Immunocontraception in Wild Horses (Equus caballus) Extends Reproductive Cycling Beyond the Normal Breeding Season

Abstract

BackgroundAlthough the physiological effects of immunocontraceptive treatment with porcine zona pellucida (PZP) have been well studied, little is known about PZP's effects on the scheduling of reproductive cycling. Recent behavioral research has suggested that recipients of PZP extend the receptive breeding period into what is normally the non-breeding season.Methodology/Principal FindingsTo determine if this is the case, we compiled foaling data from wild horses (Equus caballus) living on Shackleford Banks, North Carolina for 4 years pre- and 8 years post-contraception management with PZP (pre-contraception, n = 65 births from 45 mares; post-contraception, n = 97 births from 46 mares). Gestation lasts approximately 11–12 months in wild horses, placing conception at approximately 11.5 months prior to birth. Since the contraception program began in January 2000, foaling has occurred over a significantly broader range than it had before the contraception program. Foaling in PZP recipients (n = 45 births from 27 mares) has consistently occurred over a broader range than has foaling in non-recipients (n = 52 births from 19 mares). In addition, current recipients of PZP foaled later in the year than did prior recipient and non-recipient mares. Females receiving more consecutive PZP applications gave birth later in the season than did females receiving fewer applications. Finally, the efficacy of PZP declined with increasing consecutive applications before reaching 100% after five consecutive applications.Conclusions/SignificanceFor a gregarious species such as the horse, the extension of reproductive cycling into the fall months has important social consequences, including decreased group stability and the extension of male reproductive behavior. In addition, reproductive cycling into the fall months could have long-term effects on foal survivorship. Managers should consider these factors before enacting immunocontraceptive programs in new populations. We suggest minor alterations to management strategies to help alleviate such unintended effects in new populations.

Contraception can Lead to Trophic Asynchrony between Birth Pulse and Resources

“…The protracted breeding season of PZP-vaccinated females found in our study, as well as by Nuñez et al [38], indicates that horses that would normally conceive during their first or second reproductive cycle of the year continue to cycle throughout the year and then only conceive when contraception sufficiently decays. This inherently extends the breeding season and challenges the physiological constraints assumed to be regulating fertility.…”

“…This indicates surprising plasticity in birth phenology of temperate latitude horses given the physiological mechanisms thought to be driving reproductive cyclicity. At Shackleford Banks, North Carolina, USA, female feral horses exhibited estrus beyond the normal breeding season after being vaccinated annually with PZP for 1–6 years [38]. Based on the observed birth phenology in our study, this phenomenon occurred in our subjects as well; however, we recorded 81.4% of 328 documented births between March 1 and June 21.…”

“…Parturition phenology for North American feral horses has been shown to peak during May [36]–[38]. We estimated parturition from untreated females peaked May 10–19 across the 3 populations, which was 22–43 days before spring forage availability began to decline, and placed conception roughly 7–15 days before the longest day of sunlight (or 323–332 days from previous summer solstice).…”

Fertility control to mitigate human–wildlife conflicts: a review

“…Possible negative effects of ZP vaccines include speciesspecific ovarian pathology and multiple infertile oestrous cycles (in polyoestrous species), leading to extended breeding season, increased movements, potential late births and disruption of social hierarchy Curtis et al 2007;Kirkpatrick et al 2009Kirkpatrick et al , 2011Nuñez et al 2009Nuñez et al , 2010. Other studies on white-tailed deer and feral horses have reported that treatment with ZP vaccines does not affect time budget, social behaviour and body condition (Miller et al 2001;Hernandez et al 2006;Ransom et al 2010).…”

Understanding population dynamics of feral horses in the Tuan and Toolara State Forest for successful long-term population management

“…As a result, females lose body condition, which in turn decreases offspring survival and female fertility (Nuñez et al 2010). Puberty may be delayed and, in severe circumstances, female reproductive cyclic activity may cease, and increased adult mortality may occur (Gaillard et al 2000a).…”