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

Trophy rams are heavy beasts with rapidly growing horns a valuable commodity to hunters. 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 are able to reproduce. The effect is like plucking the biggest fish from the sea.

Biology Prof. David Coltman of the University of Sheffield and his colleagues studied 30 years of genetic data from a bighorn sheep population in Alberta.

Hunters on Ram Mountain, 180 kilometres from Calgary, can legally shoot any ram that has reached a minimum size.

"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 problem arises since hunters often shot rams before they reached their prime breeding period, which starts at six years of age.

Coltman said by targeting a genetic trait such as large horns or elephant tusks, it becomes rarer in the population. The researchers found from 1971 to 2002, horn size fell by about 25 per cent.

He notes that in the case of rams, the response happened in 30 years, a short time in evolutionary terms.

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.

Nevertheless, the researchers suggest wildlife managers should look for alternatives to minimize further weakening of genetic quality in bighorn sheep, such as limiting the number of hunting permits.