With an infection to the pack leader: The notorious pathogen of toxoplasmosis can have an amazing effect on wolves, researchers report. Affected animals more often achieve dominant status in the pack and are more willing to take risks than uninfected individuals. This emerges from investigations and behavioral analyzes of wolves in the American Yellowstone National Park. The finding expands what is known about how the parasite can affect its hosts, the scientists say.
Toxoplasmosis is an extremely widespread infectious disease: in addition to many animal species, humans can also become infected with the unicellular parasite Toxoplasma gondii. The carriers are mostly excrement from cats, because these animals are the definitive hosts of the pathogen, in which it can complete its development cycle. However, the infection is usually harmless and goes unnoticed. It can only become dangerous for people with a weak immune system, and caution should be exercised in pregnant women because the infection can damage the unborn child. However, it is also known that the pathogen can permanently settle in certain body tissues in humans and animals.
Parasites of amazing power
This is also the case with the mouse - an important intermediate host for the toxoplasma, through which they can enter cats. An amazing effect was found in the rodents: the pathogen uses certain mechanisms to cause mice to lose their natural fear of cats. The parasites turn the intermediate host into a kind of puppet for their reproduction. Because the toxoplasma reach their final host particularly efficiently via the carelessly taken prey animals. Earlier studies have already shown that the behavior-changing influence of the parasites is not limited to mice: there is evidence of a connection between latent toxoplasmosis infections and mental disorders in humans. In addition, a study of hyenas has already shown that infected young animals are more risk-averse and therefore have a higher mortality rate.
In order to uncover further possible effects on wildlife, a US research team has now targeted social predators: the wolves of Yellowstone National Park in Wyoming. This population has been intensively studied for almost thirty years. Extensive data was collected on the state of health, various characteristics and the individual behavior of many animals. This also includes antibody tests against Toxoplasma gondii, which indicate a latent infection if the result is positive. As the researchers report, of the approximately 230 wolves in the focus of the study, an average of around 30 percent were Toxoplasma positive.
First they then investigated how the animals caught these infections. The pumas (Puma concolor) of Yellowstone National Park, which act as definitive hosts for the pathogen, were the main possible origins. The examination of 62 specimens of these representatives of cats showed an infection rate of around 50 percent. Comparing the spatial distribution of the cougars in the park also showed that wolves living in areas that overlapped with a higher cougar density were comparatively more frequently infected with T. gondii. This indicates that the big cats appear to be an important source of infection.
Risk-taking and dominant
The researchers then investigated the possible behavioral influences of the Toxoplasma infections on the wolves. The analyzes of the individual behavior of the animals in combination with their infection status showed: Infected animals often occupy a dominant position in the herds, which is statistically striking. "For positive wolves, the probability of being promoted to a leadership position is almost twice as high as for negative individuals," the scientists summarize this finding. It was also shown that infected individuals are much more inclined to the fundamentally natural behavior of leaving their packs. This reflects increased daring, the researchers explain. Although the outliers have the opportunity to have their “own career”, the loss of the protection of the community entails considerable risks.
Infected animals therefore seem to be comparatively courageous or "bold". An effect that is consistent with previous results. According to the researchers, this is probably also linked to the increased chance of infected wolves to have a high status in the pack. Because these characteristics can play a role in assertiveness towards other group members. As the researchers point out, the pack leader effect may also have broader implications: Infected leaders may be more apt to place their packs in risky situations. Thus, Toxoplasma gondii could have complex effects on the wolf population in the Yellowstone area, the researchers say.
The study thus provides another example of the amazing effects that parasites can have on their hosts. There are probably many more. Possible effects of infections on individuals, groups, populations and ecosystem processes should therefore also be considered in wildlife research, say the scientists.
Source: Communications Biology, doi: 10.1038/s41587-022-01528-3