Predation vs. longevity: a counterexample

Why do bats live longer than comparably sized and metabolically similar rodents? One idea is that chiropterans can escape predation more easily (because they can usually, though not always, fly away), thereby decreasing one major source of extrinsic mortality and exposing later life to selection pressure. In contrast, mice are low on the food chain and rather likely to meet a bad end; consequently, there’s very little opportunity for genes that enhance longevity to benefit the animal (or even get expressed in the first place). Based on examples of this kind, biologists of aging generally predict that lower extrinsic mortality from predation will be a positive influence on the evolution of longevity.

Hence, it is surprising that guppies exposed to predation actually appear to live longer (and age more slowly) than similar fish from a similar environment without predators (see the original report here). Mitteldorf and Pepper review the data in light of evolutionary theories of aging:

How can evolutionary theory accommodate recent empirical results on organismal senescence?

According to a prominent recent report, guppies collected from sites lacking predators are inferior in every aspect of their life history profile to those evolved in other, nearby sites with predators present. This is an exception to two classical predictions of evolutionary theory: that low extrinsic mortality should be associated with longer life span, and that higher fertility should be associated with shorter life span. Some theorists have tried to accommodate this and other anomalous results within the standard framework, but we argue that the exceptions they carve out do not explain the results at hand. In fact, the findings suggest that population regulation has been selected at the group level, though this is a mechanism that most theorists regard with suspicion. We conclude by relating the present result to other experiments that seem to point in the same direction.

These reviewers raise the possibility of group selection — the idea that life history (here, time to maturity and ultimate lifespan) could be selected to benefit the species as a whole but not necessarily the individual — while acknowledging that such theories have been widely discredited, especially in regard to the evolution of longevity.

Then again, most arguments against group selection in aging assume the opposite case, namely, positive selection for more rapid aging, perhaps to “clear the boards” for the younger generation by preventing competition from older, larger animals of the same species. (At the risk of oversimplification, the argument goes something like this: If a population were selected to have a short lifespan, there would be a game-theoretical incentive for “defectors” to arise; these defectors would live a longer life, throughout which they would continue to reproduce; their offspring would reap the benefits of diminished competition resulting from their cohorts helpfully dying off at a young age.) We rarely consider the idea that group selection might result in longer lifespans (except in big smart mammals like whales whose experience benefits the community; this “grandmother selection” is still usually rationalized by appeal to the selection advantage to individuals and their closest relatives, who share their genes).

So, is the guppy result a specific example of how fish age funny, or a broadly applicable counterexample to a well-loved evolutionary principle? Certainly, the findings demand a closer look at the role of predation (and other forms of extrinsic mortality) on the evolution of life history and longevity, in additional species in a variety of environments.