You Wouldn’t Like Me When I’m Hangry
Scientists introduce new model of cannibalism that depends on hunger and desperation in high-density populations
July 13, 2022
For immediate release
Contact: Heidi Swanson, (202) 833-8773 ext. 211, heidi@esa.org
Most humans are familiar with the notion of being “hangry:” of being so hungry a person becomes irritable, angry, or even unreasonable. Most humans, however, don’t resort to cannibalism. That’s not true for all the animals on the planet.
Cannibalism has long been understood as a density dependent dynamic. Ecologists Jay Rosenheim and Sebastian Schreiber have partnered to discover the array of factors that underlie a population’s fluctuating cannibalism rate, and used mathematical models to examine the consequences for population dynamics of the cannibal. Their study was just published in the Ecological Society of America’s journal Ecology.
“As population density rises, cannibalism goes up,” Rosenheim said. “Most species, when conditions are good, express a reluctance to mount an attack on a conspecific. But when conditions are bad, when population density is high, animals get stressed and hungry. And if an animal is hangry enough, one eventual outcome is cannibalism.”
Cannibalism, like that demonstrated by this female praying mantis, is often dependent on population density, because density can shape an individual’s hunger and aggression. Photo by Leah Rosenheim.
The key factor that makes cannibalism rare when conditions are good is that cannibalism is risky – on a number of levels – so the stakes involved must be high enough to overcome the risks.
Any predator–prey interaction is dangerous for both parties. More so for the prey, usually, but risks to the predator exist, too. But in situations where both individuals belong to the same species, the risks are often elevated. Cannibalism is more dangerous than other kinds of predation because the playing field may be more nearly even.
“If you are a big, gnarly predator, and you’re thinking about eating a member of your own species, they’re also a big gnarly predator,” said Rosenheim. “You may have big teeth. But then, so do they.”
The other risk is less visible and more internal: parasites and pathogens are much more likely to be shared between predator and prey when they’re the same species. Many diseases are host-specific and don’t transfer between members of different species.
The final risk involves relatedness: Eating a conspecific increases the chance that you are eating a relative.
“As an animal, the last thing you want to do is eat your own offspring,” said Rosenheim. “If you’re eating a different species of prey, you don’t have to worry about that. A good way to avoid eating your relatives is just to avoid eating conspecifics.”
Only in situations where the risks – of death, injury, disease, or compromising reproductive success – outweigh the benefits do animals consistently turn to cannibalism.
“The baseline response of many organisms is, ‘Don’t mess with members of your own species,’” Rosenheim said. “But when population density rises, things often start to get bad: there is not enough food to go around, not enough nest sites to go around. Then the cost of cannibalism starts to get ignored, because the potential benefits outweigh it. Not only are you bumping into your conspecifics more often, but your willingness to mount an attack is also increasing — which leads to cannibalism.”
To explore the consequences of density-dependent cannibalism at the population level, Rosenheim worked with his colleague Sebastian Schreiber.
“Schreiber is an incredibly good modeler and theoretician,” Rosenheim said. “I showed him what I was working on, and we examined some of the likely consequences of these ideas. He added a beautiful and elegant analysis synthesizing these ideas about cannibalism and what it means for population dynamics.”
A review of the literature showed that the rate of cannibalism in many animals is controlled by a highly conserved hormonal pathway involving octopamine in invertebrates and epinephrine in vertebrates. As populations become denser, living conditions become worse, food becomes scarcer, and these hormone levels rise, leading to increased aggression and cannibalism.
“This one hormone forges a physiological link between hunger and cannibalism,” said Rosenheim. “And our models explore how cannibalism can help to regulate a population’s numbers.”
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