Does Hunting Make Animals Evolve Smaller Antlers and Horns?

Beginning in the early 2000s, scientific research and popular articles began suggesting the idea that hunting efforts that select for a specific trait (likes antlers or horns) can cause an evolutionary change in populations. Simply put, by selectively removing males that have the biggest and most impressive racks, hunters may be inadvertently and negatively changing the size of antlers and horns in hunted populations through time. But, is this actually happening in wild populations? And, if there are changes, are they always a result of hunter selection and changing genetics?

Back in 2015 when I first started graduate school, I began to dive a little bit deeper into these questions. But to really understand how hunting might be affecting the size of antlers and horns through time, I first had to take a step back and consider all the factors that go into creating a buck with huge antlers, or a ram with an impressive full curl. For ungulate species, there are three primary factors that determine how large a male can grow—age, nutrition, and genetics.

First, age has an important, and often easy to identify, influence on the size of antlers and horns. Typically, older animals have larger headgear. A yearling or 2-year-old mule deer is never going to hold the same rack on his head that you’d see on a mature 6- or 7-year-old buck. Age plays an even bigger role for horned species like bighorn sheep or mountain goats, who do not shed and regrow their horns each year like the antlers on deer, elk, or moose. More years simply allow more time to grow.

Second, nutrition plays an important, but often underappreciated, role in determining the size of antlers and horns. And, it’s not just the food on the ground in a given year that influences how big a male can grow—nutrition during early life and even when an animal is still developing in the womb can have lifetime effects. Research on whitetail deer has shown that the nutritional condition of a doe while she is pregnant directly influences the size of her son’s antlers for his whole life.

Finally, genetics are certainly necessary for animals to reach impressive size—it has to run in the family. Those genes are passed from parents to offspring, allowing a buck with huge antlers to likely sire sons with the ability to grow equally impressive antlers.

Age, nutrition, and genetics all work in tandem to determine the size of a male’s antlers or horns in a given year. If any of those three aspects is deficient for an individual, you won’t see large antlers or horns on him. An animal could have the genetics to grow into the next world record buck, but he can’t reach that point unless he is mature and has had consistent access to high quality food (as well as a mom that was relatively fat when she was pregnant with him).

The concern surrounding hunters removing males with impressive sized antlers and horns and the effect that can have on the genetic makeup of the population is not entirely unfounded—the harvest of a trophy male does indeed remove that individual and, consequently, his genes from a population (but, his genetics likely still exist in the population if he has sired any offspring). For evolutionary changes to occur on a specific trait (like antlers and horns), there must be a strong selection on that trait combined with frequent removal of animals that possess that trait. We see this often in fish populations. But, for ungulate species, reaching a harvest pressure that is highly selective and highly intensive enough to cause changes to the size of antlers and horns in wild populations can be difficult.

Selection for large antlers and horns can only occur on males. Yet, an individual’s genetics are passed down from both its mother and father. Females aren’t harvested based on the sons they produce, and thus, a doe that produces trophy bucks is not any more likely to be harvested than a doe who produces small bucks. So, even if selection for males with impressive racks is high, that selective pressure simply cannot exist on the females in a population because they don’t outwardly express that trait. When only one sex possesses the trait that is being selected for, it lowers the selective pressure on that trait by a considerable amount.

Further, while there are many hunters who do value the size of what’s on a male’s head, for many others, size isn’t quite as important. For many folks, filling the freezer with a small buck often holds much greater value than chasing after a monster for the entire season, only to end up with tag soup.

Finally, there are natural selective pressures that act in the opposite direction of hunting, and favor the persistence of males with the largest antlers and horns in a population. You may have heard the unmistakable crashing of two rams or the rattle of antlers as two bucks spar—more often than not the bigger male comes out on top. Antlers and horns are indicators of individual genetic quality, and higher quality individuals (animals with bigger antlers or horns) are often the ones who are able to breed females and pass on their genes to future generations. That’s a damn good reason to grow bigger antlers and horns.

Still, sometimes we do see changes to the size of horns and antlers of harvested males over time. But those changes can’t always be attributed to changing genetics—especially if other factors haven’t been rigorously investigated.

If declines in antler size are accompanied by declines in the average age of harvested bucks, then changes might simply be related to a shift in the age structure of the population. If the average buck shot in 2000 was 5 years old, and the average buck shot this year was 3, then you will undoubtedly see what might seem like alarming declines in the size of antlers. But that 3-year-old that you shot this year could very well be similar to the size of a 3-year-old in 2000.

Alternatively, nutrition and population size can have a very important role on determining how big males can get. If a population has reached or exceeded the number of animals that the landscape can support, competition for food becomes high. When animals are working to secure enough food for their own survival, they often invest what they are able to eat into maintaining fat or muscle (which is critical to their survival over winter) and invest less into growing antlers and horns. And, if population sizes have grown substantially, you might very well see declines in the size of antlers or horns over time. Those declines, however, have nothing to do with genetics and everything to do with nutrition. In an instance like this, hunting (and harvesting females in particular) actually can help to reduce population numbers and competition between individuals for food, which could help boost nutrition, and subsequently, the antler or horn size of animals in that population.

Even though there are many factors besides hunting that can explain changes to antler and horn size, and there are many factors that buffer against genetic changes due to hunting, there is still strong evidence that hunting has caused genetic changes in some populations. But populations where these genetic changes are occurring often have management practices that promote highly selective and intensive hunting.

During my first few years at grad school, I worked with a team of researchers, scientists, and managers from across North America to really understand how hunting affects horn size in bighorn sheep. In North America, there are a handful of bighorn sheep populations where you can see genetic declines in horn size through time—but those changes are likely an artifact of high hunting pressure combined with a size requirement for harvest. When animals are required to reach a minimum size before they are legal (e.g., a full curl), it forces high selection on animals. That selective pressure, combined with high harvest rates, has been shown to result in genetic changes to the size of horns through time.

Think about, for example, a ram that can grow big horns quickly—he could reach legal status by the time he turns 5. When harvest pressure is high and there are many hunters on the ground searching for legal animals, that ram has a very low probability of surviving the hunting season. Alternatively, a ram that grows slowly and doesn’t reach legal status until he is 7 or 8 is going to be able to survive many more hunting seasons than the fast-growing 5-year old. Thus, that slow-growing ram is able to breed females and pass his genes on to the younger generation. Over time, rams that have slow-growing or smaller horns might become more common in the population, and genetic changes then might be detected.

In our study, we didn’t find declines in horn size for most populations of bighorn sheep in North America, but we did detect it in areas with highly selective and intensive harvest practices. It’s likely that we did not detect changes in most populations because harvest is almost never as intensive or selective as it is in those few populations where changes have been detected previously. For most big game populations in North America, harvest simply does not occur at the level of selection or intensity that would be necessary to result in genetic changes through time.

So, next time you hear someone say that hunting big males is causing declines in the size of antlers or horns of the population, you should think carefully about all the other factors that play a role in determining how big those males are actually getting. You might want to tell them about the positive effects hunting can have on population health.

Feature image via John Hafner.