Toxoplasma gondii Makes Yellowstone Wolves 46 Times More Likely to Become Pack Leaders

Analysis of 26 years of wolf behavioural records, alongside blood tests from 229 wolves, has found that infection with the parasite Toxoplasma gondii makes wolves 46 times more likely to take on the role of pack leader.

The findings suggest that the parasite’s effects in the wild have been severely overlooked, as has its influence on ecosystems and animal behaviour.

Cat owners may already be familiar with this parasite. This microscopic organism is able to reproduce sexually only inside feline bodies, although it can infect and flourish in almost any warm-blooded animal.

Humans are among its hosts. In people, it causes toxoplasmosis, a parasitic illness that is generally symptom-free but can still prove fatal.

How Toxoplasma gondii changes host behaviour

After entering a non-feline host, T. gondii must somehow get its offspring back into a cat to avoid becoming an evolutionary dead end. It appears to improve those odds in a rather unsettling manner.

Rats carrying the parasite, for instance, become more prone to risk-taking and may even develop a fatal attraction to the smell of feline urine, increasing their chances of being killed by cats.

In bigger animals such as chimpanzees, infection is associated with a greater likelihood of encountering a large cat such as a leopard. Hyenas infected with T. gondii are likewise more likely to be killed by lions.

Yellowstone wolves and cougar territory

Grey wolves (Canis lupus) in Yellowstone National Park are not normally prey for cats. However, their range can overlap with cougars (Puma concolor), which are known carriers of T. gondii. Both species hunt elk (Cervus canadensis), bison (Bison bison) and mule deer (Odocoileus hemionus) in the park.

Wolves may therefore acquire the infection, perhaps by occasionally consuming dead cougars or ingesting cougar faeces.

Behavioural and population data gathered from the wolves over almost 27 years provided an unusual opportunity to examine the parasite’s effects on a wild intermediate host.

The scientists, led by Yellowstone Wolf Project biologists Connor Meyer and Kira Cassidy, also examined blood samples from wolves and cougars to estimate rates of T. gondii infection.

They discovered that wolves whose territories overlapped extensively with cougar ranges were more likely to carry T. gondii.

There was also a behavioural effect: infected animals showed a substantially greater tendency to take risks.

Toxoplasma gondii and wolf pack leadership

Wolves with the infection were 11 times more likely to disperse from their pack into unfamiliar territory. Infected males had a 50 percent probability of departing their pack within six months, whereas uninfected males typically left after 21 months.

Among females, those infected had a 25 percent chance of leaving within 30 months, compared with 48 months for females without the parasite.

Infected wolves were also far more likely to become pack leaders. T. gondii might raise testosterone levels, potentially increasing aggression and dominance-characteristics that could enable a wolf to establish itself as a pack leader.

That has two significant implications. Pack leaders are the individuals that reproduce, while T. gondii can be transmitted congenitally from mother to offspring. The parasite may also reshape the dynamics of an entire pack.

"Due to the group-living structure of the gray wolf pack, the pack leaders have a disproportionate influence on their pack mates and on group decisions," the researchers write in their paper.

"If the lead wolves are infected with T. gondii and show behavioral changes … this may create a dynamic whereby behavior, triggered by the parasite in one wolf, influences the rest of the wolves in the pack."

For example, a pack leader that follows the smell of cougar urine while boldly moving into new territory could encounter the parasite more often. This could raise T. gondii infection rates across the wolf population, creating a feedback loop of increased territorial overlap and infection.

The evidence is compelling: small, poorly studied agents can exert an enormous effect on ecosystem dynamics.

"This study demonstrates how community-level interactions can affect individual behavior and could potentially scale up to group-level decision-making, population biology, and community ecology," the researchers write.

"Incorporating the implications of parasite infections into future wildlife research is vital to understanding the impacts of parasites on individuals, groups, populations, and ecosystem processes."

The study was published in Communications Biology.

An earlier version of this article was published in November 2022.