PARASITES MANIPULATING THE behaviour of their hosts to benefit themselves is something we’ve known about for quite a while. However new research has found that the freeloaders can even influence healthy, uninfected, animals into doing their bidding.
The tapeworm Schistocephalus solidus has a complicated life cycle. It lives and eats inside stickleback fish however it reproduces in the intestines of waterbirds, such as kingfishers.
Inside the stickleback the parasite grows to such an enormous size that it can make up half of its host’s weight. Despite this the fish continues to live with the parasite inside it.
As the tapeworm grows it doesn’t just feed off the fish and leave it to its own devices, it changes the stickleback’s behaviour so that it swims towards warmer water, which the parasite prefers, and it also causes it to venture into open water where it is easy prey for waterbirds.
This is exactly what the tapeworm wants because once it’s inside the bird’s guts it can lay its eggs and complete the cycle.
A team of evolutionary biologists have demonstrated for the first time that the tapeworm doesn’t just influence the behaviour of infected fish in a shoal – it can also cause healthy fish to engage in equally risky behaviour.
The research found that if the majority of fish in the shoal are infected, and begin swimming into open water, even the healthy fish will follow, putting themselves at increased risk of being eaten.
“The reason for this ‘wrong’ decision on the part of the non-infected sticklebacks presumably has something to with shoaling behaviour,” one of the researchers, Dr Jörn Scharsack, said in a statement.
The urge to remain in the group is stronger than exercising caution against any attack by a bird independently.
However, the opposite is not true, when infected fish are in the minority, they continue to engage in risky behaviour despite the more conservative patterns of the rest of the group.
The scientists suspect that the ability of the tapeworm to influence the behaviour of healthy sticklebacks could also have a wider effect on stickleback and bird populations.
Because more fish could be lured into dangerous waters, more waterbirds could congregate there. The predators’ urge to eat fish could thus increase, and ultimately more tapeworms could get into the birds’ intestines to reproduce.
The research is published in the current issue of the journal Proceedings of the Royal Society B.
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