Natural History

"Der November" by Joachim von Sandrart (1643).

Matt Walker of the BBC writes about a very interesting paper in the journal Evolution that discusses why animals have not been able to evolve against human predators.  Predators and prey are normally locked in an evolutionary arms race in which predation places selection pressures upon prey to evolve defenses. The two most common ways that prey species evolve defenses against predation are to grow really big or produce toxins, but neither of these are particularly good defenses against human predators:

“The spread of modern humans represents one of the great ecological and evolutionary transformations in the history of life,” Prof [Geerat] Vermeij writes in Evolution.

We hunted and gathered on land, but soon began exploiting intertidal zones, taking shellfish and fish. Such intertidal zones were important food sources for prehistoric human populations living in places as far and wide as South America, South Africa, California and Oceania.

Then we started taking big animals. When we did the very adaptations that offered protection against natural predators attracted rather than deterred human hunters. The huge size of mammals such as bison or whales made them juicy targets for meat-hungry humans for example.

Other defensive ornaments became disadvantageous as humans evolved into super-predators. Elephants were killed for ivory, crabs and lobsters fished for their large meaty claws. These once advantageous traits became liabilities in the modern, human-dominated world.

We didn’t just take large species, we also preferentially harvested out the largest individuals of smaller species, a problem that persists today.

Prof Vermeij has examined the degree to which this happens.

He looked at one group of animals, marine molluscs and echinoderms such as starfish, and surveyed all the scientific research into how they have been exploited by humans. We select the largest individuals among 35 of 40 species studied, he discovered.

That means that size is no longer a refuge. Whereas growing big may have been one defence against natural predators, it offers no defence against human super-predators.

Sticking to rocks, as limpets do, is no good either as humans have invented picks and knives to prise them off.

Prey animals may do better to become toxic instead, and there is evidence that some marine species have become poisonous to people, either producing their own toxins, or by harnessing toxins produced by microbes. Reef fish and crabs are often toxic to people because they contain unpalatable, and sometimes lethal, dinoflagellates, for example.

But humans have found ways to get around this too. Many toxins need to be concentrated into organs such as the liver. And humans have learnt to remove these, to avoid their ill effects.

In short the way humans hunt appears to be the main factor preventing animals evolving adaptations to defend themselves from us.

Animals do respond to selective pressures, even over short time scales, and many species have responded to humans being super-predators, says Prof Vermeij.

By eliminating large apex predators, secondary predators have boomed. As cod numbers crashed in the 20th Century, their place was taken by an abundance of shrimp, lobster and crabs, which in turn feed on marine snails. As a result, these snails may have evolved thicker shells to protect themselves against these marauding shell-crunching crustaceans.

But we hunt on too grand a scale, with too much ingenuity, targeting the biggest animals.

“Our arrival and technological history has engendered an enormous change in the evolution of most species on Earth,” says Prof Vermeij.

In evolutionary terms, we leave our prey with nowhere to go. They have no way to defend themselves and simply cannot respond.

And that, says Prof Vermeij, represents a cataclysmic shift for species on this planet, the implications of which, he adds, we have barely begun to understand.

I  have long wondered why predation by humans is not considered more carefully in our discussions about evolution.

I have often thought that the reason why people have such a hard time taming wolves now and may not have had such problems in the past is that wolves experienced an unbelievable selection pressure in the form of human persecution.  When wolves became too wary to kill with weapons, we resorted to poisons and traps, and all of these pressures left behind only the most “paranoid” and reactive individuals to pass on their genes.   This might go a long way to explaining why it was so easy for ancient man to tame wolves, but it is now quite difficult for us to do so. And even if we wind up having a wolf that is imprinted upon humans, the chances of it being more like a friendly and docile dog are very low. However, there are and were wolves like this in modern times– Wags and Romeo are good examples.

I’ve also wondered about the behavior of white-tailed deer when they see me approach. I am from the only species that can kill a deer as soon as I get a good clear view of it. No other animal that can kill a deer can do that. All the rest– be they bobcats or coyotes– most put their teeth on the flesh. So when a deer sees me approach, it must know that the threat is that much more than if it sees a pack of coyotes approaching at the same distance.

I’ve also wondered about the way squirrels run away from people. They always try to stay on the opposite side of the trunk from where a person is standing.  Any other animal that would pursue a squirrel into the trees– like a fisher or marten (which used to be found here)–would follow the squirrel course almost exactly. It would have no reason to try keep itself on the other side of the trunk. Such behavior would have some advantage against raptors, but raptors would come down on the squirrel.  The threat from a human hunter would come from the other direction. Somewhere along the line, they evolved this strategy to avoid being shot.

The behavior of ruffed grouse has also changed dramatically as the result of human hunting.William Harnden Foster wrote the first important treatise on sport hunting ruffed grouse in New England, simply entitled New England Grouse Shooting.  Writing in the early twentieth century, Foster discusses the evolution of ruffed grouse behavior in response to rapacious market hunting. Market hunting was always a major threat to American wildlife, and although today we like to castigate Africans for their “bushmeat” trade, it was actually perfected in America. As Americans gained more wealth as a result of our industrial revolution, they came to want wild game meat.  Fine restaurants and markets in major cities offered the flesh of all sorts of wild animals, and many species, including white-tailed deer and Eastern wild turkeys nearly became extinct through this early American bushmeat trade. Grouse were always in demand for the table, and in the early days, they were easily killed. Ruffed grouse apparently originally lived in small flocks, and when a dog flushed them, the whole flock took refuge in a tree, where a hunter could easily pick them off with his fowling piece.  Over time, this selection pressure left behind only grouse that took off and flew greater distances from where they were flushed. And it also made the grouse much more solitary. Today, it is very rare to find more than one grouse in the same spot, unless it is a hen and her poults.

In no case has these animals evolved a very effective defense against human predators.  So long as we have guns, we can get them.

We’ve even been able to kill the largest whales very effectively. In the later days of whaling in Newfoundland and the Maritimes, the blue whale and the finback were rather easily subdued with harpoons that had extra blades that pojected out as soon as they main shaft lodged itself in the whale’s flesh.  These extra blades caused the whale to bleed out  and die more rapidly. All the whalers needed was a strong cord connected to the harpoon and a very stout boat to wait it out.

It is only now that we’re beginning to realize what human predation is actually doing to the evolution of animals and to the ecosystems in which they inhabit.

It is really humbling to realize that we could have this effect.

And it is also unusually disquieting– even a bit terrifying.