Emmanuelle Chrétien, Jeremy De Bonville, Joëlle Guitard, Sandra Ann Binning, Élizabeth Melis, Alexandra Kack, Ariane Côté, Maryane Gradito, Amélie Papillon, Victoria Thelamon, Marie Levet, Marie Barou-Dagues

1. Wild animals have parasites. This inconvenient truth has far-reaching implications for biologists measuring animal performance traits: infection with parasites can alter host behaviour and physiology in profound and sometimes counterintuitive ways. Yet, to what extent do studies on wild animals take individual infection status into account?
2. We performed a systematic review across eight scientific journals primarily publishing studies in animal behaviour and physiology over a 5-year period to assess the proportion of studies which acknowledge, treat or control for parasite infection in their study design and/or analyses.
3. We explored whether parasite inclusion differed between studies that are experimental versus observational, conducted in the field vs the laboratory and measured behavioural vs physiological traits. We also investigated the importance of other factors such as the journal, the trait category (e.g. locomotion, reproduction) measured, the vertebrate taxonomic group investigated, and the host climatic zone of origin.
4. Our results show that parasite inclusion was generally lacking across recent studies on wild vertebrates. In over 680 filtered papers, we found that only 21.9% acknowledged the potential effects of infections on animal performance in the text, and only 5.1% of studies treated animals for infection (i.e. parasite control) or considered infection status in the statistical analyses (i.e. parasite analysis). Parasite inclusion, control and analysis were higher in laboratory compared to field studies and higher for physiological studies compared to behavioural studies but did not differ among journals, performance trait categories and taxonomic groups. Among climatic zones, parasite inclusion, control and analysis were higher in tropical, subtropical, and temperate zones than in boreal and polar zones.
5. Overall, our literature review suggests that parasites are sorely under-acknowledged by researchers in recent years despite growing evidence that infections can modify animal performance. Given the ubiquity of parasites in the environment, we encourage scientists to consider individual infection status when assessing performance of wild animals. We also suggest ways for researchers to implement such practices in both experimental and observational studies.