this is a true horror story: some parasites change the host's behavior and "mind control" them. Moreover, this control seems to affect not only infected individuals.

Today's story is about a tapeworm called Schistocephalus solidus. Like many parasites, its parasitic experience is so complex that it takes three-tier hosts to complete the cycle of reproduction. These tapeworms reproduce in the digestive tract of birds, and the eggs are excreted with bird feces. The hatched larvae first find some copepod crustaceans, then infect the fish through the food chain, and then the waterbirds eat the infected fish.

(that's about the whole process)

in the process, tapeworms don't just quietly wait for their master to be eaten. The study found that the behavior of fish infected with these tapeworms changed. Infected fish swim to warmer waters (an environment conducive to tapeworm growth) and, more importantly, become "adventurous", more willing to swim to dangerous areas where there is no shelter, and less alert to predators. As a result, birds that eat fish are more likely to catch infected fish and eat parasites and fish together. This is exactly what tapeworms are trying to achieve: birds are their ultimate hosts, and they reproduce in the guts of waterbirds. The tapeworm eggs are excreted through the bird's feces and continue into the next cycle after hatching.

interestingly, it is not just infected individuals who are affected by parasites. A recent study found that uninfected fish in populations are also affected by parasite-controlled partners. The researchers simulated bird attacks in fish tanks and set up different proportions of parasites in schools of fish to observe their behavior. As a result, fish that are not infected by parasites escape to the bottom of the tank, while those infected stay in a "danger zone" with indifference. Not only that, when the majority of infected people in the group, those who were "clear-headed" fish will also begin to follow the crowd and take more risky behavior. If, in turn, the uninfected fish account for the majority, then the infected fish will not wake up and their risky behavior will not change. In an animal group, it is often wise to follow a large army, but in this case, it is the parasite that affects the uninfected individual.

apart from lamenting the rugged living style of parasites, I would also like to complain a little bit about the device of this experiment: the experiment used to scare schools of fish was done with fake birds. It's not even a bird, it's just a fake beak on the handle. According to the researchers, it's still made of Lego, but it does work well in the experiment, so it doesn't matter how it looks.

(if the experimental device is shown in the picture, you may not be able to see that the above thing is simulating a bird)

PS: every time you say "parasites make animals adventurous," I think of astronomers who send signals to the universe. Although it doesn't matter (probably)

Source: https://www.theatlantic.com/science/archive/2018/06/what-tapeworms-really-want/563189/

original paper: http://rspb.royalsocietypublishing.org/content/285/1881/20180956

cover: a three-thorn fish (Gasterosteus aculeatus), is also a species in the study mentioned in this article. Photo: WWU/J ö rn Scharsack

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