Intervention Report: Wildlife Contraception


Although wildlife contraception is under-researched and many specific details are highly uncertain, wildlife contraception has the potential to robustly improve animal welfare in a cost-effective way. Two forms of wildlife contraception, immunocontraception and ContraPest, are discussed. The use of ContraPest by individuals is not recommended at this time, pending further study. Immunocontraception is ready to be deployed by wildlife managers, but due to regulatory and other issues few populations are currently managed using wildlife contraception. The creation of an advocacy movement to support the use of wildlife contraception is briefly discussed.

How It Works

There are two primary forms of contraception that are both effective in wildlife and acceptable from a welfare perspective: immunocontraception and ContraPest.


Immunocontraception uses the animal’s own immune system to prevent reproduction (Gray & Cameron, 2010, p. 45). In response to certain injections, the animal’s body produces antibodies that interfere with reproduction (ibid: 45). Adjuvants– organic or inorganic chemicals, macromolecules, or cells of specific killed bacteria– are typically used to amplify the immune response (Giovanna Massei & Cowan, 2014, p. 4).

While immunocontraceptive vaccines against ova, sperm, and gonadotropins have been developed, the most commonly used vaccine is porcine zona pellucida, or PZP (Gray & Cameron, 2010, p. 45). PZP stimulates the production of antibodies to zona pellucida, the membrane surrounding a mammalian ovum before implantation, which prevents sperm binding to the zona pellucida sperm receptors and thus prevents fertilization (ibid: 45). Some scholars consider ZP vaccines to deliver the best welfare outcomes of any contraceptive (Hampton, 2017, p. 183).

Vaccines against GnRH are also sometimes used (Gray & Cameron, 2010, p. 46). GnRH stimulates production and release of follicle-stimulating hormone and luteinizing hormone and thus plays an important role in reproduction (ibid: 46). When a female animal is vaccinated against GnRH, she enters an anestrous period of varying length (ibid: 46).

Both commonly used immunocontraceptive vaccines appear to be effective across mammalian species, with success rates ranging from 70% to 100% over a period of three to seven years (Gary Killian, Diehl, Miller, Rhyan, & Thain, 2006, p. 68; Jay F. Kirkpatrick, Lyda, & Frank, 2011, p. 44; Miller, Fagerstone, & Eckery, 2013, p. S86; J. G. Powers et al., 2014). However, effectiveness studies are generally conducted on captive animals. In general, vaccines are less effective in the wild than in captivity (J. G. Powers et al., 2014, pp. 653–654). Wild animals often have lower immune response than captive animals, which may be due to physiologic stress, such as poor nutritional status, parasite load, or pathogen exposure (ibid: 654). It is hypothesized that high parasite load may reduce hosts’ immune responses to vaccination (Miller et al., 2013, p. S92). This phenomenon has been noted both in humans and in animal models (ibid: S92). In addition, immunocontraceptive vaccines, like all vaccines, have significant variation in efficacy among different individuals, with some animals having reduced or no immune response (Miller et al., 2013, p. S88).

Immunocontraceptive vaccines take a long time to reduce population sizes. Some estimates suggest that as many as ten years of treatment may be required to cause a significant decrease in a deer herd (Warren, 2011, p. 263). However, the stabilization of a wild horse population occurred within two years (Jay F. Kirkpatrick & Turner, 2008, p. 514). Immunocontraception may be more rapidly effective when used to stabilize a population at a particular size rather than to decrease a population (ibid: 514).


ContraPest is a liquid fertility control bait for rats which has recently become available and which impairs spermatogenesis in males and ovulation in females; it may also be effective in other mammals (Siers et al., 2017, p. 1). ContraPest contains two chemicals, 4-vinylcyclohexene diepoxide and triptolide (ibid: 1).

The compound 4-vinylcyclohexene diepoxide increases the rate of follicular atresia, destroying primary and primordial follicles, depleting the ovary of most existing follicles, and causing ovarian senescence (Fagerstone, Miller, Killian – Integrative …, & 2010, 2010, p. 17). Treated females have similar hormonal profiles to females going through menopause (ibid: 17). Ten days of treatment has some effect, and the effect is complete after thirty days of treatment (ibid: 17). It is being explored for use in dogs and cats (ibid: 17), in addition to its use in rats.

Triptolide is the active ingredient in the traditional Chinese medicinal herb Tripterigygium wilfordii (Witmer et al., 2017, p. 81). It lengthens the interval between estrous cycles and increases the amount of apoptosis (normal cell death) in ovarian secondary follicles (ibid: 81). It may inhibit the expression of estrogen-synthesizing enzymes (ibid: 81). It causes infertility in male rats (ibid: 81).

