Biology Magazine

Did Body Odor Evolve to Scare of Predators?

Posted on the 09 October 2018 by Reprieve @EvoAnth

Hopefully, you don't notice the body odor of most people you meet. However, that's more because our noses are rubbish than people smelling nice. We're actually quite pungent.

Did body odor evolve to scare of predators?

Although this reaction can be cute in cats, it might save your life when dealing with larger animals. The famous palaeoanthropologist Louis Leakey learnt this hard way when a lion broke into his tent. It gave him a good sniff before leaving, leading Leaky to conclude:

I seriously believe that one of things which protected many early primates, including
early man, in the defenseless days before he had weapons or tools, and when he was living on the ground, was that he was unpalatable to the carnivores

Now, the idea our smell evolved to scare off predators was based on an anecdotal observation from more than 50 years ago. Since then, have we found any better evidence?

Tigers still eat people

Paul Weldon has taken it upon himself to re-examine Leaky's "smelly people" hypothesis (although he gives it a more scientific name).

On the surface, this idea doesn't seem to have much support. At least 800 people a year were killed by tigers during the 20th century. In previous decades it was even higher. So much for us smelling offputting.

At the same time, many species - even large predators - take steps to avoid encountering humans. As such, our scent alone can be enough to scare away bears and lions. But this may just be because they've learnt to fear us and our weapons.

Thus, evidence from the wild seems to be circumstantial. We need a more controlled experiment to see how predators deal with human body odour. Sadly, it's quite hard to get ethical approval for testing "will a lion eat this dude".

That said, Weldon does recount one chap in 1919 who had designs on such a test, who reports

I was much disappointed at not securing a lion or a leopard for my trials [for the eating] of t he larger mammals, particularly of man

I am rather concerned that he had more trouble finding a lion than a human to feed them.

Flying around body odor

Fortunately, Weldon did manage to find experiments that actually went ahead. And didn't evolve people being eaten.

In one, people approached African elephants wearing various clothes. They found that the elephants were averse to clothes laced with the scent of the Maasai people, who often hunt the elephants. However, they still fled from people wearing cleaner clothes. So it seems the elephants do have some aversion to human scents, but some of this is learnt.

A similar test happened with deer in Connecticut, but with hair rather than clothes. People there wanted to protect growing trees from being grazed upon, so tied bits of human hair to them (in keeping with the grizzly theme, Weldon doesn't report where they got the hair from). Sure enough, deer avoided the trees that smelt of people.

Of course, people also hunt these deer. So like with the elephants, learning that human body odor is a bad smell can't be ruled out.

Perhaps the best evidence for our smell having a natural repellent effect doesn't come from animals, but bugs. Many insects avoid human scents unless very hungry. Now, flies aren't famous for their learning ability so this does suggest an innate drive to avoid our smell.

Square one

So, it seems we're back to square one. The evidence that our body odor evolved to scare away other animals is still lacking. However, the idea does have some support. So it isn't implausible as it might first seem, given how many people fall victim to predators despite their scent.

Maybe someone needs to time travel to 1919 and give that fellow a lion.

Weldon, P.J., 2018. Are we chemically aposematic? Revisiting LSB Leakey's hypothesis on human body odour. Biological Journal of the Linnean Society, 125(2), pp.221-228.

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