Uncovering variation in the patterns of aging.

Do all species experience the declines of old age? No. Do we currently have theories that accurately predict which species escape these declines? In PNAS, Warner et al. (1) demonstrate that the answer to this question is also “no.” Understanding the breadth and diversity of the patterns of aging across the tree of life is central to life history evolution, and recent studies demonstrate that there is considerable variation in the age trajectories of traits across species (2–4). In general, the decline in function and increase in mortality that is associated with increasing age is expected, according to classic evolutionary theories, because of the decline in the strength of selection with age (5–7). In other words, the alleles that predispose us to heart disease or dementia have effects primarily at late ages, after we have reproduced, thus their impact on Darwinian fitness is minimal and these deleterious traits—and many others like them—persist. In natural populations, as we look across species, a central premise of the evolutionary theories of aging is that the rate of aging in different species will depend on levels of environmentally imposed mortality (7). All living organisms are subject to a constant risk of mortality posed by accidents, predation, and disease. This environmental, or extrinsic, mortality will influence the probability that individuals will survive to a given age. The hypothesis is that species with the highest levels of environmental mortality should evolve high levels of early reproduction and rapid aging. Conversely, slower rates of aging are expected in populations or species with low environmental mortality, for example in species such as turtles, which have protective shells that reduce predation. In this case natural selection will act to eliminate late-acting deleterious alleles because those individuals who can live longer will have higher lifetime reproductive success. Additionally, turtles increase their size with increasing age, and larger-sized individuals have higher fecundity that may help to maintain a high level of selection on late-age traits. Warner et al. (1) show that the story about aging in turtles is not this simple. In their study with the painted turtle (Chrysemys picta) (Fig. 1), the authors marked more than 1,000 individual turtles living in the backwaters of the Mississippi River, and these individuals were followed for 24 y. Over this long time period, patterns of reproduction and mortality were tracked to determine how the trajectories of these traits changed with advancing age of the individual turtles. Based on evolutionary theories, Warner et al.’s expectation had been that reproduction would increase with age as the turtles grew larger, and that there would be no age-dependent increase in mortality. However, what they found was that reproduction declined andmortality increased as the turtles aged. The oldest females produced eggs with reduced hatchling success. Warner et al.’s analysis also shows acceleration in the probability of mortality across the entire lifespan. Specifically, themortality rate for the turtles doubled every 13.8 y. Human populations have an approximate mortality rate doubling time of 7.2 y (8), which means that after age 30, our chances of dying double approximately every 7.2 y. The painted turtles are thus aging slower than humans, but these Fig. 1. A study with the painted turtle (Chrysemys picta) shows age-related declines in a reptile with a long lifespan. Image courtesy of Clare Isabel Ming-ch’eng Adams (Iowa State University, Ames, IA).