Abstract
Abiotic inputs such as photoperiod and temperature can regulate reproductive cyclicity in many species. When humans perturb this process by intervening in reproductive cycles, the ecological consequences may be profound. Trophic mismatches between birth pulse and resources in wildlife species may cascade toward decreased survival and threaten the viability of small populations. We followed feral horses (Equus caballus) in three populations for a longitudinal study of the transient immunocontraceptive porcine zona pellucida (PZP), and found that repeated vaccinations extended the duration of infertility far beyond the targeted period. After the targeted years of infertility, the probability of parturition from post-treated females was 25.6% compared to 64.1% for untreated females, when the data were constrained only to females that had demonstrated fertility prior to the study. Estimated time to parturition increased 411.3 days per year of consecutive historical treatment. Births from untreated females in these temperate latitude populations were observed to peak in the middle of May, indicating peak conception occurred around the previous summer solstice. When the post-treated females did conceive and give birth, parturition was an estimated 31.5 days later than births from untreated females, resulting in asynchrony with peak forage availability. The latest neonate born to a post-treated female arrived 7.5 months after the peak in births from untreated females, indicating conception occurred within 24–31 days of the winter solstice. These results demonstrate surprising physiological plasticity for temperate latitude horses, and indicate that while photoperiod and temperature are powerful inputs driving the biological rhythms of conception and birth in horses, these inputs may not limit their ability to conceive under perturbed conditions. The protracted infertility observed in PZP-treated horses may be of benefit for managing overabundant wildlife, but also suggests caution for use in small refugia or rare species.
Highlights
Phenology of most biological phenomena is influenced by natural abiotic events and is reflected in traits evolving to maximize fitness [1]
All data collection was conducted with the permission of the Bureau of Land Management (BLM) on public lands they administer, and involved routine observations of feral horses, which are protected under The Wild Free-Roaming Horses and Burros Act of 1971 (U.S Public Law 91–195, as amended)
A similar result was reported for horses at Assateague Island National Seashore, MD and VA, USA, where 68.8% of 32 female horses treated for three consecutive years with porcine zona pellucida (PZP) became pregnant 1–4 years after the last treatment [8]
Summary
Phenology of most biological phenomena is influenced by natural abiotic events and is reflected in traits evolving to maximize fitness [1]. Feral horses (Equus caballus) in the northern hemisphere typically begin reproductive cyclicity in early spring and continue until late autumn; we may posit that conception should naturally peak near the longest day of sunlight (summer solstice) and parturition should peak 335–342 days later (duration of equine gestation [5]). This pattern should result in synchrony of the birth pulse with spring, when climate and forage availability for the dam can contribute to increased neonate survival
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