Heel, roll
over, survive. German shepherds are easier to train
than beagles—and for their size, they live longer,
too.
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.