Parasite manipulates more than we know

You’ve probably heard of the parasites that transmit malaria or ringworm. These predators are organisms that live on in a host. In many cases, they can influence the behaviour of the host to their own benefit, sometimes causing them to act with more risky tendencies.

Influencing the host to act with less caution means that the parasite is more easily able to complete its life cycle. So, a parasite won’t kill the host, but under its influence, it might die. But is a parasite able to indirectly influence the behaviour of healthy individuals in a species population? Researchers from the University of Munster think so.


Parasite Life Cycle

They designed a study to test this theory in three-spined sticklebacks and the tapeworm, Schistocephalus solidus. When infected with the tapeworm, sticklebacks act carelessly and swim into deep open waters where they are more likely to get eaten by a bird – progressing the parasites life cycle. They were interested to see whether the healthy fish were affected by this odd risky behaviour shown by the infected individuals.

 They bred sticklebacks in the laboratory and infected some of them with tapeworms. Their behaviour observed in the aquarium was recorded, before and after a threat from a dummy bird. Food was delivered to the top of the aquarium – this was designated the “dangerous” area because the fish would more likely be eaten in this area. The researchers realized that after contact with the bird, the healthy sticklebacks acted cautiously, avoiding the deep open waters – as a stickleback should – even though the food was there. The infected fish, however, were quick to return to the top, fearlessly.

Interestingly, in groups where the majority of the fish were infected, a new pattern arose. The healthy individuals followed the lead of the infected sticklebacks. They displayed the same careless behaviour by returning to the top of the aquarium.

It is suspected, by the researchers, that these fish are more interested in remaining in the group than in protecting themselves from the potential threat of the bird. This however, is not translated to the infected fish – they show no interest in remaining in the group.

It is known that humans act in a similar group cohesion fashion, in that when wrong answers are presented by more than two people, we are more likely to agree with the group regardless of if we think the answer is wrong. Social networks seem to be an important part of life not only for humans but other species as well.

The researchers state they to the best of their knowledge, this is the first work published on how a parasite with a complex life cycle can indirectly manipulate the behaviour of the uninfected individuals of its host species. This work is important because it illustrates just how complex the inner workings of social networks may be, and how much we still don’t know about parasites, and even life in general.

Published Paper:

Demandt N, Saus B, Kurvers RHJM, Krause J, Kurtz J, Scharsack JP. (2018) Parasite-infected sticklebacks increase the risk-taking behaviour of uninfected group members. Proc. R. Soc. B 285: 20180956.


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