Small dogs generally live longer than big

问题：

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 (36) how long dogs survive. Personality influences life span, too, according to a new study that might help explain how animal (37) evolve.
Research on animals from ants to apes has found that different (38) have different personalities. Some are timid, others (39) . Biologists have proposed that temperaments evolved along with life history. (40) , violent animals use a lot of energy fast in (41) short lives, the thinking goes, whereas calmer animals last longer, saving themselves to (42) later in life.
But it’s hard to run evolutionary (43) on these personalities in anything longer lived than a fruit fly. So evolutionary biologist Vincent Careau of Universit6 de Sherbrooke in Canada came up with an idea: dogs. "All these breed differences reflect an experiment on artificial selection," says Careau. (44) , but from generations of humans crossbreeding and selecting animals with traits---the ability to chase foxes into holes, or herd sheep, or sit attractively on a sofa.
But evolutionary physiologist Joseph Williams of Ohio State University in Columbus isn’t convinced that (45) . "For me, it still remains to be seen. Dogs are contrary to what you would expect in nature in terms of longevity," he says. Elephants live for decades, (46) . On the other hand, a Chihuahua will generally outlast a Saint Bernard.