Wildlife contraception and welfare
Contraception as a promising tool for welfare improvements
When resources are scarce, wild animals sometimes die of starvation (Gordon et al. 1988), dehydration, or exposure due to a lack of suitable shelters (Sibly & Hone 2002, Hill et al. 2019), all of which cause significant suffering (Gregory 2008). In many species, the competition for resources is especially strong among juvenile animals (Sol et al. 1998, Ward et al. 2006). Naively, making more resources available seems like it would solve the resource scarcity problem, but increasing the amount of available resources will often lead to higher fertility rates and juvenile survival rates, which eventually leads to larger populations instead (Prevedello et al. 2013, Ruffino et al. 2014), such that the competition for resources does not decrease in the long term.
One simple approach to find out how many animals are negatively affected by resource scarcity is to measure the fraction of deaths that are directly attributed to starvation. However, such an approach plausibly underestimates the pervasiveness of resource scarcity, due to interactions among different types of causes of death. For example, predation appears to be the most common cause of death for several taxa and life stages (Hill et al. 2019, Collins & Kays 2011), but whilst it is the proximate or final cause when it happens, it may not always be the ultimate cause — the underlying reason the lethal event occurred. For example, an animal that is starving may be prone to taking risks (Anholt & Werner 1995), or too weak to run away, which causes them to be more exposed to predators, making them more likely to be eaten. Therefore, even for some proximate causes of death that do not seem to be related to resource scarcity, there could be an underlying scarcity-related risk factor. In that case, a reduction of one cause of mortality (e.g. predation) might be partially or fully ‘compensated’ by the increase of another cause of mortality (e.g. starvation) such that life expectancy doesn’t change, known as compensatory mortality (Bergman et al. 2015). If everyone who is starving is eaten by a predator just before they would otherwise die of starvation, it would superficially appear like starvation wasn’t a problem at all, if we base our judgment solely on the proximate cause of death (predation in this case). Nevertheless, resource scarcity does appear to be a fairly common phenomenon (Prevedello et al. 2013), and irrespective of the ultimate cause of death, living a life of near starvation is a negative experience and of welfare concern. However, since resource limitations often are restraining population sizes (Prevedello et al. 2013), many populations will simply grow in response to increased resource availability, resulting in a larger population with similar levels of starvation.
Contraception provides one mechanism by which resource scarcity could be reduced, without inducing a corresponding increase in population size (Hecht 2021). Moreover, contraception may improve individual well-being through the alleviation of resource scarcity in other ways as well. For example, contraception could improve the welfare of the offspring by reducing sibling competition for resources (Mendl 1988, Andersen et al. 2011, Hudson et al. 2011). It could also reduce the burden of gestation and childrearing for some parents by allowing them to save energy and invest more in maintaining body condition (Kirkwood 1977, Kirkwood & Rose 1991, Lemaître et al. 2015). The cost of parental care, and thus benefits of reducing it, are dependent on the species and sex of the individual (Goldberg et al. 2020, Santos & Nakagawa 2012), which will cause additional variation in the net welfare effect of contraception.
There are several studies in which the effects of contraceptives on proxies for welfare have been investigated in wild animals (see Supplementary Table 2 in the review by Gray & Cameron 2010), and they seem to show both negative and positive welfare effects. The negative effects that were detected for some species/methods are related to maladaptive social behavior and inflammation at injection sites, among other things (Gray & Cameron 2010). The plausibly positive effects that have been found are related to survival and body weight, which are the types of benefits that we would predict given the model outlined in the previous paragraph. Several of the studies on survival detected increases (see for instance: Twigg et al. 2001, Turner & Kirkpatrick 2002, Williams et al. 2007), but one of them found no effect (Saunders et al. 2002), and two of them observed mixed results (Ransom et al. 2013 and Bromley & Gese 2001). Many studies have looked for changes in body weight, and several of them have found increases in treated females (see Table S2 in Gray & Cameron 2010). Such increases in body weight could serve as a proxy for welfare through improved nutritional status, but it could also be a reflection of changes in satiety and the foraging risk-reward trade off, where the ultimate effect on welfare is less clear. Many studies have had very small sample sizes, lacked control groups, and focused on a narrow set of welfare relevant traits, so it is hard to draw any strong conclusions. However, it seems very likely that the type, target and delivery mechanism of contraceptives have contributed to the observed differences in welfare-related outcomes among studies.
It is important to note though, that wildlife contraceptives haven’t been developed or applied with the goal of improving wild animal welfare by alleviating resource scarcity. As an analogy: even though a squirt gun is ineffective at putting out large fires — it was never intended for that use — water delivered in the right way is nonetheless very useful when there is a fire. Similarly, it is perhaps no surprise that the currently available evidence on the welfare effects is inconclusive, if no one has intentionally tried to improve wild animal welfare with it. It is the compelling theoretical potential that wildlife contraception offers for significant welfare improvement for large numbers of individuals that makes us excited to see further research in this area.
