Being the buffest guy on the block is often one of the surest ways to score a mate in the animal kingdom. But new research indicates that brawniness can have a serious downside. Bucks, bulls, and other large hoofed males have relatively small teeth, which wear out quickly and impair their ability to digest food.
Researchers have long puzzled over the shorter life span of larger male ungulates, or hoofed mammals. Two years ago, Juan Carranza, a behavioral ecologist at the University of Extremadura in Cáceres, Spain, and colleagues showed that molars in male red deer were relatively smaller than those in the more petite females, and that this difference was correlated with a shorter life. Researchers had assumed that the size difference was due to the fact that male and female teeth wore down at different rates, perhaps because of different eating habits between the sexes. But Carranza's work suggested that male teeth were never that big to begin with.
To confirm this hypothesis, Carranza and colleagues combed the literature for information on body size in ungulates and obtained measurements of the chewing surfaces of molars and premolars from museum specimens. They compared 123 species--almost half of the known ungulates--studying some in which males and females were the same size, such as roe deer, as well as species in which males were bigger than females, including Barbary sheep and the Nilgai, an antelope.
When males and females were about the same size, so were their teeth. But in species in which larger males evolved, tooth size increased relatively little. Thus, females ended up with larger chewing surfaces for their size than did males, the researchers report in the September issue of American Naturalist. The team concludes that teeth probably didn't grow at the same rate as body size because males can successfully compete for females only in their prime. Once teeth wear down, they become ineffective, and the animal gets weaker and more susceptible to disease or injury. But that doesn't matter to these males, as once they are too old to beat out rivals for mates, there's no need to live a long life. When it comes to how many offspring a male can father, "it seems that compared to body mass, tooth size is relatively unimportant," says Joanne Isaac, a mammalogist at James Cook University in Townsville, Australia, who was not part of the study team.
The large size of the survey "is simply impressive and exciting," says Atle Mysterud, a mammalogist at the University of Oslo in Norway. And Dan Nussey, a vertebrate evolutionary ecologist at the University of Cambridge, U.K., thinks the work makes an important contribution to understanding aging: "There are few comparative analyses of aging rates taking an ecological perspective in the way that Carranza's work does."