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Behavior-changing parasite moves wolves to the head of the pack

An image of 3 wolves in a snowy landscape.

Toxoplasma gondii is a ubiquitous protozoan parasite that can infect any warm-blooded species. In lab studies, infection with T. gondii has been shown to increase dopamine and testosterone levels along with risk-taking behaviors in hosts including rodents, chimps, and hyenas. Oh, and humans.

But its effects have not really been studied in the wild, so some researchers decided to assess how infection impacts gray wolves in Yellowstone National Park. They found that “the odds that a seropositive [infected] wolf becomes a pack leader is more than 46 times higher than a seronegative wolf becoming a pack leader.”

In the wild

Serum samples have been taken from the wolf packs in Yellowstone since 1995. These scientists assayed samples from 229 individual wolves taken over the years—116 males, 112 females, and one hermaphrodite—to try to correlate the presence of antibodies against the parasite with demographic factors and specific behaviors. (The relationship between antibodies and infection is complicated, given that the parasite can persist at low levels indefinitely after infections.)

Gray wolves and cougars are intermediate hosts and definitive hosts, respectively, of T. gondii, meaning the parasite grows to sexual maturity in wolves but needs to infect cougars to reproduce sexually. The two carnivores have some overlapping territory within Yellowstone, especially along its northern edge, and they compete for the same prey. Living in an area of high cougar overlap was the single biggest predictor of a wolf being infected with the parasite, more than any demographic factors like the wolf’s age, sex, or coat color.

Wolves with antibodies against the parasite were significantly more likely to disperse (leave their packs and set out on their own) and to become pack leaders. Pursuing both of these courses of action constitutes aggressive and risky wolf behavior, and they represent the two biggest decisions in a wolf’s life.

Parasites in charge?

Because gray wolves live together in groups, pack leaders have a disproportionate effect on their collective decisions. An infected leader may increase the overall number of infected wolves, both because pack leaders have a reproductive advantage and because risk-taking leaders might be less hesitant to lead their packs into cougar territory, where they can pick up their own infections.

Plus, wolves are social creatures who learn from and emulate their leader’s behaviors. So T. gondii-infected, aggressive, risk-taking pack leaders can yield “a more assertive, risk-embracing pack culture even though only a few key individuals are actually infected.”

Of course, increased engagement in risky behaviors is dangerous, so some of these hyper-aggressive wolf leaders and the packs that copy them are more likely to get themselves killed. Regardless, the selfish genes dictating their behaviors and their fates aren’t even their own genes. Parasites are the puppeteers.

communications biology, 2022. DOI: 10.1038/s42003-022-04122-0

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