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Wolves With a Parasite Become More Daring, Study Shows

New research shows wolves with Toxoplasmosis gondii take more risks.

Key points

  • New research shows that a parasite can change the behavior of wolves and make them take more risks.
  • Having T. gondii might lead to increased levels of testosterone and aggression and help an individual rise to become a leader of a pack.
  • The significance of this research cannot be overstated: A bug could wag a wolf's tail.
patrice schoefolt/Pexels
Source: patrice schoefolt/Pexels

Yesterday morning someone wrote to me and asked me to write a "short weekend read" about a recent study showing that a parasite, Toxoplasmosis gondii, can make wild gray wolves take more risks. I had heard about this study but hadn't read it, so I did and I agree the results of this research are profoundly interesting and important. The original research is reported in a paper by Connor Meyer and his colleagues, including some of the world's leading experts on wolf behavior, called "Parasitic infection increases risk-taking in a social, intermediate host carnivore." An excellent, easy-to-read summary can be found in an essay by Jake Buehler titled "A parasite makes wolves more likely to become pack leaders."

Wolves display many different personalities; some are timid and shy and some are bold and daring.1 The researchers studied wild wolves living in Yellowstone National Park for whom there is an amazing amount of data on all aspects of social and other types of behavior.

A little background is needed. It's known that the parasite Toxoplasmosis gondii can be transferred between wild spotted hyenas and lions (called felids) and the wolves in Yellowstone share space with cougars who also are felids. Using this information, the researchers wondered if wolves whose homes overlap with those of cougars would show increased transmission of this parasite because of direct or indirect contact. They used 26 years of serological (analyses of blood) and behavioral data in their study. They also wanted to know if being infected with T. gondii affected wolf behavior by making them more daring and taking more risks, as it does among felids, rodents, and chimpanzees who become bolder and more daring when they're infected.

The wolf researchers learned that spatial overlap with cougars increased infection rates among wolves. Wolves who had at least 42.1% overlap with dense populations of cougars were more infected than those wolves who had less contact.

They also learned that there were changes in behavior due to being infected with T. gondii. Dispersing wolves, those who moved from one place to another, had almost twice as much seroprevalence—loosely equivalent to the amount of the parasite—than non-dispersers and were 11 times more likely to leave their pack. Importantly, bolder dispersing wolves also were 46 times more likely to become pack leaders. Having T. gondii might lead to increased levels of testosterone and aggression and help an individual rise to become a leader of a pack. It's not known if the parasite affects survival or reproduction of individual wolves.

Why This Research Is So Important

The researchers conclude: "This study is the first to examine the relationship between T. gondii infection and wolf behaviour and decision-making, finding a link between parasitic infection and wolf ecology. Dispersal is an important function of wolf population dynamics, but represents a risky decision as those that disperse suffer higher mortality rates." Kira Cassidy, a wildlife biologist at the Yellowstone Wolf Project and a co-author on the study, notes, “I think people are just starting to really appreciate that personality differences in animals are a major consideration in behavior... Now we add a parasite-impacting behavior to the list.” I couldn't agree more because comparative research across many different species is showing how important individual personalities are in all sorts of social encounters.

These data also have some practical on-the-ground importance because when wolves are moved from one location to another in reintroduction projects, such as the one planned in my home state of Colorado, it's important to know how individual wolves in new packs that are assembled by humans will interact with one another. Now, we know that there might be a hidden cause of behavior in the form of a small bug that could play a role, so as part of their health checks wolves should be screened to see if they have this parasite.

Who would have thought a parasite—a tiny little hidden bug—could result in such profound changes in behavior in individual wolves that could then spread through a population? I find the results of this study to be fascinating and incredibly important. I also wonder if dogs living with cats could also be infected and show these sorts of behavioral changes.2

All of the nitty-gritty details of this study are contained in the research paper, but the significance of this research cannot be overstated: A bug could wag a wolf's tail. I look forward to more similar research on a wider array of animals. There are many hidden causes of social behavior and now a parasite is known to be a major player.


1) For details about wolf behavior, emotions, and personality see, for example, four outstanding books by wolf expert Rick McIntyre, the last of which is reviewed in a piece called "The Power and Legacy of Yellowstone's Alpha Female Wolf 06." See also The Story of Yellowstone Wolf 8: From Underdog to Alpha Male, The Reign of Wolf 21, Yellowstone's Benevolent Alpha Male, and The Redemption of Yellowstone's Renegade Alpha Wolf 302. For more discussion on how wolves can get toxoplasmosis from cougars and what it means click here.

2) The results may help to explain some data my students and I collected on wild coyotes some 40+ years ago. In a nutshell, we found that some young coyotes cheated when they played, often by being more assertive and aggressive than other youngsters, and eventually dispersed from their natal pack into which they were born. We hypothesized that they left of their own accord—they were not forced out—because other coyotes either avoided them or did not accept their invitations to play. The dispersers suffered around four times higher mortality but they left because they were no longer welcome-—they didn't follow the Golden Rules of fair play. We have no idea whether they had T. gondii or if there was some other hidden cause of their being more daring. This has come to be called the "social cohesion" model of dispersal.

Yellowstone Wolves: Everything You Want to Know and More.

When Dogs Play, They Follow the Golden Rules of Fairness.

Flegr, J. and I. Hrdy. Influence of chronic toxoplasmosis on some human personality factors. Folia Parasitologica 41, 122-126, 1994.

Rodriguez, Toni. Common Parasite Linked to Personality Changes. Scientific American, September,ber 1, 2012.

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