A very brief history of wildlife contraception research
As of 2011 (Kirkpatrick et al. 2011), wildlife contraception had been developed and successfully tested in 85 species of animals (mostly mammals), and work on different forms of wildlife contraception have continued since then (Asa & Moresco 2019). There are several different forms of contraceptives available for wild animals. Vaccine-based contraceptives (so-called immunocontraceptives) like the porcine zona pellucida (PZP) vaccine and the gonadotropin-releasing hormone (GnRH) vaccine were developed for feral horses and other wild mammals in the 1980s, and are still being used today (Asa & Moresco 2019). Steroid hormonal contraception methods have also been investigated, although high costs, toxicity, and problems with delivery (Kirkpatrick et al. 2011) have caused these methods to mostly be abandoned. Surgical sterilization has also been used to reduce fertility in wild populations of animals (Denicola & Denicola 2021), but because of the invasiveness of the procedure, it is likely more detrimental to animal welfare than other methods (Hampton 2017). It is also oftentimes more expensive per treated animal (Kirkpatrick et al. 2011). More recently, contraceptives have been developed for birds (Nicarbazin), such as pigeons and geese, and 4-vinylcyclohexene diepoxide (VCD) developed for rats (See box). These contraceptives prevent fertilization (in the case of Nicarbazin) or development of germ cells (in the case of VCD), and have only recently become available as commercial products. Products like these open up the potential for large-scale application of wildlife contraceptives, given the abundance of pigeons and rats in the world, and the potential for use in other related species.
Future research
Although scientists have conducted wildlife contraception research for decades, the focus of this research has largely not been on the welfare impacts of contraception, most likely because the primary goal has been to reduce population sizes of pest or managed species, rather than to improve welfare. Consequently, more research is needed to address the many unanswered questions regarding the possible connection between contraception and welfare. Examples of such questions include: can we expect better or worse welfare outcomes in certain taxa? If there is strong interspecific competition, will contraception lead to compensatory population growth and resource use by competitor species, eliminating the potential positive effects?
In addition to ecological considerations of contraceptive use, each of the methods requires additional scrutiny. How do different delivery methods, such as oral baits, injections and surgical sterilizations differ in their welfare effects? What are the welfare related side-effects, and in what contexts do they outweigh the possible positive welfare benefits? What are the long-term welfare effects to the treated animals? What are the indirect effects from changes in population sizes on the welfare of individuals belonging to other species?
Finally, the inter-individual dynamics of different animal populations generate additional research questions. For example, do the effects of contraception on welfare differ between solitary and social species? One reason we might expect different welfare outcomes in social and solitary species is that for social species, fewer offspring could affect the collective gathering of resources or protection from outside threats. A smaller group could be suboptimal relative to a larger social group, and contraception could therefore reduce the group size below its optimum and have negative welfare implications. Conversely, individuals of solitary species might mostly experience competitive interactions with conspecifics, where a smaller population size would be more likely to have positive welfare effects through reduced competition.
We outline a few potential projects that could help answer some of the questions regarding the welfare aspects of wildlife contraception:
Project idea 1: Empirically test the welfare effects of a contraceptive agent, with a focus on both the direct and indirect effects on individuals within the population. See an example for such a project on pigeons. Another target could be rodents (using, for example, the product ContraPest) or other species that have high fertility (with potential to improve the welfare of many individuals).
Project idea 2: Empirically or theoretically test whether and how sociality interacts with the potential welfare effects of contraception.
Project idea 3: Empirically test whether different delivery methods have different average welfare outcomes, and whether it is possible to predict which method will suit different types of species.
Project idea 4: Empirically explore the factors that determine perceptions of wildlife contraceptives, and determine public support for making improvements beyond the ‘natural’ welfare-level as opposed to just minimizing harms.
For all of the possible projects mentioned above, studying both adults and juveniles would be highly valuable when possible, given that we expect somewhat different mechanisms to mediate the welfare effects in adults compared to the welfare effects on juveniles. For instance, we would expect any reduction in sibling competition to primarily affect juvenile welfare.
Box 1
When looking for ways to test hypotheses about the welfare effects of contraceptives on wild animals, we want to look for projects where there is a potential for improvement to a lot of individuals. Rats (Rattus spp.) seem highly promising in that regard, given that they have high fertility rates. The reproductive strategy of rats results in a large number of ‘excess’ individuals who will most likely die before they reproduce. By reducing competition, contraception might increase the fraction of juveniles surviving to reproductive maturity, even if there is no change in the size of the adult population. Humans are also already exploring contraceptive interventions for them. ContraPest (developed by SenesTech) is a type of contraceptive that can be used for rats and other rodents in the United States. Testing the welfare effects of ContraPest on rodents, or indeed trying to further cut costs or develop additional types of contraceptives for rodents, seem like very promising avenues for further research.
Pigeons (Columba livia) are another promising target species for studying the welfare effects of wildlife contraceptives, as they are also very common, and there is a commercially available contraceptive for them (OvoControl, produced by Innolytics). The fertility rate of pigeons is lower than the fertility rate of rats, which therefore likely affects the potential for juvenile welfare improvement. Nevertheless, the available contraceptive for pigeons appears to be somewhat cheaper than for rats, as is indicated by my rough cost estimate for applying OvoControl and ContraPest to populations of pigeons and rats, respectively. In addition, public support for pigeon welfare (and other bird species) improvements is likely to be greater than for rats (Jaeger & Wilks 2021), so studying the welfare effects of OvoControl might have greater potential for widespread implementation. Hence, studying the welfare effects of pigeon contraception might be the most promising option, in order to eventually achieve welfare improvements for wild animals through contraception.
While currently available data does not clearly demonstrate welfare improvements from contraceptive applications, there are good theoretical reasons to expect that contraception could be extremely effective under the right conditions. Therefore, if we explicitly try to use contraception to improve wild animal welfare, it has the potential to improve the living conditions of many wild animals, especially in populations where resource scarcity is pervasive. Wildlife contraception holds great potential to improve wild animal welfare, and research to address the key theoretical and empirical questions should be a priority.
