Shortly after gray wolves were reintroduced to Yellowstone National Park in 1995, Daniel MacNulty was puzzled by something. The breeding pair in one of the packs frequently stopped during their elk hunts to rest. "They sat on the sidelines while their offspring did the work," says MacNulty, an ecologist from Michigan Technological University in Houghton. "After their kids made the kill, they would amble up to feed."
Laziness? Not at all. The two were almost 5 years old, which MacNulty has learned is fairly old age for wolves. His new study is one of the first to look at the effects of aging in predators, and it raises questions about current methods of controlling wolf populations. "It's an exciting paper and should become a classic," says ecologist John Fryxell of the University of Guelph in Canada.
MacNulty has followed 94 radio-collared wolves in Yellowstone for 13 years, closely monitoring their hunts for two 30-day periods during each of those years. His research on these individual canids shows that wolves age rapidly. Indeed, by age 2 they're in their hunting prime, drawing on youthful endurance and sudden bursts of speed to take down elk. But just as quickly, they lose that talent, MacNulty's team reports online in Ecology Letters. "Wolves are old when they're 4," he says. The median life span for wolves in Yellowstone is 6 years, although some have lived as long as 10. Those older wolves manage to survive because the younger ones in their pack pick up the slack, killing elk and letting all the pack members feed. Older wolves are also heftier and may come in at the end of a hunt to use their weight to help pull down the elk, says MacNulty.
As one might expect, aging predators are good news for prey. The wolves' kill rate on elk in Yellowstone declined significantly as the number of geriatric hunters in the wolf population increased. And that could have cascading effects on the ecosystem. For instance, elk may linger and browse on woody plants when elderly wolves are around. More browsing could slow the recovery of willows and aspen trees, which have come back since the wolves' reintroduction.
Fryxell says ecologists are starting to realize that age needs to be included in models of predator-prey abundance. Game managers should pay attention as well. Most managers who want to boost numbers of elk and deer think all you need to do is kill wolves, says ecologist Christopher Wilmers of the University of California, Santa Cruz. "But this study shows you're probably increasing your problem, since you'll end up with younger wolves that kill more prey." That's because when a pack vanishes or is weakened and loses its territory, he says, younger wolves often move in. "You're better off leaving the wolves alone."