Parasites can do all sorts of weird and disturbing things to their hosts. They can turn rats, and maybe even humans, into daredevils, make crickets commit suicide, convert caterpillars into head-banging bodyguards and even transform poor little ladybugs into zombie babysitters. Now, to add to this chilling list, researchers have found that a common parasite can drive cannibalistic behavior. But don’t worry, the study looked at shrimps, not humans, and they think it “seems unlikely” that a parasite would influence cannibalism in our species. Good to know. The study has been published in Royal Society Open Science.

While it may be frowned upon in today’s society, cannibalism can be found pretty much anywhere you look in nature, having been documented in more than 3,000 species. That’s because the act of killing and feeding on conspecifics actually comes with a variety of benefits, such as boosting growth and survival and the elimination of competitors. It may also help a population persist when resources start running low. That being said, cannibalism is a double-edged sword as gobbling your own kind also runs the risk of acquiring parasites, should the victim be infected.

Since it is known that certain parasites are able to profoundly influence the behavior of their hosts, including altering the rate of predation, scientists wondered whether parasitism could also be important in determining cannibalistic interactions. To investigate this idea further, researchers from the University of Leeds, Queen’s University Belfast and Stellenbosch University teamed up and designed a study aimed to examine the effects of infection with a tiny common parasite called Pleistophora mulleri. This organism is specific to a freshwater shrimp that is indigenous to Northern Irish waters, known as Gammarus duebeni celticus.  

They chose to investigate this pair since up to 90% of this shrimp harbor the parasite, and there is strong evidence to suggest that this species commonly engages in cannibalism. Furthermore, the only known route of infection is through oral transmission, or more specifically, the consumption of an infected shrimp, dead or alive.

For the study, researchers gathered adult male and juvenile shrimps from a river in Northern Ireland, determined whether or not they were infected and then documented their behavior towards one another. They found that infection with the parasite increased the rate of cannibalism by adults towards juveniles, with parasitized shrimps consuming twice as much of their own kind as uninfected individuals.

Interestingly, when given the choice, uninfected adults were less likely to cannibalize infected juveniles than uninfected individuals, whereas infected shrimp did not avoid parasitized conspecifics, probably because they have no motive to do so. Uninfected individuals, on the other hand, would benefit from this behavior because it means they get the nutritional benefit from cannibalism but also avoid infection.

As pointed out by lead author Mandy Bunke, the parasite represents a huge burden on the host with millions of them residing in the shrimps’ muscle, all relying on the host for food. It’s therefore possible that the increased demand for food resulting from infection could be driving cannibalistic behavior. Furthermore, infection can be debilitating, possibly reducing the shrimp’s ability to catch prey, so cannibalizing smaller conspecifics may be the easiest way to meet their food needs.