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

Abstract

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 hours on average (range = 2.4-20.5 hr).Males spent more time vigilant (estimate = 12.431, P = 0.016, 𝑥̅ squeal = 30 s, 𝑥̅ 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, 𝑥̅ before = 26 s, 𝑥̅ during = 39 s, 𝑥̅ 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.

Out of the stable: Social disruption and concurrent shifts in the feral mare (Equus caballus) fecal microbiota

“…Fecal samples were collected opportunistically (Altmann, 1974) during two other investigations addressing (1) the prolonged effects of repeated PZP treatment on mare fertility (in 2015; Nuñez et al, 2017) and (2) stallion behavior and stress physiology (in 2016; Jones et al, 2020; Jones & Nuñez, 2019, 2023). We only collected samples when we were certain of the mares' identity and the samples' location; all samples were still warm at the time of collection.…”

The Social and Reproductive Challenges Faced by Free-Roaming Horse (Equus caballus) Stallions

“…After two years of staying barren, an individual mare was observed to leave the harem taken over by her son and voluntarily join the harem of an unrelated stallion [ 91 ]. Similarly, contracepted mares were found to be more likely to change harem stallions [ 93 , 94 ]. This indicates that mares can change harems despite any guarding behaviour of the stallion.…”

“…Due to high pressure from other males during the mating season, stallions must frequently defend their mares from joining another male. In times of high turnover of contracepted mares, stallions are especially alert [ 93 ]. When male squeals were played on a speaker, stallions were found to be more vigilant and more likely to approach the loudspeaker compared to when a control sound was played [ 93 ].…”

“…In times of high turnover of contracepted mares, stallions are especially alert [ 93 ]. When male squeals were played on a speaker, stallions were found to be more vigilant and more likely to approach the loudspeaker compared to when a control sound was played [ 93 ]. The stallion is often the individual that is most vigilant when other members of the group are resting or sleeping [ 27 , 95 ].…”

“…The GonaCon-B contraceptive vaccine had no measurable effect on the social behaviour of horses, but a reduction in the number of individuals due to removals affected stallions’ behaviour: they herded and guarded their harems less when the population was reduced [ 111 , 152 ]. It was also found that PZP-contracepted mares showed changed time budgets [ 94 ], lower harem fidelity, and changed social groups more often than did control mares [ 80 , 93 , 94 , 154 ]. As stressed by the authors, a change in group composition often provokes conflicts between resident and incoming mares, changes in the relations between the harem stallion and resident mares, and a disruption to group stability, which is fundamental to reproductive performance [ 45 ].…”