Many researchers are involved in the study of aging in different species with varied life spans, with the aim of identifying important factors in the molecular biology of aging. What drives these differences in life span? Researchers are interested in finding answers from both the evolutionary and cell biology perspectives. This article focuses on the relationships between species life span, predation, and size:
Aristotle's observation that bigger animals tend to live longer has lasted. Indeed, it's the only trend today's scientists agree on. "All of the other hypotheses have fallen apart," says Steven Austad, a biogerontologist at the University of Alabama, Birmingham. One of the most popular ideas of the past 100 years has been that animals with higher metabolic rates live shorter lives because they run out their body clock faster. But "it hasn't held up," Austad says. Parrot hearts can beat up to 600 times per minute, for example, but they outlive by decades many creatures with slower tickers. Other assumptions, for example that short-lived animals generate more tissue-damaging free radicals or have cells that stop dividing sooner, also lack strong evidence. "A lot of simple stories have vanished."
By the mid-1980s, Austad was observing opossum behavior in Venezuela as a postdoc when he began to notice how quickly the marsupials aged. "They'd go from being in great shape to having cataracts and muscle wasting in 3 months." Austad also noticed something even more intriguing: Opossums on a nearby island free from predators seemed to age slower - and live longer - than their mainland counterparts. The observation helped explain why Aristotle's key insight continues to hold true. Large animals tend to live longer, says Austad, because they face fewer dangers. It's not a simple question of survival, he says, but rather the result of millions of years of evolutionary pressure. Whales and elephants can afford to take their time growing because no one is going to attack them, he explains. And that means they can invest resources in robust bodies that will allow them to sire many rounds of offspring. Mice and other heavily preyed-on small animals, on the other hand, live life in fast-forward: They need to put their energy into growing and reproducing quickly, not into developing hardy immune systems.
When it comes to our pets, the bigger-is-better theory gets flipped on its ear. Cats live an average of 15 years, compared with about 12 years for dogs, despite generally being smaller. And small dogs can live twice as long as large ones. Yet the lesson of Austad's opossums may still apply. Gray wolves, the ancestors of dogs, live a maximum of 11 or 12 years in the wild, whereas wildcats can live up to 16 years. This suggests that the two species face different evolutionary pressures, Austad says. Wolves are more social than cats and thus more likely to spread infectious disease, he says; wildcats, on the other hand, keep to themselves, reducing the spread of disease, and are adept at defending against predators. "Cats are so incredibly well-armed, they're like porcupines" - an animal that notably also has a long life span for its size, more than 20 years. Indeed, two other small animals that are good at avoiding danger, naked mole rats and bats, can live 30 and 40 years, respectively. (Mole rats spend most of their time underground, whereas bats can simply fly away.)
Despite the differences between cats and dogs, both pets are living longer than ever before. Dog life expectancy has doubled in the past 4 decades, and housecats now live twice as long as their feral counterparts. The reasons can largely be chalked up to better health care and better diet. Americans will spend $60 billion on their pets this year, with a large chunk of that going to humanlike health care (think annual physicals and open-heart surgery) and premium food. "The same things that allow us to live longer also apply to our pets."