Wolves infected with a common parasite are more likely than uninfected animals to lead a pack, according to an analysis of more than 200 North American wolves1. Infected animals are also more likely to leave their home packs and strike out on their own.

The parasite, Toxoplasma gondii, makes its hosts bold — a mechanism that increases its survival. To reproduce sexually, T. gondii must reach the body of a cat, usually when its host is eaten by one. That becomes much more likely if the parasite alters the host’s behaviour, making it foolhardy. Research results are mixed, but in rodents, infection generally correlates with decreased fear of cats and increased exploratory behaviour. Physical and behavioural changes have also been found in people: testosterone and dopamine production is increased and more risks are taken.

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