While the behavioral and physiological effects of immunocontraception have been studied extensively in various species, the potential consequences for non-target animals have received less attention. We explored this issue in a population of feral horses on Shackleford Banks, North Carolina, USA that has been managed using the immunocontraceptive agent porcine zona pellucida (PZP). On Shackleford, mares previously treated with PZP change groups more often than untreated mares, disrupting the social stability within this population. We explored how this decreased female fidelity affects the behavior and physiology of stallions. We used behavioral observations to assess whether stallions experiencing varying rates of female turnover engaged in different rates of male-male contests, rates of aggressive or reproductive behaviors exhibited toward females, and/or time spent vigilant. We found that stallions experiencing higher rates of female turnover engaged in more frequent and more highly escalated contests and spent more time vigilant. To further explore the consequences of decreased female fidelity for male behavior in a more controlled context, we conducted a playback experiment to assess stallion responsiveness to signals from rivals. We played squeals (aggressive male vocalizations) and controls (men reciting, “hello, horse”) to all stallions and recorded all responses. Males spent more time vigilant and were more likely to approach the speaker following squeals than control trials, suggesting that squeal playbacks served their intended purpose by eliciting responses more typical of encounters between rival males. Furthermore, males exhibited heightened responsiveness to squeals during and after female group changes compared to before they experienced any group changes. Finally, we examined the effect of female turnover on male stress levels by assessing fecal cortisol levels and found that males experiencing higher turnover rates exhibited higher cortisol levels. Altogether, these findings demonstrate that increased female turnover leads to increases in male-male aggression, vigilance, and cortisol levels. As females previously treated with PZP change groups more often, stallions associating with these females are more likely to exhibit these changes in behavior and stress physiology. Such consequences of immunocontraception management for non-target animals are important to consider if maintaining animal welfare and naturally functioning populations is a management goal.