Research: Bighorn Sheep

Harvest Records Project

Harvest Records Project


The Harvest Records Project is focused on identifying the effects of harvest on horn size of mountain sheep. Hunting is an important part of wildlife management, but there is concern over how harvest might affect the size of horns and antlers of ungulate species over time. This project evaluated how the size of horns of hunted bighorn sheep changed through times, and identified the mechanisms that caused change.

Our hypothesis:

If horn sizes are becoming smaller over time, we have a few ideas for why this is happening: 1.) Demographic Shifts (older rams with larger horns are being harvested more frequently, leaving younger, smaller horned rams as the main demographic remaining); or 2.) Selective harvesting (Since harvest tends to target rams with larger horns, older rams will start growing smaller horns to alleviate the pressures of harvest.) or environmental effects (climate change and habitat fragmentation decrease nutrition & eventually horn sizes).

harvest records project chart showing predictions

What's the issue?

Horns and antlers of ungulates play an important biological role and hold important sociological significance. From signaling strength to rivals and potential mates, to the allure they provide hunters seeking to hang impressive trophies on their living room walls, the size of horns and antlers are important. The interest in harvesting animals based on the size of horns and antlers recently has raised concern over the potential effects that hunting may have on the size of those structures over time.

Because of the biological and cultural importance of horns and antlers, if hunting is reducing their size through evolutionary change, wildlife agencies may need to adjust how they manage big game hunting. Bighorn sheep have been at the center of this controversy, in large part because scientists observed declines in horn size due to heavy harvest pressure in one population in Canada. That observation triggered the need to evaluate this question more broadly to determine whether management of big game is sustainable for maintaining animals with large horns and antlers.

How're we tackling it?

State and provincial wildlife management agencies have been recording age and horn measurements of all hunted bighorn sheep for decades. To investigate whether hunting is changing horn size, the researchers evaluated nearly 25,000 records of bighorn sheep harvested from 72 hunt areas throughout nine U.S. states and one Canadian province over the 35 years spanning 1981 to 2016.

The researchers first assessed whether horn size was changing through time in each hunt area. Where changes did occur, they tried to determine the cause for that change. Horn size of bighorn sheep is primarily shaped by three mechanisms: age, genetics, and nutrition. 

Age Mechanism – older sheep have bigger horns than younger sheep.

Genetics Mechanism – horn size is influence by genetics, and some animals have genes for bigger horns than other animals.

Nutrition Mechanism – sheep with access to plenty of good forage have bigger horns than those with limited resources.

To examine if hunting is affecting horn size through time, the researchers evaluated if populations were experiencing changes in horn size, and identifying which of these mechanisms affected that change. The researchers evaluated whether hunters were harvesting younger animals over time, whether the horn size of consistently aged sheep (7-year-olds) was declining over time, and whether vegetation and weather data indicated declining resources available to sheep over time.

What are our findings?

Among 72 hunt areas, horn size remained unchanged in 44, decreased in 19, and actually increased in 9. The researchers found evidence that all three mechanisms caused changes in horn size, depending on the hunt area in question. In fewer than half of hunt areas where horn size declined, did evidence point to hunting as the cause of evolutionary change. In the others, nutrition and changing age structure appeared to be driving down horn size. In all, current harvest regimes in most hunt areas do not appear to be reducing horn size in bighorn sheep.

People working on the project:

Tayler LaSharr, Ryan Long, James Heffelfinger, Vernon Bleich, Paul Krausman, R. Terry Bowyer, Justin Shannon, Robert Klaver, Clay Brewer, Mike Cox, A. Andrew Holland, Ane Hubbs, Chadwich Lehman, Jonathan Muir, Bruce Sterling, and Kevin Monteith

Funders

Utah Division of Wildlife Resources, the National Wild Sheep Foundation (WSF), Wyoming Wild Sheep Foundation, Alberta Wild Sheep Foundation, California Wild Sheep Foundation, Arizona Desert Bighorn Sheep Society, Wyoming Governor’s Big Game License Coalition, Iowa Foundation for North American Wild Sheep, Utah Foundation for North American Wild Sheep, and the Pope and Young Club.

Publications

LaSharr, T. N., R. A. Long, J. R. Heffelfinger, V. C. Bleich, P. R. Krausman, R. T. Bowyer, J. M. Shannon, R. W. Klaver, C. E. Brewer, M. Cox, A. A. Holland, A. Hubbs, C. P. Lehman, J. D. Muir, B. Sterling, and K. L. Monteith. 2019. Hunting mountain sheep: Do current harvest practices affect horn growth? Evolutionary Applications.

LaSharr, T. N., R. A. Long, J. R. Heffelfinger, V. C. Bleich, P. R. Krausman, R. T. Bowyer, J. M. Shannon, E. M. Rominger, C. P. Lehman, M. Cox, and K. L. Monteith. 2019. Biological relevance of antler, horn, and pronghorn size in records programsJournal of Mammalogy.

Find our other publications on bighorn sheep Bighorn sheep outline

Project updates

  • Project PDF download here