Rising up to the challenge of their rivals: Mare infidelity intensifies stallion response to playback of aggressive conspecific vocalizations


Management tools like immunocontraception can alter the behavior of target animals, but the extent to which they affect non-target individuals has received less attention. The feral horse (Equus caballus) population on Shackleford Banks, North Carolina is an ideal system with which these questions may be explored, as management of the population with the immunocontraceptive agent porcine zona pellucida (PZP) has resulted in an increased propensity for females to change social groups and thus, decreased social stability. During the study, on average, females made 1.4 group changes per day (range = 0–18.5 group changes per day): females previously treated with PZP made 1.8 group changes per day, while females that had never been treated made 1.2 group changes per day. Between May and August 2017, we used playbacks of aggressive male vocalizations (squeals) and human voices (reciting “hello horse”) to assess changes in stallion responses to male rivals versus socially irrelevant stimuli in the context of female turnover. Over the course of the study, males were observed for 9.4 h on average (range = 2.4–20.5 h). Males spent more time vigilant (estimate = 12.431, P = 0.016, x¯ squeal = 30 s, x¯ control = 19 s) and were more likely to approach the speaker following squeal playbacks than controls (estimate = 2.325, P = 0.039). Males’ latency to return to normal behaviors varied depending on whether the playback was conducted in the weeks before, during, or after group changes occurred (P = 0.025, x¯ before = 26 s, x¯ during = 39 s, x¯ after = 53 s). Male responses were not affected by the total number of female group changes a male experienced during the observation period (P > 0.4), suggesting the effects are more context-dependent and not long-lasting. These findings suggest mare turnover can impact stallion responsiveness to potential rivals. As previously contracepted mares change groups more often than untreated mares and stallions exhibit prolonged responses to aggressive vocalizations after experiencing a female group change, contraception-induced changes to mare behavior may lead to increased male aggression in response to intruding rivals, which could be associated with greater energy expenditure. Finally, our work demonstrates that playback experiments are a useful tool for studying feral horse behavior in the wild. As the need for population control of different species continues to expand, rigorous investigations of immunocontraception's effects on non-target animals are critical if agencies are to manage populations most effectively.

Publication Title

Applied Animal Behaviour Science