The idea of cannibalism tends to make people squeamish. The immediate reaction is usually, “I couldn’t do that.” But why not?

Scientists have puzzled over that question for a long time. It turns out that there are biological reasons why eating our own kind is taboo, not only among humans, but among most living creatures throughout the world.

David Pfennig, professor of behavioral ecology and evolutionary biology, uses cannibalistic animals to study kin recognition and disease transmission.

In his study of cannibalism, he found that the ability of an individual to recognize another as kin is important.

“There has to be a way for an organism to recognize relatives,” Pfennig says. “Biologists have now found kin recognition in a wide array of organisms. For instance, we see that primates can recognize their kin even in situations where these related organisms have never met.”

Pfennig explains that, in most cases, the sense of smell allows creatures to tell siblings from non-siblings, parents from strangers. “There have been interesting studies showing that in humans, if you give a small child a soiled garment of a newborn brother or sister, they can recognize the newborn as family,” Pfennig says. “They can literally smell it.”

Kin recognition can be learned in different ways. Animals can learn what they smell like from themselves, from their nest mates, or even from their environment. Later in life, if they smell those same smells again on another individual, then they will treat that individual as kin.

Even so, what can be gained by recognizing others of the same ilk and by helping those most like you survive?

An ant colony gives one answer. Worker ants do not get a chance to mate and pass along their genes to the next population. Why do these workers promote the health of the colony when they do not receive the one benefit that biology demands-ensuring their genes live on in the next generation?

In a way, they do receive that benefit because the worker is genetically related to the queen-she’s her daughter. If she cannot pass along her own genealogy, then the next best thing is to help the queen reproduce again and again. The worker passes along her genetic makeup through the birth of others like herself.

“If we think what’s really important in evolution is the ability to perpetuate your genes, then by helping relatives reproduce, we have an alternate way of aiding the survival of our genes,” Pfennig says. “If you reproduce, your kids would share half of their genes in common with you, so you’ve transmitted half of your genes into the next generation. But you could do the exact same thing in terms of number of genes by helping a brother or sister reproduce. As far as passing on your genes into the next generation, the two are equivalent.”

Recognizing kin and helping them survive strengthens the family’s genetic position in the population. But there is another reason why kin recognition, and the avoidance of cannibalism, is important.

“I started wondering why cannibalism is so rare in nature because it’s a great way of simultaneously getting a meal and getting rid of the competition,” Pfennig says. “Well, it turns out that cannibalism is also a great way to get diseases.”

A cannibalistic organism runs the risk of contracting lethal pathogens from the meal it just swallowed. “If a pathogen is able to infect another person, that means it is basically able to unlock the human immune system, and it’s possible that it is going to also infect me,” Pfennig says. “In general, I think we can predict that animals should avoid eating things that are very similar to them.”

And mostly, they do. Pfennig and fellow researchers conducted an experiment in which they fed cannibalistic tiger salamander larvae either diseased members of their own species or diseased members of a closely related species, the small-mouthed salamander. As a control group, they fed other larvae either healthy tiger salamanders or healthy small-mouth salamanders. The cannibals that ate the diseased members of their own species were less likely to survive to metamorphosis and grew significantly less than those that ate the diseased small-mouthed salamanders. Also, when given the choice between species members and non-species members, both healthy, the larvae chose to eat the non-species.

The first known case of disease spreading by cannibalism in humans happened in the 1950s in the New Guinea highlands. Tribes of isolated natives were dying from Kuru, a disease with symptoms including loss of motor control and dementia. Infectious agents called prions, also responsible for Mad Cow disease, cause Kuru. Prions are believed to be an aberrant form of a protein, and unlike bacteria and viruses, contain no nucleic acids, making them pernicious and also a scientific surprise.

When scientists descended on the highlands to search for the cause of Kuru, they found that tribe members ground up the brains of deceased relatives during a religious ceremony and ate them in a soup. Once scientists warned the natives against this practice, the disease dropped nearly out of sight.

Pfennig thinks the avoidance of cannibalism can be taken even further; he feels animals should refrain from eating any species closely related to them. Eating organisms that are genetically farthest from oneself provides the least disease risk. Following this, the best possible diet for disease avoidance in humans should be plants.

“You could say that I am making an argument for vegetarians, but that is not necessarily so,” Pfennig says. “There is another really important factor. If you were extremely hungry and I gave you a choice between a steak and a salad, which would you go for?”

Food isn’t only about disease risk; it is also about nutrition. And here lies the contradiction, because cannibalism provides the perfect nutrients.

“Eating your own species allows you to obtain the necessary nutrients for your own growth and maintenance, and they have already pre-packaged those materials for you in the proper proportions,” Pfennig says. “This is one of the paradoxes of why cannibalism isn’t more common.”

Pfennig says the optimal diet is one somewhere in the middle, close enough to one’s own species so that the individual garners sufficient nutrients, but far enough away so the disease risk is low.

“Experiments have been done that show that organisms don’t prefer to eat their own kind, they don’t prefer to eat things that are really different, but they prefer to eat things that are pretty intermediate,” Pfennig says. “The highest fitness return should be on the intermediate diet choice.”

Pfennig’s new research continues in the direction of diseases. He wants to investigate whether diseases can pass more easily through populations that are genetically similar to one another. As an example, Pfennig points to the decimation of New World populations, thought to be highly inbred and genetically homogenous, when faced with Old World diseases for the first time.

But he has answered his initial question about why most species tend to shy away from eating their own kind. Not only would it turn the stomach of most people to gnaw on a forearm, but it would be a health risk, too.

Brady Huggett was a student who formerly contributed to Endeavors.