in species with frequent or intense male-male conflict, males incur significant mortality risk and therefore don't live as long as females. Alternatively, if females experience increased mortality in producing or rearing young, males should be expected to live longer. If either sex is the predominant disperser, and mortality risk during dispersal is high, this too could cause inter-sex differences in capacity for longevity. Finally, a variety of hypotheses have been put forward arguing that either the larger sex, or the smaller sex, should tend to live longer.

Using comparative primate life-history data, we examine the variability of sex-biased longevity in primates. We further examine the degree to which level of paternal care, grandmaternal care, male-male conflict, and mass dimorphism predict sex-biased longevity. Finally, we examine the patterns of correlated evolution among these variables in a phylogenetic context.

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