Ecology: Parasite-infected wolves more likely to become leader of the pack

Grey wolves in America’s Yellowstone National Park exposed to Toxoplasma gondii — the parasite that causes the disease toxoplasmosis — are over 46 times more likely to become a pack leader than uninfected wolves. The findings, published in Communications Biology, are the first to demonstrate parasite infection affecting decision-making and behaviour in wolves.Grey wolves in America’s Yellowstone National Park exposed to Toxoplasma gondii — the parasite that causes the disease toxoplasmosis — are over 46 times more likely to become a pack leader than uninfected wolves. The findings, published in Communications Biology, are the first to demonstrate parasite infection affecting decision-making and behaviour in wolves.

Connor Meyer, Kira Cassidy and colleagues studied grey wolves (Canis lupus) living in Yellowstone National Park, Wyoming, USA, looking at the association between risk-taking behaviours and infection with Toxoplasma gondii (which often has no negative effects on the fitness of healthy individuals, but can be fatal to the young or immunosuppressed). The authors analysed 26 years (1995–2020) of behavioural and distribution data as well as blood samples from 229 anaesthetised wolves, which were screened for antibodies to T. gondii. As cougars (Puma concolor) in the park are known to be hosts of the parasite, the authors also used a previously developed spatial model of cougar distribution and screened blood samples from 62 cougars.

The authors found that wolves occupying areas that overlapped with a higher cougar density were more likely to be infected with T. gondii than those not living near cougars, suggesting that wolves may become infected with the parasite as a result of direct contact with cougars and their environments. They also identified associations between parasite infection and high-risk behaviours in both males and females that can affect the fitness of individual wolves and wolf populations. Wolves that tested positive for T. gondii were 11 times more likely to disperse from their pack and more than 46 times more likely to become a pack leader than uninfected wolves. Males reached a 50% probability of leaving their pack within a six-month period if infected with the parasite, but after 21 months if uninfected. Females reached a 25% chance of leaving their pack within 30 months if infected, extending to 48 months if uninfected.

As previous research has identified associations between T. gondii infection and increased boldness in hyenas as well as increased testosterone production in rats, the authors speculate that similar mechanisms could drive the risky behaviours observed in wolves that tested positive for the parasite. They theorise that infection could have wider implications for the Yellowstone’s wolf population as infected pack leaders could lead their packs into more high risk areas overlapping with cougars. This potentially increases the risk of further infection for uninfected wolves, creating a feedback loop driving further risky behaviours.