Why we don't eat each other

Cannibalism is commonplace in nature; there aren't many animals that won't eat their own kind under the right conditions. But humans have a very good reason to stay hungry.

 Illustration by Steven Driessen

Illustration by Steven Driessen

Almost every animal species on the planet, from bees to lions to goldfish, will turn to cannibalism in one circumstance or another — and humans are no exception, a gruesome fact that has spelled the death of many an unwary pioneer. But what do all these disparate species have in common? Individuals consume the flesh of their own species for myriad reasons, but almost always driven by a singular goal: to survive and perpetuate their genes to the best of their abilities.

There are several distinct categories of cannibalism. Sexual cannibalism is performed almost exclusively among insects and arachnids, wherein the female will consume the male during or after insemination. In this scenario, the male’s body provides direct nutrients to the female, allowing the offspring inside her to benefit, as well.

Sarah Huber, a professor of animal behaviour at the College of William & Mary, noted that “males will sacrifice themselves with the hope that the resulting offspring will be stronger and more plentiful, carrying their genes into the future even if they themselves don’t survive.” In redback spiders, for example, the cannibalised male has a 65% increase in paternity, a significant leg up on those males that escape with their lives.

 
  Female redback spiders ( Latrodectus hasseltii ) will consume the smaller male during mating.   Doug Beckers/Flickr  (CC BY-SA 2.0)

Female redback spiders (Latrodectus hasseltii) will consume the smaller male during mating. Doug Beckers/Flickr (CC BY-SA 2.0)

 

In contrast, there are also plentiful examples of matriphagy — eating one's own mother — as a regular part of a creature’s life cycle. After the young of many species of insects, arachnids, and other short-lived organisms are born, they may consume all or part of their mother to give them a nutritious head start in life. 

“It’s interesting that sexual cannibalism and matriphagy occur primarily in species with a short lifespan,” said Dr Huber. "The primary driver for these behaviours is probably the species’ short lifespan, which imposes a deadline. That is, they may only get one opportunity to mate in their short life, and so must make the most of it to ensure that their DNA lives on.”

For organisms with a longer lifespan and many more chances to reproduce, cannibalism may be a way to establish dominance or ensure that a chunk of territory is exclusively their own. Male lions that have recently defeated the ruling dominant male and have taken over the pride will often devour the previous male's cubs. They do this for two reasons: firstly, to push the cubs' mother back into reproductive activity so she can carry the new male’s offspring; and secondly, to eliminate traces of the previous leader’s DNA, ending his bloodline in the pride and reducing competition among offspring. And it doesn’t hurt that the cubs provide a high protein meal for the incoming male. Male members of primate species such as chimpanzees and orangutans will often cannibalise young for the same reasons, and male polar bears will kill and eat cubs (both their own and those of rival males) in times of low resources. 

It’s not just males that get violent — mothers will occasionally eat their own offspring, as well. In prairie dog colonies, lactating mothers consume some of their young at peak breeding season, sacrificing a few from the litter to provide the energy it takes to feed the remaining babies.

  For many animal species, like the lion (left) and prairie dog (right), cannibalism is an ordinary fact of life.   Lip Kee/Flickr  (CC BY-SA 2.0);  Asiir/Wikimedia Commons  (CC BY-SA 2.5)

For many animal species, like the lion (left) and prairie dog (right), cannibalism is an ordinary fact of life. Lip Kee/Flickr (CC BY-SA 2.0); Asiir/Wikimedia Commons (CC BY-SA 2.5)

In species that reproduce more slowly and less often than the prairie dog, incidences of maternal infanticide are more exceptional, and are usually the result of some kind of abnormality, such as stress or starvation. Occasionally, however, if a mother has too many infants to provide for, or senses that some of her young are weak or sick, she will kill and eat them to be able to better care for the strong ones that are left. 

Dr Huber observed this behaviour first-hand in Darwin's finches during a research trip to the Galapagos Islands. "It was a time of drought and very few resources for the birds," she said. "We were noticing a high mortality among the hatchlings due to a parasite, and one day I observed a mother eating her own dead nestling and feeding it to the chicks. It’s just a testament to the ultimate need for survival, that environment can shape and change behaviour to result in phenomena like cannibalism."

Siblings can also be the perpetrators of cannibalism: in several species, including sharks, a newborn may hatch from an egg (still inside its mother) before its siblings, and consume some or all of the other offspring still contained in its mother’s brood pouch. This allows the cannibalistic youngster to grow before it exits its mother’s body, making it less vulnerable to predation.

Bees and ants are known to be very efficient in their disposal of the dead: they are carried back to the hive or the colony and eaten by other workers for a nutrient-packed energy boost. Bees also calculatedly consume some types of larvae for sex-ratio control and extra nutrients in times of pollen deficiency. No sentimentality needed.

  Sand tiger sharks (left) and honeybees (right) both gain survival advantages from engaging in cannibalism.   Brian Gratwicke/Flickr  (CC BY 2.0);  Nick Pitsas/CSIRO  (CC BY 3.0)

Sand tiger sharks (left) and honeybees (right) both gain survival advantages from engaging in cannibalism. Brian Gratwicke/Flickr (CC BY 2.0); Nick Pitsas/CSIRO (CC BY 3.0)

Accounts of cannibalism in the brutal world of non-human animals are seemingly endless, and occur often as an essential part of natural survival mechanisms. But what is it that sets humans apart? We are a species that has a sense of personhood, the capacity for empathy, and the complicated additions of culture, language, and spirituality to our base animal instincts. Popular examples of cannibalism in humans usually include dire circumstances in which people are forced to consume other people or face starvation, as in the case of the Donner party of California.

