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The efficacy of contraceptive treatments has been extensively tested, and several formulations are effective at reducing fertility in a range of species. However, these formulations should minimally impact the behavior of individuals and populations before a contraceptive is used for population manipulation, but these effects have received less attention. Potential side effects have been identified theoretically and we reviewed published studies that have investigated side effects on behavior and physiology of individuals or population-level effects, which provided mixed results. Physiological side effects were most prevalent. Most studies reported a lack of secondary effects, but were usually based on qualitative data or anecdotes. A meta-analysis on quantitative studies of side effects showed that secondary effects consistently occur across all categories and all contraceptive types. This contrasts with the qualitative studies, suggesting that anecdotal reports are insufficient to investigate secondary impacts of contraceptive treatment. We conclude that more research is needed to address fundamental questions about secondary effects of contraceptive treatment and experiments are fundamental to conclusions. In addition, researchers are missing a vital opportunity to use contraceptives as an experimental tool to test the influence of reproduction, sex and fertility on the behavior of wildlife species.Reproduction (2010) 139 45-55

Context. Contraception is increasingly used as a management technique to reduce fertility in wildlife populations, but the feasibility of contraceptive formulations has been limited until recently as they have required multiple treatments to achieve prolonged infertility.Aims. We tested the efficacy and evaluated potential side effects of two contraceptive formulations, a porcine zona pellucida (PZP) formulation, SpayVac®, and a gonadotrophin releasing hormone (GnRH) formulation GonaCon-B™, in a population of free-roaming feral horses (Equus caballus). Both formulations were developed to provide several years of infertility with one injection. 2Methods. Females were treated in June 2005 with either GonaCon-B™ (n=24), SpayVac® (n=20), adjuvant only (n=22), or received no injection (n=18). Females were monitored for fertility status year round for three years post treatment.Key results. Both contraceptive treatments significantly reduced fertility for three years.Fertility rates for GonaCon-B™ mares were 39%, 42% and 31% respectively and 37%, 50%, and 44% for SpayVac® mares. During the same seasons, 61%, 67% and 76% of control females were fertile. We found no significant effects from contraceptive treatment on the sex ratio of foals, birthing season, or foal survival.Conclusions. These results demonstrate that both vaccines are capable of significantly reducing fertility for several years without boosters.Implications. Contraceptive vaccines examined in this study represent a useful tool for the management of feral horses, due to their being efficacious for three years in the absence of booster immunisations.

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