had no offspring during the study. That must be borne in mind particularly in interpreting long-term impacts of contraception (e.g., repeated years of reproductive “failure” due to contraception).
Gray (2009) and Gray et al. (2010, 2011) studied the effects of a liquid-PZP vaccine on behavior of free-ranging horses in Nevada during breeding and nonbreeding seasons. There were no treatment effects on activity budget, rates of sexual behavior, proximity between stallions and mares, attempts to initiate proximity, aggression given or received, or band changing by mares. Powell (1999) found no differences in spatial relationships, dominance rank, or aggression between mares currently on PZP and those not currently on PZP on Assateague Island; however, at the time of Powell’s studies, all mares had been treated with PZP at some point in the past, so true controls were not available. On Shackleford Banks, an island where some mares were never treated with PZP, changes in time budgets were observed. Many factors—such as the presence of a foal, the size of a harem, and features of the male associated with the harem—affected time spent in various activities, but a female’s contraceptive status also affected time budgets. In “best fit” general linear models attempting to identify individual and group characteristics that account for variation in the proportion of time spent in grazing and standing, a female’s contraceptive status and an interaction involving contraceptive status and a harem male’s identity had significant effects, as did total harem size and the interaction of male identity and total harem size. In general, PZP-treated females and females in large harems graze less and stand more than non–PZP-treated females and females in smaller groups, but these effects are related to the particular males with which they interact (Madosky et al., in review).
In a study of liquid and pelleted PZP in three populations of horses in the western United States, Ransom et al. (2010) found no effect of treatment on activity budgets, but they did find that treated females engaged in significantly more reproductive behavior (0.05 behavior per hour in control mares versus 0.11 behavior per hour in treated mares), which could be expected with a contraceptive that causes females to cycle repeatedly during the breeding season. Powell (1999) also found no difference in activity budgets between mares currently on PZP and those not currently on PZP. Nuñez et al. (2009) saw significantly more sexual or courtship behavior in treated mares than in controls outside the breeding season but also cited data on other temperate equids that showed that out-of-season cycling is known to occur. Powell (1999) found a nonsignificant trend for currently treated mares to engage in more social behavior overall; however, when only sexual behavior was considered, there was no effect of current contraception status on behavior (Powell, 2000). Turner et al. (1996) did not discern any differences in reproductive behavior between liquid-PZP– treated burros and untreated burros, but they did not provide quantified behavioral data. No other studies of PZP contraception in burros have been published.
The effects of liquid PZP on harem stability in horses have been studied in Nevada during breeding and nonbreeding seasons by Gray (2009) and on Shackleford Banks during the nonbreeding season by Nuñez et al. (2009) and during the breeding season by Madosky et al. (2010). Stability was also assessed on Assateague Island by National Park Service staff (A. Turner, Assateague Island National Seashore, email communication, December 13, 2011). The studies on Shackleford Banks suggest that PZP is associated with increased harem-changing by mares, whereas the Nevada and Assateague studies found no differences between treated and untreated mares in harem-changing. The studies all differ in methodological approaches, definitions of treated and untreated animals, and ecological and social contexts. No studies have been able to control all the factors that could affect harem stability in the field, which could include age, pregnancy status, characteristics of other mares and stallions in the harem, distribution of resources, stallion turnover rates,