Sport hunting hits evolutionary traits in bighorn sheep

December 10, 2003

Trophy hunting is driving down the horn size of big horn sheep--an evolutionary response caused by killing the largest rams before they reach their breeding peak, according to research produced in part at the University of Alberta.

By shooting the most desirable rams--those with large horns and high body weight--before they are able to achieve high rates of paternity, hunters have depleted genes for big horns and fast growth for the population. Scientists have noticed similar results with fish, but this research marks the first time investigators have discovered these findings in bighorn sheep. The research appears in this week's edition of the prestigious scientific journal, "Nature."

"Because you don't have the best rams mating, they aren't reproducing and the population isn't seeing the best genetic variability," said Dr. Curtis Strobek from the Faculty of Science at the University of Alberta and one of the authors on the paper. The lead author, Dr. David Coltman, did much of the DNA fingerprinting as a post-doctoral fellow in Strobek's lab. Currently at the University of Sheffield, Coltman will be returning to the U of A in June as a researcher.

Coltman's research team studied 30 years of data from a harvest bighorn sheep population at Ram Mountain, Alberta. Fifty-seven rams were shot since 1975, most before reaching eight years of age and nine were shot as early as the age of four. Mating success increases in bighorn sheep by six years of age when rams become capable of holding their own against females during the rut. The scientists used quantitative genetic analyses to explore the evolutionary response to hunter selection on ram weight and horn size.

"Unrestricted harvesting of trophy rams has contributed to a decline in the very traits that determine trophy quality," wrote the researchers. Wildlife managers should look at alternative strategies "to minimize further deterioration of the genetic quality of bighorn sheep."

A world-class trophy ram is a valuable commodity and hunting permits have been auctioned for hundreds of thousands dollars. One sport hunter paid more than $1 million in 1998 and 1999 for special permits to hunt trophy rams in Alberta. In many parts of North America, sport harvest of mountain sheep is often restricted only by the availability of rams whose horns reach a minimum size prescribed by regulations. Although the use of income generated from sport hunting goes towards enhancing and conserving mountain ungulate habitat can be seen in a positive light, so far little attention has been paid to the potential evolutionary consequences. A similar effect may be impacting African elephants where tuskless males are becoming more common.
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University of Alberta

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