Potential Benefits

Domain One: Nutrition

While immunocontraceptive vaccines improve body condition in a handful of species in a handful of studies, in general, vaccines have no effect on body condition (Gray & Cameron, 2010, p. 48). The effect of ContraPest on body condition or nutritional status has not been studied.

If the population is maintained below the carrying capacity using contraception, the population would no longer be limited by food. We would expect animals to have abundant food and perhaps have improved body condition and nutritional status, similar to what happens given supplemental feeding. In winters and emergency situations, it would be possible to provide supplemental food without concern about increasing populations to an excessive degree. However, since contraception has never been used to maintain a population below the carrying capacity, this has never been studied.

Domain Two: Environment

Wildlife contraception does not appear to have any effect on the animal’s experience of the environment (air quality, temperature, etc).

Domain Three: Health

The primary effect of wildlife contraception will be on health.

The most obvious benefit of wildlife contraception is that, if large wildlife populations cause human-wildlife conflict, contraception allows humans to control wildlife populations without having to use traps, poisons, translocation, or other forms of control which would cause much more suffering. Contraception is likely to cause less suffering than many other possible kinds of wild-animal control.

If we use wildlife contraception to maintain populations below the carrying capacity, we will no longer need predator species to kill prey to prevent their deaths by starvation. Therefore, it will be possible to expand predator control programs.

There are many theoretical reasons one would expect an effect of wildlife contraception on disease transmission (Gray & Cameron, 2010, p. 50; G. Killian, Fagerstone, Kreeger, Miller, & Rhyan, 2007, p. 268). If animal populations fall below a threshold density, the disease will no longer be maintained in the population (G. Killian et al., 2007, p. 268). Suppressing reproductive behavior eliminates contact associated with courtship, estrous, or intramale competition, thus reducing the risk of disease transmission (G. Killian et al., 2007, p. 268). Fertility control eliminates the risk of vertical (mother-to-child) transmission (ibid: 268). Suppressing reproductive behavior also prevents venereal disease and diseases related to pregnancy (G. Killian et al., 2007, p. 268; Miller et al., 2013, pp. S90–S91). Conversely, if contraception leads to an increase in sexual interactions, disease transmission rates may increase (Gray & Cameron, 2010, p. 50).

However, most studies find no effect on disease transmission rate (Gray & Cameron, 2010, p. 50). In at least one species contraception increased disease transmission rate (Ransom, Powers, Thompson Hobbs, & Baker, 2014, p. 261). Why reality differs from theory in this way is uncertain and should be studied in more detail (see “Areas of Uncertainty” below).

While few studies have examined contraception use in the wild for a long enough period to notice the effects on survival and longevity, three-quarters of studies that did examine it found an increase in survival or longevity (Gray & Cameron, 2010, p. 50). Contraception is particularly likely to improve survival among females (Ransom et al., 2014, p. 261). In ungulates, contraception often improves both survival and health condition (Giovanna Massei, Dave Cowan, Douglas Eckery, USDA APHIS National Wildlife Research Center, & Authors, 2014, p. 221). Wildlife contraception may increase survival by causing animals to spend fewer resources on reproduction and by reducing the level of competition faced by the offspring of fertile animals (Gray & Cameron, 2010, p. 50).

While understudied, the effects of wildlife contraception on survival can be striking. For example, PZP treatment has such an effect that many treated horses began to live past the age of 25, which was never seen before treatment (Gray & Cameron, 2010, p. 50). If this trend holds for other species managed through wildlife contraception, it is likely to be the largest source of improved welfare from wildlife contraception.

Domain Four: Behavior

In general, wildlife contraception has few effects on behavior. However, PZP may permit animals to have more sexual activity without experiencing pregnancy (Gray & Cameron, 2010, pp. 48–49; G. J. Killian & Miller, 2000, p. 286; Miller et al., 2013, p. S87), which may cause them pleasure.

Domain Five: Affective Experience

No studies have directly examined the effects of wildlife contraception on animals’ affective experience. However, I believe the effects will be positive, due to increased longevity and (potentially) decreased likelihood of disease, predation, or malnutrition.

Potential Costs to Welfare

In general, contraceptives are safe and have few effects on welfare (Fagerstone, Miller, Killian – Integrative …, et al., 2010, pp. 22–23; Gray & Cameron, 2010, p. 47; J. F. Kirkpatrick et al., 2009, p. 153).

Domain One: Nutrition

In general, wildlife contraception has no effect on an animal’s body condition or nutritional status. However, animals with abscesses due to immunocontraception (discussed below) may experience poor body condition (Krause, Kelt, Gionfriddo, & Van Vuren, 2014, pp. 20–21). In one study, few deer treated with PZP have lower levels of bone marrow fat, usually depleted in severe cases of malnutrition during winter, in spite of their overall good to excellent body condition (Curtis, Richmond, Miller, & Quimby, 2007, p. 4627); it is uncertain what caused this or what the welfare implications are.

