When wild animal populations approach their carrying capacities, or the maximum population size the ecosystem can sustain, individuals within these populations experience negative impacts. A common example of these impacts is resource scarcity, which can lead to poor welfare as animals starve and become weakened, increasing their risk of exposure and susceptibility to disease.

In response, lethal control measures, such as hunting, poisoning, and trapping, are often used to manage overabundant wild animal populations. However, these approaches are increasingly seen as unethical, especially in light of non-lethal methods that can achieve the same outcomes.

Non-lethal mechanisms, such as the use of wildlife contraceptives, are a viable alternative for controlling wild animal populations. Much of the scientific literature on contraceptives has focused on possible harms, yet fertility control could have wide-ranging welfare benefits — both for the individuals who receive the treatment and for non-treated animals. These benefits, however, are under-researched.

This short paper provides a theoretical basis for further investigations into the potential direct and indirect welfare benefits of wildlife contraceptives in targeted populations. Based on knowledge of ecological feedback loops and population dynamics, the authors have made hypotheses around the nature of these benefits.

For instance, alleviating competition for resources through a reduction in reproductive rate could:

• Decrease aggression between individuals
• Lower disease transmission
• Increase juvenile survival in populations where juvenile survival is density-dependent (that is, where fewer juveniles survive at higher population densities)
• Increase survival in treated individuals, as the energy that would be used for reproduction can be invested in body condition instead

The authors also explore the contexts in which wildlife contraceptives should be avoided. The contraceptive approach could be less effective for:

• Species whose population sizes are limited by something other than resource scarcity (by predation, for example)
• Species for whom a reduction in energetic investment into reproduction doesn’t lead to increased investment in body condition in adults
• Social and cooperative species for whom survival is linked to group size

The authors recommend research into life history traits that may impede the benefits of contraceptives, such as the degree of parental care, fertility, dispersal patterns, age of maturation, and sociality.

Given the theoretical nature of this paper, an understanding of the practicalities of using contraceptives — such as the scale required for effective intervention and its reversibility — is lacking. For example, potential impacts on non-target species are not addressed, but this would be critical for any real-world application. As such, empirical studies (in which evidence is collected in the field or lab) are required to test the authors’ hypotheses.

Though theoretical, this paper is helpful for wild animal welfare advocates looking to understand the implications of non-lethal methods of population control. Rather than focusing on just the risks, it’s important to consider how contraceptives might actually improve the welfare of individual wild animals while also having benefits at the population level.