I'm not sure if this has been discussed before but I thought it was interesting.
Nature 426, 595 (11 December 2003) | doi:10.1038/426595a
Sheep horns downsized by hunters' taste for trophies
John Whitfield, London
The horns of some bighorn sheep are getting smaller, because hunters are picking off the most impressive rams before they reach their breeding peak.
A study of one sheep population in Canada shows that hunting can harm the gene pool of a species over just a few years. That means there should be tougher restrictions on what animals can be taken, says David Coltman of the University of Sheffield, UK. "For selection to be having this effect is of fundamental importance," he says.
Biologists have long suspected that hunting can affect animal evolution. Elephant poaching, for example, is thought to have led to an increase in the number of tuskless animals in Africa. And in Canada, the hunting of moose seems to have resulted in animals with smaller antlers.
To pin the relationship down, Coltman and his colleagues studied the sheep of Ram Mountain, Alberta. This Canadian province is home to the world's biggest bighorn sheep, and is a magnet to hunters. Since 1975, 57 of Ram Mountain's rams have been shot — about 10% of all males in the population each year from 1975 to 1996. In 1996, the government restricted hunting to males with a large, 'full curl' of horns, which has reduced the cull to zero in recent years. Coltman looked at rams from 1971 to 2002, and found that horn size fell by about a quarter over this period (see page 655). Despite the recent drop-off in hunting, horn size has not recovered.
Large horns are generally correlated with large, healthy rams, says Coltman, so the effect on the population's genetics is probably deeper than the effect on horns alone. He suspects that hunting is also influencing mating behaviour, with fewer rams butting heads to fight for partners.
One reason for the change is that hunters prefer rams with large horns, as they make for more impressive trophies. But it could also be an accidental side effect of some hunting regulations. Restricting hunting to males with large horns is meant to limit the killing of animals that are not old enough to breed, but it also encourages the culling of animals that grow large horns early in life. "You force every hunter to harvest the very animal that you're trying to grow," says Kevin Hurley, a wildlife biologist with the Wyoming Fish and Game Department and executive director of the Northern Wild Sheep and Goat Council.
A better strategy may be to limit the number of hunting permits, argue both Hurley and Coltman. In the western United States, it is common to offer a limited number of permits for bighorn sheep by lottery or auction — the right to shoot a single ram frequently fetches as much as $100,000.
Hunters are generally sympathetic to the need for management, says Kelly Semple, executive director of Hunting for Tomorrow, a coalition of hunting groups based in Edmonton, Alberta, as hunters do not want to drive large-horned animals into extinction. But she warns against generalizing Coltman's results to all species and locations.