Why cannibalism is a common behavior in some animals

In November 1820, a sperm whale attacked and sank the Whaler Essex while the 20-man crew scrambled into smaller ships. Within weeks, the men ran out of water, food and hope of rescue. As crew members began to die of starvation and dehydration, survivors turned to their bodies and bones for food.

Cannibalism has long been a taboo subject in the United States, and European missionaries or explorers abroad used practice as an excuse to justify violence.

But it’s common in the natural world. Animals hunt and eat their own species out of necessity, due to environmental stresses, or simply because they like the taste. Scientists began to study cannibalism in the 1970s. Since then, they’ve learned more about how it happens and if there are limits to who isn’t on the menu. Researchers find that cannibalism is not binge eating. Depending on environmental factors, cannibalism can be predictable.

Eliminate cannibalism

The first scientists to research cannibalism studied patterns among cannibalistic creatures. In 1981, a study in the Annual review of ecology and systematics reviewed data from hundreds of studies regarding cannibalism, or “intraspecific predation,” as the author called it.

The author identified five main patterns among cannibalistic behaviors. For starters, the young were more likely to be eaten than the mature adults. Eggs, in particular, were the most vulnerable because they were both defenseless and nutrient-dense. Researchers have observed cannibalism in nearly all egg-laying creatures, from spiders to lizards to birds. The study also revealed that many animals did not recognize their own species. To them, an egg was just an egg.

The study also revealed that women were more cannibalistic than men. When researchers looked at both predator and prey sex, they found that females nibbled on males 88% of the time, and 76% of this was related to courtship and mating rituals. . Killing a mate could benefit a female’s offspring. If a female devours the male after mating, she would eliminate any possibility for the male to mate with another female and create competition for her young.

The study also found that although cannibalism increased with hunger, scarcity was not a requirement for the behavior. But cannibalism was directly linked to overpopulation. The more an animal felt the pressure of overcrowding, the more likely it was to seek out a meal of the same species.

Since this groundbreaking study was published in the early 1980s, researchers have worked through the framework and found more nuanced behavior. Some species, for example, prefer not to eat their family members.

Spare siblings

For some animals, cannibalism begins soon after hatching or birth, and their siblings are one of their first snacks. Researchers from the University of Jyväskylä in Finland were curious if the coloring poison dart frog (Dendrobates tinctorius) exhibited cannibalistic behavior. They are confined to small ponds, which is an environment conducive to cannibalism.

Dyed poison dart frog fathers pick up their tadpoles one by one and drop them into small pools of water. The tadpoles stay there until they metamorphose and jump out. These pools are not exclusive and some older tadpoles might still be there. Any given pond may have tadpoles of varying ages and parentage.

In a study 2022 Posted in Behavioral ecology, Jyväskylä researchers wondered if poison dart tadpoles were as likely to eat a sibling as a stranger. In their lab, they created small pools and then paired tadpoles that were either siblings, half-siblings, or unrelated. The researchers put on plexiglass to separate the tadpoles, then put them together to observe.

If any of the tadpoles became too aggressive, such as biting for two seconds or more, the researchers would break off the fight. The researchers found that larger tadpoles were more likely to attack smaller ones, but full siblings were less likely to fight to the death. The concept of half siblings didn’t make much sense to tadpoles and didn’t stop aggressive behavior.

In the wild, tadpoles with parental support, i.e. parents who bring food to the pool, are less likely to turn to cannibalism to survive. If the parents slack off, the nutrition from eating a pool mate can support metamorphosis and help the tadpole leave the problematic pool sooner. The study concluded that family ties have their limits, and although tadpoles are less likely to eat their own, they will if necessary.

Undesirable side effects

For animals like the tint poison dart frog, cannibalism in the early years can provide needed nutrients. But research shows that cannibalism can also lead to unwanted side effects.

Scientists disagree on the impact parasites and deadly diseases have on cannibals. Some claim that cannibals have better luck of exposure to parasites or deadly diseases when they eat one of their own. As in parasites evolve to manage to survive in certain species and develop defenses against the animal’s immunity.

Others suggest that cannibalism kills the infected host, thereby limiting the parasite or disease and its spread.

Another disadvantage of cannibalism is that it decreases inclusive fitness. Parents, siblings, and offspring share their genetic makeup, and eating them means removing more of the cannibal’s DNA from the gene pool. For species that recognize siblings, this also means weeding out others who might have recognized the cannibal as a parent and sparing them from a similar fate.