Some parasites can make an animal more likely to eat its own kind, finds a new study that is among the first to link parasites to cannibalism.
The findings, published in the journal Royal Society Open Science, add to the growing body of evidence that certain parasites may sometimes influence, and even directly control, the behavior of those that they infect.
Mandy Bunke of the University of Leeds and her team point out that both parasites and cannibalism are ubiquitous in nature, with eating of one's own kind documented in more than 3000 species, including humans.
Bunke told Discovery News that "increased demand for food by the parasites may drive the host to be more cannibalistic."
Co-author Mhairi Alexander of Stellenbosch University added that while many questions about the phenomenon remain, "we do know that parasites can affect foraging behavior and also vulnerability to predation in a range of species."
For the new study, the researchers focused on the shrimp Gammarus duebeni celticus, which is native to waters off of Ireland. The tiny parasite Pleistophora mulleri often lives off of the shrimp.
Bunke and her team collected the shrimp from Downhill River at County Antrim, Northern Ireland. They then observed their interactions, noting the presence or not of the parasites.
"We found that parasites had a surprising effect on their shrimp hosts, making them stronger cannibals," Bunke said. "Adult shrimp with parasites ate more young shrimp of the same species than the uninfected shrimp did."
She explained that although each of the parasites is infinitesimally small--just 5 micrometers long--millions of them invade the host shrimp's muscles. Once inside, the parasites severely damage the shrimp's muscles and continue to crave more nutrients, with the host becoming hungrier too.
Senior author Alison Dunn of the University of Leeds told Discovery News that "being more cannibalistic might help the host to deal with the cost of the infection as it gains more food."
"Interestingly," she continued, "we have also found in earlier work that infected shrimp may be able to catch and eat less prey of other animal species, so perhaps cannibalism of smaller shrimp is the only way these sick animals can survive."
People can become infected after consuming raw shrimp, fish and other edibles that may harbor parasites, but human victims clearly are not driven to cannibalism afterward. It could be, however, that the behavior of past societies that regularly practiced cannibalism was influenced by parasites.
The idea is not as far fetched as you might at first think.
"Research by our colleague Glen McConkey has shown that infection by the brain parasite Toxoplasma gondii, found in 10-20 percent of the U.K.'s population, directly affects the production of dopamine, a key chemical messenger in the brain," Dunn said, adding that "there is evidence that T. gondii makes humans less risk averse."
A study in the American Journal of Epidemiology found that this parasite is equally, if not more, common in U.S. residents. The research, led by Jeffrey Jones at the National Center for Infectious Diseases, found that prevalence is highest among those in the Northeast (29.2 percent) than in the South (22.8 percent), Midwest (20.5 percent), or West (17.5 percent).
Mark Torchin of the Smithsonian Tropical Research Institute has also studied parasites.
"Certain parasites have been shown to strongly alter host behavior," Torchin told Discovery News, adding that the new study "builds on growing literature demonstrating the importance of parasites in ecosystems and the strong impacts they can exert on host populations."
Dunn suspects that parasites could be affecting the behavior of invasive species, whose costs to agriculture and wildlife are estimated at 120 billion per year in the U.S. alone.