Decreased female fidelity alters male behavior in a feral horse population managed with immunocontraception

Abstract

In social species like the feral horse (Equus caballus), changes in individual behavior are likely to affect associated animals. On Shackleford Banks, North Carolina, USA, mares treated with the contraceptive agent porcine zona pellucida (PZP) demonstrate decreased fidelity to their band stallions. Here, we assess the effects of such decreased mare fidelity on male behavior and address potential interactions with habitat visibility, a component of the environment shown to significantly affect feral horse behavior. We compared the frequency and escalation of male-male contests, rates of aggressive and reproductive behaviors directed toward females, and the percentage of time spent vigilant among males experiencing varying levels of mare group changing behavior. We found that regardless of habitat visibility, males experiencing more female group changes engaged in contests at a higher rate (P = 0.003) and escalation (P = 0.029) and spent more time vigilant (P = 0.014) than males experiencing fewer group changes. However, while visibility had a positive effect on aggression directed by stallions toward mares (P = 0.013), female group changing behavior did not influence male-female aggressive or reproductive behaviors (P > 0.1), showing that decreases in mare fidelity altered male-male but not male-female interactions. These results have important implications for feral horse management; PZP-contracepted mares demonstrating prolonged decreases in stallion fidelity may have a disproportionate effect on male behavior. Moreover, our results shed light on the relative influences of female behavior and environmental factors like habitat visibility on male behavior. Such findings can ultimately improve our understanding of how the social and physical environments interact to shape male-male and male-female interactions.

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

“…Group changing behavior can induce stress responses in Shackleford mares (Nuñez et al, 2014). Mares changing groups are often chased, and sometimes kicked and/or bitten as resident stallions attempt to herd them back to the group (Madosky et al, 2010); they are subject to increased levels of reproductive behavior by both resident and new stallions (Nuñez et al, 2009); and often find themselves near highly escalated male–male conflicts (Jones & Nuñez, 2019). Moreover, mares making successful changes to new bands are frequently subject to aggression from resident females (C. M. V. Nuñez, unpublished data).…”

“…More recently, mares treated with PZP immunocontraception have changed groups more often, making approximately 10 times more group changes than untreated mares and joining (at least temporarily) five times as many groups (Madosky et al, 2010; Nuñez et al, 2009). At the time of this study, 6–7 years after suspension of contraception management, previously treated mares continued to change groups more often than mares that had never been treated (Jones & Nuñez, 2019; Nuñez et al, 2017), providing ample variation in this behavior for comparison.…”

“…The western portion of the island contains two primary water sources and is dominated by high dunes and dense brush that limit visibility; animals in this area share resources and exhibit overlapping home ranges. The eastern portion is flat and open, with high visibility; historically, horses in this region were territorial (Rubenstein, 1981), though more recent decreases in available water have necessitated significant home range overlap (Jones & Nuñez, 2019; Marr, 1996). The central portion's terrain is similar to that of the east, facilitating high visibility, though water sources are distributed more evenly, allowing horses to maintain more bounded territories (Jones & Nuñez, 2019, Rubenstein, 1981).…”

“…Mare transfer activity was rarely witnessed directly (2015, n = 1; 2016, n = 1); therefore, mare absence from a band was an important metric with which we measured the number of transfers between groups (Jones & Nuñez, 2019;Nuñez et al, 2009Nuñez et al, , 2017. We remained with each group for at least 30 min to ensure that individuals recorded as absent were not actually nearby, but out of our sight.…”

“…Stallions respond to female group changing behavior aggressively with chases, and sometimes kicks and bites to the females as they attempt to bring them back to the group (Madosky, 2011). Moreover, females engaging in this behavior often find themselves near highly escalated male–male conflicts (Jones & Nuñez, 2019). In addition, females that manage to change groups are often subject to aggression from resident females (C. M. V. Nuñez, unpublished data).…”