It’s Survival of the Weak and Scrawny
Researchers see 'evolution in reverse' as hunters kill off prized animals with the biggest antlers and pelts.
Some of the most iconic photographs of Teddy Roosevelt, one of the first conservationists in American politics, show the president posing companionably with the prizes of his trophy hunts. An elephant felled in Africa in 1909 points its tusks skyward; a Cape buffalo, crowned with horns in the shape of a handlebar mustache, slumps in a Kenyan swamp. In North America, he stalked deer, pronghorn antelope, bighorn sheep and elk, which he called "lordly game" for their majestic antlers. What's remarkable about these photographs is not that they depict a hunter who was also naturalist John Muir's staunchest political ally. It's that just 100 years after his expeditions, many of the kind of magnificent trophies he routinely captured are becoming rare.
Elk still range across parts of North America, but every hunting season brings a greater challenge to find the sought-after bull with a towering spread of antlers. Africa and Asia still have elephants, but Roosevelt would have regarded most of them as freaks, because they don't have tusks. Researchers describe what's happening as none other than the selection process that Darwin made famous: the fittest of a species survive to reproduce and pass along their traits to succeeding generations, while the traits of the unfit gradually disappear. Selective hunting—picking out individuals with the best horns or antlers, or the largest piece of hide—works in reverse: the evolutionary loser is not the small and defenseless, but the biggest and best-equipped to win mates or fend off attackers.
When hunting is severe enough to outstrip other threats to survival, the unsought, middling individuals make out better than the alpha animals, and the species changes. "Survival of the fittest" is still the rule, but the "fit" begin to look unlike what you might expect. And looks aren't the only things changing: behavior adapts too, from how hunted animals act to how they reproduce. There's nothing wrong with a species getting molded over time by new kinds of risk. But some experts believe problems arise when these changes make no evolutionary sense.
Ram Mountain in Alberta, Canada, is home to a population of bighorn sheep, whose most vulnerable individuals are males with thick, curving horns that give them a regal, Princess Leia look. In the course of 30 years of study, biologist Marco Festa-Bianchet of the University of Sherbrooke in Quebec found a roughly 25 percent decline in the size of these horns, and both male and female sheep getting smaller. There's no mystery on Ram Mountain: male sheep with big horns tend to be larger and produce larger offspring. During the fall rut, or breeding season, these alpha rams mate more than any other males, by winning fights or thwarting other males' access to their ewes. Their success, however, is contingent upon their surviving the two-month hunting season just before the rut, and in a strange way, they're competing against their horns. Around the age of 4, their horn size makes them legal game—several years before their reproductive peak. That means smaller-horned males get far more opportunity to mate.
Other species are shrinking, too. Australia's red kangaroo has become noticeably smaller as poachers target the largest animals for leather. The phenomenon has been most apparent in harvested fish: since fishing nets began capturing only fish of sufficient size in the 1980s, the Atlantic cod and salmon, several flounders and the northern pike have all propagated in miniature.
So what if fish or kangaroos are smaller? If being smaller is safer, this might be a successful adaptation for a hunted species. After all, " 'fitness' is relative and transitory," says Columbia University biologist Don Melnick, meaning that Darwinian natural selection has nothing to do with what's good or bad, or the way things should be. Tusks used to make elephants fitter, as a weapon or a tool in foraging—until ivory became a precious commodity and having tusks got you killed. Then tuskless elephants, products of a genetic fluke, became the more consistent breeders and grew from around 2 percent among African elephants to more than 38 percent in one Zambian population, and 98 percent in a South African one. In Asia, where female elephants don't have tusks to begin with, the proportion of tuskless elephants has more than doubled, to more than 90 percent in Sri Lanka. But there's a cost to not having tusks. Tusked elephants, like the old dominant males on Ram Mountain, were "genetically 'better' individuals," says Festa-Bianchet. "When you take them systematically out of the population for several years, you end up leaving essentially a bunch of losers doing the breeding."