Trophy hunters affecting size of sheep horns

Trophy hunters who target rams with the biggest horns are speeding up a process that ultimately reduces the size of the horns they cherish, researchers say.

Biology professor David Coltman said by targeting a genetic trait, such as large horns or elephant tusks, it becomes rarer in the population.

"Unrestricted harvesting of trophy rams has contributed to a decline in the very traits that determine trophy quality," Coltman's team wrote in Thursday's issue of the journal Nature.

The University of Sheffield researcher was working out the University of Alberta.

Trophy rams are heavy beasts with rapidly growing horns – such a valuable commodity to hunters that provincial hunting permits have been auctioned off for hundreds of thousands of dollars.

Hunters tend to shoot rams with the largest horns before the sheep reach their prime breeding period, which is typically six years of age. Hunters on Ram Mountain, 180 kilometres from Calgary, can legally shoot any ram that has reached a minimum size.

Wildlife biologist Bill Wishart says the study only applies to the small Ram Mountain herd of 50 animals isolated for Coltman's paper.

"It would take an awful lot of hunters and an awful lot of hunting, to do what they did at Ram Mountain," said Wishart. Other wildlife biologists say sheep habitat has a greater effect on horn size than hunters.

Coltman and his colleagues studied 30 years of genetic data from a bighorn sheep population in Alberta. He says they noticed a genetic response in those three decades, a short time in evolutionary terms.

The researchers suggest wildlife managers should look for alternatives to minimize further weakening of genetic quality in bighorn sheep.