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.

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.

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.