Cannibalism is not uncommon in the animal kingdom. Many different types of organisms occasionally prey on members of their own species – house mice, monarch butterfly larvae, wandering spiders, crows, and many kinds of fish, just to name a few. As Laurel Fox, the author of a scientific paper entitled “Cannibalism in natural populations,” (the source of the examples of cannibalistic species listed above) wrote in 1975, “[c]annibalism is not an aberrant behavior limited to confined or highly stressed populations, but is a normal response to many environmental factors.”
Fox details a number of reasons why cannibalism can occur among animal populations, including a lack of other sources of food, overcrowding, stress, and simple availability. “In many examples initiation and control of cannibalism has not been ascribed to any obvious factor,” Fox writes, “and in these cases cannibalism may be a response primarily to the presence of vulnerable individuals.”
In fact, the behavior is so common among zebrafish (a small, freshwater species popular in both living room fish tanks and research labs) that online guides dedicated to the care of pet zebrafish warn their readers to keep adults separate from eggs and larvae. It’s also common enough that when a group of researchers was searching for a predator of zebrafish larvae to use in a study, they settled on the adult form of the species.
Those researchers recently published the findings of their study – an investigation of the mechanisms behind the ability of larval zebrafish to evade predators – in The Journal of Experimental Biology. Based on earlier experiments, the scientists already knew that the larvae sense their predators by feeling the flow of water that the larger fish push before them as they move rather than by seeing them coming, but in order to successfully avoid being eaten, the larvae need to avoid their predators as well as sense their presence.
This, the researchers found, is exactly what zebrafish larvae do; based on the cues they got from the water flowing around a predator (a dead adult zebrafish that had been preserved with formalin, and was guided through the experimental tank with a robotic arm), the “larvae direct[ed] their escape away from the side of their body exposed to more rapid flow. This suggests that prey fish use a flow reflex that enables predator evasion by generating a directed maneuver at high speed.”
Zebrafish larvae are equipped with the ability to feel a predator coming, and the reflex to swim away from it, even when that predator is an adult zebrafish.