Laissez-Faire Stallions? Males’ Fecal Cortisol Metabolite Concentrations Do Not Vary with Increased Female Turnover in Feral Horses (Equus caballus)

Abstract

Stress responses can be triggered by several physical and social factors, prompting physiological reactions including increases in glucocorticoid concentrations. In a population of feral horses (Equus caballus) on Shackleford Banks, North Carolina, females previously immunized with the immunocontraceptive agent porcine zona pellucida (PZP) change social groups (bands) more often than unimmunized females, disrupting the social stability within the population. We assessed the effects of increased female group changing behavior (or female turnover) on individual male stress by comparing fecal cortisol metabolite (FCM) concentrations among stallions experiencing varying amounts of female group changing behavior. FCM concentrations did not significantly correlate with female turnover. Similarly, FCM concentrations were not dependent upon the timing of female group changing behavior. These findings suggest that female turnover rate has little influence on physiological measures of stress in associated stallions. That said, Shackleford stallions experiencing increased female turnover do engage in behaviors typically associated with stress (increased vigilance, highly escalated male-male conflicts). Future work should compare FCM concentrations across time within populations and among populations managed under different strategies to better isolate factors influencing stallion stress physiology. Such studies are especially important if we are to determine how changes in female behavior related to immunocontraception impact physiological and behavioral indicators of stress for non-target animals. Finally, our study highlights the importance of considering both physiological and behavioral measures when investigating animal responses to potentially challenging situations.

Hair Cortisol Concentrations in Feral Horses and the Influence of Physiological and Social Factors

“…Alternatively, it could suggest that band movements are not undertaken unless the individual is in a better physiological state to take on the additional stress of establishing themselves in a new social hierarchy. The lack of a significant relationship between band changes and HCC among males is consistent with the findings of a study looking at feral horses in another population that did not detect significant changes in stallion fecal glucocorticoid levels despite inter-band movements by harem females [55].…”

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

“…This result was not confirmed by [ 78 ], but this study did find higher faecal metabolite concentrations of oestradiol and epi-androstenediol, which are metabolites of testosterone, in harem stallions when compared with bachelors. No differences in ACTH levels and hair cortisol, hormones commonly related to acute and chronic stress responses, were reported between harem and bachelor stallions [ 79 , 80 ], suggesting that bachelor status is not correlated with a higher stress load in stallions. Since bachelor groups enable social living for defeated or elderly stallions, this unimale formation evidently satisfies social needs for reproductively non-active horse males.…”

“…This could have a significant impact on body condition due to less time being available for foraging and other maintenance behaviours. However, a recent study did not confirm increased levels of fecal cortisol, and thus chronic stress, in stallions who, mostly due to contracepted mares’ infidelity, experienced frequent changes in their groups [ 80 ].…”

“…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 ].…”