Treatment with VCD, a component of ContraPest, does not affect rat body weight, but VCD-treated rats eat less when food is provided ad libitum (Muhammad et al., 2009, pp. 48–49).

Domain Two: Environment

Wildlife contraception is not believed to have any effect on the animal’s experience of its environment.

Domain Three: Health

The primary negative health consequence of immunocontraceptive vaccines is injection site reactions such as abscesses and granulomas. An abscess is a collection of pus which has built up within the tissue of the body; an abscess is usually painful. A granuloma is a collection of immune cells known as histiocytes which forms when the immune system attempts to wall off foreign substances it cannot eliminate. In humans, granulomas are sometimes painful and sometimes symptomless. The long-term effects of granulomas on welfare in free-living species are unknown, because many animals are not monitored long-term (Hampton, 2017, p. 171).

Injection site reactions may be more common in the wild: for example, captive white-tailed deer never have an injection site reaction, while free-ranging white-tailed deer sometimes do (Miller et al., 2013, p. S92). Deer who have poor general health or a high parasite load may be more likely to experience an injection site reaction (ibid: S92). Remote darting may force hair follicles and surface dirt into the wound, causing an abscess even if the vaccine does not (J. F. Kirkpatrick et al., 2009, p. 154).

A table is presented below of abscess and granuloma rates across species.

SpeciesVaccine% abscess% granuloma% difficulty walkingCitation
VariousPZP~1N/AN/A(Giovanna Massei & Cowan, 2014, p. 4)
ElephantPZPN/A89N/A(Delsink et al., 2007, p. 28)
HorsePZP.5-.7N/AN/A(J. F. Kirkpatrick et al., 2009, p. 154)
DeerPZP0.5N/AN/A(J. F. Kirkpatrick et al., 2009, p. 154)
Various (in zoos)PZP1.3N/AN/A(J. F. Kirkpatrick et al., 2009, p. 154)
CatGnRHN/A33N/A(Giovanna Massei & Cowan, 2014, p. 4)
SquirrelGnRH87N/A36(Krause et al., 2014, p. 18)
ElkGnRH35N/A0(J. Powers, 2011, p. 47)
DeerGnRH44940(J. P. Gionfriddo, Denicola, Miller, & Fagerstone, 2011, pp. 151–152)
DeerGnRH1812N/A(James P. Gionfriddo et al., 2009, p. 181)
CattleGnRH000(Giovanna Massei et al., 2015)
BoarGnRH000(G. Massei et al., 2008, p. 542; Giovanna Massei et al., 2012).

In addition, PZP treatment may result in loss of ovarian function, oophoritis (inflammation of the ovary), and cyst formation (Gray & Cameron, 2010, p. 47). In deer, PZP may result in eosinophilic oophoritis (the accumulation of a certain kind of white blood cell in the ovary, causing inflammation) (Curtis et al., 2007, p. 4628). For humans, ovarian cysts usually cause no pain, but may sometimes cause a dull ache or sharp pain (“Ovarian Pain: Causes, Diagnosis, and Treatments,” n.d.). Oophoritis in humans sometimes causes no symptoms, but sometimes may cause abdominal and pelvic pain or pain during urination or intercourse (“Oophoritis: Symptoms, Causes, and More,” 2017). Purified ZP proteins reduce the risk of ovarian problems related to ZP vaccines (Giovanna Massei & Cowan, 2014, p. 4).

PZP-treated white-tailed does have more estrous cycles than untreated females and may remain sexually active after the end of the usual breeding season (Gray & Cameron, 2010, pp. 48–49; G. J. Killian & Miller, 2000, p. 286; Miller et al., 2013, p. S87). In ungulates, persistent estrus may lead to weight loss, while in carnivores it may result in pathological exposure to reproductive hormones, which causes a variety of negative health consequences (Asa & Porton, 2005, p. 75). If PZP wears off late in the season, a fawn may be born in July or August, putting it at serious risk of overwinter mortality (G. J. Killian & Miller, 2000, p. 287). Repeat estrus may, in some species, result in increased aggressive, following, or chasing behavior on the past of males, with deleterious health consequences (Asa & Porton, 2005, p. 77).

