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 behaviour 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 behaviour 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.