Why do some animals turn cannibalistic?

Under the right circumstances, fruit flies can evolve cannibalistic behavior, a new study says. 

While cannibalism was considered uncommon and of little evolutionary importance in the past, it is now recognized that it can serve several functions, including nutrition, minimizing competition and regulating population density. Cannibalism has been identified in over 1300 species. "The most likely function of cannibalism in non-carnivorous species is nutrition, however it may as well serve other functions such as eliminating competitors for resources and mates", said Dr Roshan Vijendravarma, an evolutionary biologist at the University of Lausanne, Switzerland. In crickets, for example, it has been shown that cannibalism is an important way to obtain nutrients, allowing the surviving insects to migrate further. Cannibalism also contributes to population control in some species of moths or to reduce the rate of parasitism in sawflies.

Despite current advances in our understanding of cannibalism, we don't currently know how it evolves and whether it is a trait mostly influenced by the environment or determined genetically. So far, flour beetles are the only non-carnivorous insects where this behavior has been shown to have a genetic basis, whereas other studies have simply blamed factors such as poor nutrition or overcrowded conditions.

New research led by Dr Vijendravarma has found that after 118 generations, larvae of the common fruit fly (Drosophila melanogaster) developing in food-deprived conditions were twice as likely to pile around and kill injured larvae, when compared to larvae with normal feeding levels. Once a target larva is wounded, other larvae show up and engage in collective cannibalistic behavior. Older larvae are likely targeted because they are hardly able to move, making them easy targets for the young and agile. Despite being studied for over a century, such behavior has never been reported in this species.

However, while lack of food is clearly an important factor triggering cannibalism, some well-fed larvae also showed this behavior. This raises important questions, such as how does cannibalism evolve in these flies? Is it a genetic or an environmental led trait?

"Of course, all this at the current stage is speculative", says Dr Vijendravarma, but "our research demonstrates there is a heritable component at least for the propensity to cannibalize, which can evolve". Dr Vishwesha Guttal, a biologist from the Centre for Ecological Sciences of the Indian Institute of Science agrees. "The rapid evolution of cannibalistic traits provides compelling evidence for how malnourished environments can lead to the evolution of cannibalism due to sufficient genetic variation in the population", says Dr. Guttal.

"Animals have a broader and more flexible diet than we often realize. What an animal eats influences its physiology, life history, geographic range, population density and dynamics, and evolution. Carnivorous behavior by so-called non-carnivorous insects deserves a closer look", said Dr Matthew Richardson, an entomologist from the Subtropical Insects and Horticulture Research Unit of the United States Department of Agriculture, who has observed this behavior in grasshoppers.

An important implication of this work concerns future studies wanting to dissect the basis of cannibalism, as Dr Vijendravarma explains, "A number of theoretical models about the ecological and evolutionary significance of cannibalism have been proposed but have seldom been tested, especially due to the lack of a model system. The state-of-art techniques available in Drosophila open an entire new perspective on how this behavior can be investigated in depth."

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