14.1.2 14.12: What determines the average longevity in different species?

Can individuals live forever? No. Just because they are not at risk from internal, genetic sources of mortality that doesn’t mean there aren’t things in the world around them that might kill them—for example, predators. lightning, drought, or disease. And for different species, these risks are more or less serious. Rodents, for example, are at very high risk of predation nearly every minute of every day, so they are likely to be dead within a few years (FIGURE 14-20).

Because death from some external source is likely to come so early, organisms living in high-risk worlds must reproduce early: natural selection favors this because if they didn’t, they wouldn’t leave any descendants. Now, when a harmful mutation arises that causes its damage at an early age—say, one or two years of age—the rodent has probably already reproduced and passed the mutation on. And so that harmful allele increases in frequency in the population, causing individuals to age earlier.

Tortoises, by contrast, live in relatively low-risk worlds. Because they have armor-like shells that protect them from danger, death from external sources is low. Early, intensive investment in reproduction is not necessary and so has not been favored by natural selection. In such species, when a harmful mutation arises that has its effect at 5 or 10 years of age, the individual carrying it is likely to die before it has reproduced. Consequently, the mutation isn’t passed on; natural selection weeds it out of the population.

The age at time of reproduction, then, is a key factor determining longevity. Factors that favor early reproduction also favor early aging; factors that don’t favor early reproduction—or actively favor later reproduction—favor later aging. But then, what determines when an organism reproduces?

We can think of each population as evolving in a world with a specific hazard factor (see Figure 14-20). This factor includes the risk of death from all types of external forces. When the hazard factor is low, as it is for tortoises and humans, individuals of a species tend to reproduce later, and so natural selection can weed out all harmful mutations except those that have their effects late in life. Individuals of these species will live a long time before they succumb to aging. A high hazard factor, on the other hand, will lead to earlier reproduction and therefore early aging and shorter life span.

Which species should age more slowly in captivity, a porcupine or a guinea pig? The difference is large, revealing unambiguously that it’s good to be a porcupine. Few predators want to eat you. Or rather, few predators can eat you. Because of their sharp quills, porcupines have evolved in a world with little risk of predation and with a lower hazard factor.

Meanwhile, guinea pigs, who have no protective quills, live with a high risk of predation. Because they’ve been evolving in a world characterized by a high hazard factor, they reproduce much sooner, age more quickly, and die younger. A guinea pig in captivity lives less than 10 years, whereas a captive porcupine can live 28 years. In the wild the difference is even greater: 15 years for the porcupine and only 3 or 4 years for the guinea pig.

What difference in longevity would you expect between poisonous and non-poisonous snakes, or between bats (which can fly) and mice?