Snakes continue to evolve into man-eaters – what scientists think is happening

Although cannibalism may seem like a rare and unnatural event, scientists theorize that this behavior has occurred in several snake lineages throughout evolutionary history and is often triggered by environmental stressors.

Researchers reviewed 500 reports of cannibalism across snake species and found that cannibalism has evolved independently at least 11 times, according to a study published in the journal Biological Reviews on November 2, 2025.

Although these cannibalistic behaviors occur in a variety of situations, the researchers suggest that overall cannibalism is common and appears to recur because it is beneficial to snakes in situations where they need to make opportunistic food choices.

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“For us humans, we don’t consider cannibalism to be a normal thing. It’s strange and disgusting,” Bruna Falcan, lead author of the study and a graduate student in biology at the University of São Paulo, told Live Science. “But for the snakes, it’s good for them, it’s good for their ecological adaptation. … This is strategic.”

Evolutionary benefits of cannibalism

Some of the best-known examples of cannibalism in nature are seen in mating spiders and praying mantises. This is because it may be beneficial for the female to eat her mate. “Cannibalism is widespread throughout the animal kingdom,” biologist and National Geographic explorer Xavier Graudas, who was not involved in the study, told Live Science.

Scientists have previously considered this behavior to be maladaptive, meaning not beneficial to the species as a whole, but studies documenting cannibalism in animals increasingly hypothesize evolutionary reasons for the behavior. For example, it may help parents control nestling size, or it may arise as a response to limited resource availability, a form of population control, or opportunistic predation selection.

Cannibalistic behavior in snakes is usually detailed in brief, individual reports, Graudas said. For example, his research team reported that male Montpellier snakes (Malpolon monspessulanus) prey on female snakes in France. This behavior is thought to be driven by limited food resources, especially during periods of food scarcity outside the breeding season. (It is considered unusual for a male to eat a female during the breeding season, as this reduces mating opportunities.)

According to the study, cannibalism is not only widespread in snakes, but also has evolved independently in different snake lineages and regions. This study combines numerous reports to explain the behavior.

The research team collected 503 reported cases of cannibalism across 207 snake species. Reports span a wide range of snake groups as well as all continents where snakes occur, including reports of both wild and captive snakes.

“None of us expected that snakes could be so cannibalistic, and no one talked about it,” Falcao said. “The more we searched, the more cases we found.”

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The researchers found that cannibalism was most common in the families Colubridae, Viperidae, and Prunidae. Colubridae is the largest family of snakes, accounting for 29% of all reports. However, because this family is not known to commonly prey on snakes, the authors proposed that most cases of cannibalism in this group may be related to stress factors such as lack of other food sources. Members of the Viperidae family, including vipers, accounted for 21% of all cannibalism reports. However, the researchers noted that most of these cases occurred in captivity, and stress factors associated with captivity, such as being confined in a small space with limited food, may have led to cannibalism.

The serpentine family, which includes cobras, accounted for about 19% of cannibalism reports. This is not too surprising, the researchers said, as cobras are known to prey on other snakes in the wild.

According to the study, nearly half of cannibal snake species have a generalized diet, and the researchers linked this dietary flexibility to cannibalism when necessary. However, Glaudas suggested that this relationship may not be clear-cut, as researchers define only 47.7% of man-eating snake species as generalist. The higher the rate, the stronger the evidence for a relationship, he said.

“I’m a little more skeptical about the data presented regarding the idea that cannibalism may be more common in generalist species,” Graudas said.

Cannibalism appears to be correlated with jaw structure, so whether a snake has jaws large enough to prey on another snake is an important factor. There are no reports of cannibalism in snake species that do not have this ability.

When researchers analyzed cannibalistic behavior across snakes’ evolutionary history, they concluded that the behavior evolved independently at least 11 times across the snake evolutionary tree.

Most reports of snake cannibalism are anecdotal, so this study provides a useful overview, Glaudas said. This is “welcome research that will allow us to better understand the correlates of cannibalism in snakes,” Graudas said.

Snakes form a highly successful branch of the evolutionary tree. They live on every continent except Antarctica and are adapted to most ecological niches, the study authors noted. Cannibalistic behavior is seen in many different species of snakes around the world, so it may reflect the snake’s ability to opportunistically adapt to its circumstances, Falcao said. “That’s really amazing [cannibalism] “It evolved 11 times independently in the snake lineage,” she noted.

Falcao said the study could not cover all reports of snake cannibalism, but many of them are in hard-to-find old books and archives, so there is likely more to be discovered on the subject.