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Hunting pressure not causing smaller bighorn sheep

By: Bob Weber, The Canadian Press

 | Aug 11, 2014 - 3:10 PM |
EDMONTON - A new study indicates trophy hunters aren't causing Alberta's bighorn sheep to evolve into something smaller and less impressive.

Previous studies have found that the average horn size of a six-year-old bighorn has decreased by about three centimetres over the last 30 years.

Scientists have suggested that's because hunting pressure is causing a kind of reverse natural selection — as hunters continually take out the biggest males in a herd, smaller animals have more chances to reproduce and pass along their DNA.

But new research in the Proceedings of the National Academy of Sciences has concluded that inheritance isn't a big deal when it comes to size.

"Body mass is only weakly inherited," said Lochran Traill, a biologist at Imperial College in London.

"Thereby, any evolutionary response of the targeted trait to hunting is negligible."

Traill and his colleagues used a combination of field data and mathematical modelling to tease out how different factors influence the overall state of the iconic alpine ungulate, Alberta's provincial animal.

It turns out shrinking horn sizes from the bighorn hunt aren't due to any unseen biological effect. Animals with big horns are rarer simply because hunters shoot lots of them.

"Any shift in the distribution of body mass among males is demographic," Traill said. "Changes are largely driven by the removal of larger animals

"There are simply fewer males."

That means that the number of big males with fully curled horns could easily rebound if they weren't targeted as much.

"Any decreased frequency of hunting will simply mean that fewer large males are being shot, and thus the likelihood of large males being found in the population may increase," said Traill.

He pointed out that letting big males breed is also good for the overall health of a herd.

"It will always be better to let males in their prime breed," said Traill, who added it remains "prudent practice" to target older males "past their mating prime."

Alberta Environment is examining its hunting regulations around bighorns to try to determine if adjustments need to be made to improve stocks.

"We are in the process of completing a new management plan for bighorn sheep," said spokeswoman Carrie Sancartier.

"Once that is completed there is expected to be a public discussion that will include harvest."

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