“It's a lifeboat strategy,” Bill Schutt, a zoologist at the American Museum of Natural History, offered by way of explanation. “That way at least one of the chicks survives.”

This is the kind of knowledge you pick up when you're writing a book on cannibalism, as Schutt has done. For his new work, “Cannibalism: A Perfectly Natural History,” he scoured several continents and every branch of the tree of life for stories about siblings eating siblings, strangers eating strangers, parents eating children, children eating parents, mates eating mates and more.

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There are quite a lot of them. Although cannibalism is often depicted as aberrant behavior, Schutt found that it occurs all the time in the natural world. We humans have also long made a habit of consuming our own — Renaissance Europeans drank human blood as medicine; Pacific Islander communities ate the flesh of deceased relatives as a gesture of grief. Schutt even tried a taste of a human organ (don't worry, no people were harmed in the making of this book).

To learn more about what makes cannibalism “perfectly natural,” Speaking of Science gave Schutt a call. Our conversation has been edited for length and clarity.

Speaking of Science: Your background is in bat research. What got you interested in cannibalism?

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Bill Schutt: For my PhD, I studied vampire bats and my first nonfiction book was “Dark Banquet: Blood and the Curious Lives of Blood Feeding Creatures,” where I went in and sort of demystified the whole idea of vampirism. And as a follow-up book, I thought of going in and finding something that is sensationalized, that a lot of people think negative thoughts about. It would be interesting to be able to go in there and put a different spin on it from the perspective of a zoologist. And I’ve always been into strange animals and macabre behavior, so cannibalism seemed to be a very natural follow-up to blood feeding.

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SOS: I'll say. Part of your argument is that cannibalism is more natural than scientists once thought — it doesn't only occur when an animal is under extreme stress. What are some of the ways that you found cannibalism can occur?

BS: Well, there are the ones that are sort of obvious, like there’s not enough food, so there’s no alternative source of nutrition. That makes sense, and that’s the one that people know about.

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But in the animal kingdom, you’ve also got parental care, which to me was a big surprise. For example, trophic [unfertilized] eggs are laid by spiders or snails or a lot of different invertebrates. These eggs are never meant to hatch; they’re in a sense little kids meals that are readily available once your babies come along. So the spiderlings will consume those. And if they run out, [the mother] will just call the spiderlings to her and squat down — if spiders squat down — and the babies climb all over her and consume her.

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Then there’s parental care associated with things like these weird amphibians called caecilians, where some of them hatch from eggs. In those instances, the hatchlings peel their mother’s skin like a grape, and they eat that for a week or so.

And then you’ve got another group of caecilians that are live born, and scientists did some dissections and found out that the newborns have got this weird little tooth that drops off when they’re born. And the scientists wondered what is this for? So they examined the inside of the pregnant females and found that the lining of the oviduct had been eaten away. The young are using the tooth to scrape their mother's insides. This, to me, is not only an extreme form of parental care, but incredibly interesting.

There’s the lifeboat strategy, where nesting birds have asynchronous laying of eggs. So there’s an older bird and a younger bird that gets picked on, and if there’s not enough food, then not only will that bird get picked on, it will be consumed.

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And there’s a reproductive strategy in big cats, for example. Let's say a lion takes over a pride, and there’s a female with a cub. Rather than wait several years for that female to come into estrus again, the male will kill and eat the cub, and then the female will be receptive to his advances much more quickly.

It’s also used as a way to sort of hedge against unpredictable environmental conditions. I fund this incredibly cool work being done by these folks out in Arizona on Spadefoot toad larvae. Their eggs are laid in a, you know they call them ponds, but they’re really glorified puddles, and with those dry conditions those bodies of water could evaporate overnight. So it pays to get out of the pool as quickly as possible. Because if the pond dries out before these are toadlets, then everyone dies.

So what has evolved in this system is that some of the tadpoles just overnight explode in size. They have huge teeth with really heavy duty jaw musculature, shortened digestive tracts now adapted for meat consumption. And they will eat their little omnivorous brethren and develop quicker and get out of the pond faster.

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SOS: Our society has a taboo against cannibalism passed down from the time of the Greeks and Romans. But you found that consuming humans was more widespread than culture might have us believe.

BS: Yeah, when you think about human cannibalism the first thing that jumps to mind would be criminal cannibalism. Jeffrey Dahmer and all these other horror shows. Or the Donner Party. Strandings. The guys that got stuck in the Andes, the rugby team. Those are the things that immediately come to mind.

But what I found was that there’s ritual cannibalism all over the world. For example, funerary practices. There were many cultures that didn’t come under the influence of the ingrained Western taboo, that weren’t taught that cannibalism was the worst thing you could do to another person. And there you find these funerary practices where the dead are consumed, or you find instances where body parts are used medicinally. Or, sort of on the other side, there are instances where cannibalism is used to terrify your enemies.

But there’s more to cannibalism than just “we ran out of food” or “this is a crazy person,” and I tried to emphasize that.

The most surprising thing that I found in writing this book was that given the Western cannibalism taboo, which is deeply ingrained, that for hundreds and hundreds of years Europeans practiced medicinal cannibalism on a scale that was completely surprising.

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Body parts of every kind were used to treat all sorts of different maladies. From drinking blood to treat epilepsy to consuming human fat to treat burns and skin problems. People ground up powdered mummies and consumed them thinking there was a medicinal benefit. I thought that was great because it was evidently a mistranslation of an Arabic word “mummia,” which to them meant a tarry bitumen that was used to bind up wounds. … But when Europeans saw “mummia,” they thought that these guys were consuming actual mummies, and so there was a run on mummies which were suddenly in short supply.

SOS: You were invited by a woman who runs a placenta encapsulation company to taste a placenta. [Note, there's no scientific evidence for the purported health benefits of placenta-eating.]

BS: Mmmhmm.

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SOS: So — how was it?

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BS: It tasted fine. It was prepared by a chef. He cooked it osso bucco so he knew what he was doing, and he cooked it with veggies. You know he assured me all the veggies are organic. And I was like, “Thank God for that, you know, I wouldn’t want to eat human placenta with nonorganic vegetables.”

But he poured some really nice wine into it, and it smelled great, and it tasted fine. I’m sure anybody else who tasted it would have said the same thing.

SOS: And you consider that cannibalism?

BS: There is a large fetal component to [the placenta]. It’s not all maternal tissue. So therefore, in a sense, you are eating a part of another person, and it’s flesh, and in my book that translates to cannibalism.

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You know with this term, there are gray areas. For example, if you eat your fingernails, is that auto-cannibalism? I could get more gross than that but you know what I’m talking about. It's not black and white.

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For the purposes of my book, I think that it's consuming any part of an individual of the same species, whether you kill that individual or whether that individual is dead and you come upon him scavenging, or something else. As long as it's the same species, I would consider that to be cannibalism.

CANNIBALISM: A PERFECTLY NATURAL HISTORY

By Bill Schutt

Algonquin Books. 332 pp. $26.95