Small dogs generally live longer than big dogs—so that yappy Yorkshire terrier next door could be around for a long time. But body size isn't the only factor that determines how long dogs survive. Personality influences life span, too, according to a new study that might help explain how animal dispositions evolve.
Research on animals from ants to apes has found that different individuals have different personalities. Some are timid, others aggressive. Biologists have proposed that temperaments evolved along with life history. Bold, aggressive animals use a lot of energy fast in relatively short lives, the thinking goes, whereas calmer animals last longer, saving themselves to reproduce later in life.
But it's hard to run evolutionary experiments on these traits in anything longer lived than a fruit fly. So evolutionary biologist Vincent Careau of Université de Sherbrooke in Canada came up with an idea: dogs. "All these breed differences reflect an experiment on artificial selection," says Careau. The huge diversity of dogs resulted not from natural selection but from generations of humans crossbreeding and selecting animals with traits they wanted—the ability to chase foxes into holes, or herd sheep, or sit attractively on a sofa.
Careau collected data from previous studies on various breeds' energy expenditure and longevity. After factoring out body size, Careau tested for correlations with activity, obedience, and aggressiveness.
More obedient dogs like German shepherds and bichon frisés live long for their size, he reveals in the June issue of The American Naturalist. Hard-to-train dogs like beagles and Pomeranians generally die earlier than do other, similarly sized breeds. Careau found a similar relationship for energy expenditure. Peaceful dogs like Newfoundlands and Labradors tend to burn less energy per kilogram than do aggressive dogs like fox terriers and Great Danes. Presumably, the people who created these breeds were selecting dogs based on personality, not on how much they ate or how long they lived, says Careau. So he thinks that personality and metabolic demands are somehow genetically linked.
"It's an intriguing finding," says theoretical biologist Franjo Weissing of the University of Groningen in the Netherlands. "It's a little bit related to this idea 'live fast, die young.' " Weissing co-authored an influential paper in 2007 on how tradeoffs in life history, between “living speed” and longevity, can lead to the evolution of animal personalities. "The nice thing is, what we see here comes really close to what natural selection models do predict."
But evolutionary physiologist Joseph Williams of Ohio State University in Columbus isn't convinced that what happened in dogs has anything to do with evolution in nature. "For me, the jury is still out. Dogs are contrary to what you would expect in nature in terms of longevity," he says. Elephants live for decades, whereas a mouse might make it through only three seasons. On the other hand, a Chihuahua will generally outlast a Saint Bernard.