Selective hunting is the purposeful hunting of specific individuals of a species. In trophy hunting, those with the most impressive ornaments, such as large horns or antlers (known as “weapons”), are harvested. Individuals who possess specific characteristics are removed from the population and they cannot pass the genes for them onto the next generation.
This leads to the question of whether hunting has an effect on the evolution of traits in a species.
Selective hunting is more likely to cause evolution than hunting for meat or recreation due to hunters preferring certain traits over others- the gene pool is being directionally altered, potentially resulting in evolution. In a recently published paper by Festa-Bianchet & Mysterud, the circumstances under which evolution can result from hunting are explored.
In species such as bighorn sheep or red deer, where weapon size is related to reproductive success, large weapons are grown over many years and play a direct role in winning interactions which allow them to mate. Systems in which dominant males mate with many females (polygynous systems) have a higher chance of resulting in evolution because of the large genetic contribution these males make to the next generation. The removal of individuals with the largest weapons means that the mating success of the survivors is redistributed, and the next generation are likely to on average have smaller weapons. Hunter-induced evolution of smaller weapons is more likely in these species than whose weapons grow rapidly and don’t play a role in reproductive success, such as roe deer and mountain goats.
The selection pressure caused by hunting must be greater than the sexual selection that favours large weapons, as well as being consistent for many generations over a large enough area, in order for weapon size to evolve in this way. It is also worth noting that the strength of selection is half that of what it would be if both males and females were selectively hunted. Of course, evolution depends on whether those harvested have already produced offspring when they are killed, as they may have already left their genes for large weapons to the next generation. Also, if individuals with the desired characteristics immigrate from the protected areas into the hunted populations and breed successfully, evolution will be less likely to occur.
The circumstances that lead to evolution are specific and are more complex than explained above. It has been suggested that the effect hunting is having on evolution is being misrepresented by the media– articles are reporting strong correlations and predicting a dire future for many animals due to hunting, without having the data to support it.
So what needs to be taken into account when studying this link?
To conclude hunting is causing evolution, any changes observed need to be based in genetics and should therefore be heritable; studies need to demonstrate that they are not just plastic responses to environmental change (phenotypic change without the alteration of genes). For example, in hunted populations of chamois (a species of goat-antelope), a decrease in both horn length and mass has been observed, however this is most likely linked to the effects of climate change. Another example it that of the ibex, whose horns seem to grow faster in warmer climates. Many local variables can affect weapon size, such as climate as population density, as well as the age of an individual, so when making comparisons it important to take these into account as possible causes of variation.
The genetic architecture of horns and antlers is complex, meaning that a genetic response to selection is not expected unless the selective pressure is exceedingly strong.
The age of individuals at when they are harvested needs to be recorded in order for any conclusions to be valid; intense selective harvesting of those with the largest weapons can reduce the average age over time, meaning that average weapon size in the population decreases, even when there is no evolutionary response in weapon growth. When age is not taken into account this gives the appearance of evolution when really it is due to the fact that younger individuals have smaller weapons. Ideally the age at harvest for all hunted animals should be recorded, but it should especially be measured when the eldest individuals are selectively harvested, so that a temporal record of weapon size distribution can be created.
If age at harvest is decreasing but weapon size is constant over a suitable period of time, this suggests a increase in weapon growth rate, and, after ruling out phenotypic plasticity, implies evolution is taking place.
For some ungulates and carnivores, hunting accounts for more than 50% of adult male mortality. If hunting is causing evolution, it is useful for us to understand how, and suggest ways to monitor and control it. Some regulations can impose selection of their own, such as those that only allow individuals of a specific age or size to be hunted in an effort to maintain variation. It is not a good idea to ban hunting altogether- many wildlife conservation programs rely on sportsmen for funding. For example, a U.S. trophy hunter paying $110,000 to kill a rare mountain goat in Pakistan was legal through a program which allows a certain number of individuals to be hunted each year, with the revenue from this practice being used as an incentive to the local residents not to poach the species- as a result, its conservation status has been downgraded from “endangered” to “threatened”.
However, reports suggest that fewer people are hunting than ever before, and while this is beneficial when it comes to illegal hunting, it is actually harming some of these conservation efforts.
In their paper, Festa-Bianchet & Mysterud provide many examples of species where hunter-induced evolution could be possible, but conclude that there is insufficient evidence to prove it is happening. Long term studies are needed to answer this question properly, but that doesn’t mean that all this knowledge can’t be used to take precautions that help maintain the natural variation in hunted populations.