And yet, cannibalism is well-documented by paleoanthropologists from excavated remains in Mexico, Europe, North America, and almost every other continent. Neanderthals are well known to have been cannibalistic. Many human remains found at archaeological sites show the same evidence of butchering as non-human animal remains, suggesting that human meat was often treated and handled the same way as other protein sources.

Some scientists think cannibalism may have been part of an economic system of barter; others hypothesise that rival groups cannibalised each other to signify dominance, a kind of predator-prey relationship among warring factions. The Brazilian Tupinamba tribe, for example, felt that the ultimate defeat was to be eaten by one's enemy, and the people of Fiji believed that if the body of a dead adversary were consumed, their spirit would be unable reach the afterlife, and so couldn't give power and guidance to their allies. 

Cannibalism has also long been a part of religious rituals, the ingestion of another’s body or a specific body part conferring upon the eater a special power or significance. The Wari’ tribe in Brazil practised reverential cannibalism as part of a funeral rite until the late 1960s, when it was made punishable by the Brazilian government.

 
  ‘Cannibal feast on the Island of Tanna, New Hebrides’ (1891)   Charles E. Gordon Fraser/Wikimedia Commons  (public domain)

‘Cannibal feast on the Island of Tanna, New Hebrides’ (1891) Charles E. Gordon Fraser/Wikimedia Commons (public domain)

 

This act, however, viewed as horrific by many, is not restricted to seemingly isolated indigenous populations. This misconception is often used to perpetuate the idea of these peoples as ‘barbaric’ or ‘uncivilised’, a cultural inequity that has for centuries contributed to the exploitation of these populations. In fact, well into the 18th century, Western aristocrats such as Francis Bacon and Charles II consumed human bones and fats as part of a medicinal regimen. Aristocratic revenge feuds, common in medieval Europe, occasionally ended with adversaries consuming their conquest’s heart or entrails as a violent show of dominance.

Perhaps the practice of cannibalism in humans occurs for similar reasons as it does in other animals, but within the formalised structure of spirituality or war. But what makes it so horrifying? Is this a cultural or a biological taboo—and has one been subsumed by the other? In fact, there exists a very strong natural discouragement to the practice of cannibalism in the form of disease, an indication that humans may have evolved a mechanism that makes cannibalism non-adaptive.

The disease in question is called kuru, a form of transmissible spongiform encephalopathy related to Creutzfeldt-Jakob disease. Its symptoms include reduced muscle coordination (ataxia), trembling, incontinence, development of lesions and uncontrolled sporadic laughter. Kuru can be transmitted from an infected person to a non-infected person, and primarily affects the brain and nervous system.

The infectious agent in kuru is a malformed protein in the brain, called a prion, which can transmit its ‘wrongness’ of structure to other proteins. Prion is a word derived from the combination of ‘protein’ and ‘infection’, giving an accurate sense of its action. These misshapen proteins propagate and cause host tissues to develop microscopic holes, giving the affected area a spongy appearance—hence the ‘spongiform’ part of the name. The holes are also hypothesised to be the source of the symptoms, disturbing brain function. Prions are transmissible through consumption and subcutaneous exposure.

Kuru was first observed in the 1950s in the Fore tribe of Papua New Guinea, a group that practised family mortuary cannibalism, or consumption of deceased relatives. The dead were feasted upon as part of a funeral rite, particularly the internal organs and the brain. As the disease spread, those who died from kuru were in turn eaten after death, perpetuating the spread of the epidemic. Women and children, who were tasked with preparing the body, were most likely to contract the disease, potentially through contact of infected flesh with an open wound.

 
  A 10-year-old girl infected with kuru in a remote Fore village in Papua New Guinea, 1967.   John Mathews (2008)  (CC BY)

A 10-year-old girl infected with kuru in a remote Fore village in Papua New Guinea, 1967. John Mathews (2008) (CC BY)

 

Another contributing factor to the imbalance of infection was that men of this tribe believed that consuming the dead made them weaker for hunting, so were less likely to partake in this practice around the time of an expedition. After enforcement from the Australian government, the practice of family mortuary cannibalism has declined, and the last known sufferer of kuru died in 2005. Nevertheless, the epidemic has given valuable insight into the role of cannibalism in human evolution. 

A seminal paper, published in 2008, used genetic analysis of the Fore to show that certain genotypes are more susceptible to contracting and exhibiting symptoms of kuru. The authors argue that this stratification of individuals from less to more susceptible is a result of the disease — that kuru imposed a strong ‘balancing selection’ upon the population. 

When the authors compared the genotype stratification of the Fore to other populations around the globe, they found the same patterns: some individuals carry genomes coded for susceptibility, while others carry resistant genes. Resistance is made possible by a gene that inhibits the protein-protein interaction that allows prions to spread their damaging, malformed shape. These findings support the idea that the spread of kuru among the Fore is not the first event of its kind in human history, that something similar may have arisen separately in other cannibalistic communities.

If this is the case, perhaps human communities have evolved to view cannibalism as non-adaptive. This may come in the guise of a cultural revulsion for the practice, but the underlying cause may be more deeply rooted in our biology, a natural aversion like the deep-seated fear we have of snakes and spiders — it’s an ancient fear that tells us it might not be such a good idea to eat each other.

Humans are the only species in which cannibalism is known to cause disease, so this could be the reason why we are the only species with an aversion to cannibalism. In this instance, evolutionary biology may have influenced cultural practices, resulting in the taboos we see today. After all, we are — at our very core — just animals.