Abstract
Due to the extirpation of their natural predators, feral horse populations have expanded across the United States, necessitating their management. Contraception of females (mares) with porcine zona pellucida (PZP) is a popular option; however, effects to physiology and behavior can be substantial. On Shackleford Banks, North Carolina, USA, treated mares have exhibited cycling during the non-breeding season and demonstrated decreased fidelity to the band stallion, but PZP's long-term effects on mare physiology and behavior remain largely unexplored. After the contraception program was suspended in this population, we examined how prior exposure to varying levels of PZP treatment impacted (1) foaling probability and foaling dates (a proxy for ovulatory cycling) from 2009 to 2014 and (2) mare fidelity to the band stallion and reproductive behavior during 2013 and 2015. Additionally, we evaluated the effects of time since the mares' last treatment on these factors. Mares receiving any level of prior PZP treatment were less likely to foal than were untreated mares. Among mares that received 1-3 PZP applications, foaling probability increased with time since last treatment before declining, at ~6 years post-treatment. Mares that received 4+ applications did not exhibit a significant increase in foaling probability with time since last treatment. Moreover, previously treated mares continued to conceive later than did untreated mares. Finally, mares previously receiving 4+ treatments changed groups more often than did untreated mares, though reproductive behavior did not differ with contraception history. Our results suggest that although PZP-induced subfertility and its associated behavioral effects can persist after the cessation of treatment, these effects can be ameliorated for some factors with less intense treatment. Careful consideration to the frequency of PZP treatment is important to maintaining more naturally functioning populations; the ability to manage populations adaptively may be compromised if females are kept subfertile for extended periods of time.
Highlights
The extirpation of predator species in North America has precipitated the expansion of free-ranging ungulate populations (Eberhardt et al, 1982; McCullough et al, 1997), escalating human–wildlife conflict (Conover, 1995)
We investigated the likelihood that previously treated mares on Shackleford Banks, North Carolina USA, would return to baseline fertility, foaling dates and behavior after the cessation of contraceptive treatment with porcine zona pellucida (PZP)
Mares received varying numbers of total PZP inoculations, allowing us to more closely investigate the effects of treatment intensity on mare fertility and subsequent effects to mare behavior. This contraceptive has been administered to several mammalian species including, white-tailed deer (O. virginianus) (Miller et al, 1999), elk (C. canadensis) (Shideler et al, 2001), black bears (Ursus americanus) (Lane et al, 2007), bison (Bison bison) (Duncan et al, 2013), and African elephants (Loxodonta africana) (Fayrer-Hosken et al, 2000); these species have evolved an array of reproductive behaviors and social systems, which are vital to their survival and well-being
Summary
The extirpation of predator species in North America has precipitated the expansion of free-ranging ungulate populations (Eberhardt et al, 1982; McCullough et al, 1997), escalating human–wildlife conflict (Conover, 1995). Perhaps the most well-known example is that of increasing white-tailed deer (Odocoileus virginianus) populations across the United States. These deer damage crops, feast on suburban ornamentals, spread disease, and are involved in both vehicle and train collisions (Conover, 1997), resulting in human injury, the loss of human life and costs amounting to more than 1 billion USD of property damage per year (Mastro et al, 2008). Elk (Cervus canadensis) populations have increased in some areas, with severe ecological impacts. Contraceptive management has become increasingly popular (Hobbs et al, 2000; Porton, 2005)
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