Mosquitofish and guppies sometimes exhibit cannibalism when kept in captivity, but a new study shows that this behavior is not common in the wild. According to the authors of the study, rare cases of cannibalism among fish are most likely caused by intense competition for food. The study has implications not only for aquarists, but also for our understanding of evolution.
To figure out just how common cannibalism is, Professor Brian Langerhans of North Carolina State University analyzed years of data from nearly 12,000 wild fish from 17 species.
“This is data accumulated from several different projects over the years,” said Professor Langerhans, senior author of the study. “In order to determine the mechanisms responsible for this phenomenon in the wild, we needed really large sample sizes. So we were accumulating data for this work while doing other projects at the same time”
Scientists examined the stomach contents of 11,946 fish using X-rays or dissections. Of all the surveys, they found only 35 cases of cannibalism, which is less than 0.3 percent. Moreover, cases of cannibalism were found only in three species of mosquito fish.
Study co-author Rüdiger Risch of Royal Holloway University of London began work on the project as a postdoctoral researcher at Langerhans’ lab between 2010 and 2012.
“In captivity, mosquitoes and guppies practice cannibalism quite often, so there are protocols in research laboratories and aquaculture to quickly separate offspring from larger fish,” says Risch. “But when you study the diet of fish in the wild, you don’t find much evidence of this. We wanted to find out if cannibalism occurs in nature and why.”
Scientists have found that rare cases of cannibalism were not just opportunistic, but caused by extreme food competition. Cannibalism arose only in those cases when there were few predators on fish, which led to too high growth of their population.
“Competition for resources seems to be the main predictor of cannibalism,” said Professor Langerhans. “We also saw that the absence of predators has an indirect effect on cannibalism: Removal of predators allows population density to increase dramatically, resulting in a reduction in resources. This same driving factor may be responsible for many cases of cannibalism in the animal kingdom in natural conditions”
The study was published in the journal Ecology & Evolution.