Link between Toxoplasma gondii infection and behaviour in a free-living mammal

Toxoplasma gondii (T. gondii) is a trophically-transmitted parasite that is thought to facilitate its transmission by altering some of its hosts anti-predator behaviours. For example, T. gondii has been shown to cause its hosts to become less risk-adverse, more exploratory, slower reacting, and weaken their aversion to the scent of predators. Studies examining this phenomenon, however, have almost exclusively been carried out in laboratory conditions with laboratory raised animals, mainly small mammals. Fewer studies examine how T. gondii may have a role in complex behavioural in more natural contexts in free-living wild animals. Under such scenario, the goals of our study were two-fold. Firstly, to determine the rate of T. gondii infection in a population of free-living fallow deer (Dama dama) living at the edge of a metropolis. Secondly, we aimed to determine whether infection with T. gondii was linked to risk taking behaviour in these free ranging large mammals. By analysing blood samples taken from 94 individual deer, we found a seropositivity rate ranging between 15% and 35% across the 2 years of the study. We combined seropositivity with unique behavioural data on these individuals on a spectrum from deer avoiding humans (risk-avoiders) to those begging for food (risk-takers). We found that T. gondii infection was linked to an increased likelihood of begging behaviour, i.e., deer seropositive for toxoplasmosis were more likely to engage in risky behaviours by approaching humans closely. The causal mechanism behind this phenomenon needs to be further explored, as we discuss in this paper, because our study does not exclude that risk-taking animals are more likely to contract the parasite, opposed to our main hypothesis that implies that those deer contracting the parasite are more likely to display risk-taking behaviour. Besides, our study is a significant contribution on the transmission and maintenance dynamics of this very interesting parasite, offering new insights on the need to carry out longitudinal studies able to disentangle the causal mechanism and T. gondii’s ability to manipulate its host.