The health effects of ContraPest itself have rarely been studied. However, the health effects of one of its components, VCD, have been well-studied, since VCD is used as a model for human menopause. VCD does not appear to cause generalized toxicity outside of the ovary (Mayer, Devine, Dyer, & Hoyer, 2004). Treatment with VCD decreases bone mineral density (Lukefahr et al., 2012). For peripubertal rats, there is no effect of VCD treatment on hematology parameters, but for adult rats VCD treatment increases neutrophil counts and decreases hemoglobin and hematocrit, consistent with inflammation (ibid: 52-53). Peripubertal rats appear to retain normal renal function, while adult rats had significant increases in creatinine and blood urea nitrogen, which suggests a loss of renal function (ibid: 53). All rats retained normal liver function but had heavier livers (ibid: 53-54). In high doses, VCD is carcinogenic causing forestomach hyperplasia, adenomas and carcinomas, ovarian neoplasms and death (Burd, 2014, p. 24). However, much smaller doses are required for ovarian senescence (ibid: 24). High-dose VCD treatment may cause vomiting for brushtail possums (ibid: 46). Treatment with VCD reduces female rats’ levels of progesterone, plasma testosterone, and plasma DHT (Reis et al., 2014).

Domain Four: Behavior

Contraception deprives an animal of the potentially fulfilling experience of parenting (Hampton, 2017, p. 184).

Preventing natural reproductive behavior, such as courtship and mating, may cause harm to animals, because courtship and mating are generally very pleasurable for reproductively successful individuals (Hampton, 2017, p. 174). (Of course, courtship and mating may themselves cause suffering through e.g. violent competition between males, unwanted mating attempts, and lack of mating success.) GnRH vaccines prevent reproductive behavior such as estrus cycles (Gary Killian, Wagner, Fagerstone, & Miller, 2008; Miller et al., 2013, p. S88). ZP proteins do not cross-react with other tissues and protein hormones (Jay F. Kirkpatrick et al., 2011, p. 42). Therefore, they are “downstream” of many reproductive processes, leaving the animals with reproductive behavior as natural as possible (ibid: 42).

Domain Five: Affective Experience

Remote delivery of contraceptives may cause the animal stress, because they are afraid of being darted again (Jewgenow, 2017, p. 271). However, it is believed that remote delivery systems may minimize suffering, other than that associated with darting injuries or injection site interactions (Hampton, 2017, p. 169).

Treatment with VCD alone, a component of ContraPest, causes female rats to show signs of anxiety (Reis et al., 2014).

Miscellaneous Costs

PZP vaccines are derived from the ovaries of pigs slaughtered in slaughterhouses (Miller et al., 2013, p. S87). For this reason, many animal rights advocates may consider the use of PZP to be unethical. ZP vaccines can be derived from bacteria, but they are generally less effective (ibid: S87).

Areas of Uncertainty

Minimizing the pain and distress to wild animals while controlling populations is a complicated topic (Asa & Porton, 2005, pp. 6–7). Contraception may change social behaviors in ways that are beneficial, negative, or neutral (ibid: 6). It is likely that the effects of psychosocial changes on welfare are species-specific (ibid: 6-7).

For every study that finds a collateral effect of wildlife contraception, there is another study of a different species or fertility control method that finds no such effect (Ransom et al., 2014, p. 262). To understand the use of fertility control on a species, we must think about “species biology, reproductive system, physiology, behavioral ecology, population biology and ecological context” (ibid: 262).

With rare exceptions, wildlife contraception has generally only been studied in medium to large mammals (Jay F. Kirkpatrick et al., 2011, p. 40). Studies are particularly likely to concentrate on the orders Perissodactyla (especially horses), Artiodactyla (especially deer and domestic cows), and Carnivora (especially felids) (Gray & Cameron, 2010, p. 46). Most studies are relatively short-term and only address health concerns, without considering the effects of contraception on the animal’s affective states, experience of the environment, nutrition and ability to perform natural behaviors (Hampton, 2017, pp. 176–177). Much of the research on wildlife contraceptives is confined to captive populations (Asa & Porton, 2005, p. 214).

While most studies found changes in physiology and behavior, these effects may be too small to have any effect on animal welfare (Gray & Cameron, 2010, p. 49). However, few studies directly assess wild animal welfare, such as by studying chronic stress indicators (ibid: 49). Studies rarely consider the welfare of untreated animals such as males, even though contraceptives may also change their behavior (ibid: 49-50). Many studies were performed on captive animals, which do not always generalize to free-ranging wildlife (ibid: 50).

90% of studies of wildlife fertility control focus on individual-level effects (Ransom et al., 2014, p. 262). Only 6.5% consider behavioral or indirect demographic effects on population-level assessment of fertility control, and only about three percent quantitatively model these effects (ibid: 262).

When one prey species becomes less abundant due to contraception, predators may switch to predating on a different prey species (Jewgenow, 2017, p. 273). The effects of this are unknown.

It is unclear why most studies show no effect of wildlife contraception on disease transmission rate (Gray & Cameron, 2010, p. 50), since in theory wildlife contraception ought to decrease disease transmission rate substantially.

A few deer treated with PZP have lower levels of bone marrow fat, usually depleted in severe cases of malnutrition during winter, in spite of their overall good to excellent body condition (Curtis et al., 2007, p. 4627). It is unknown why this might be or what the welfare effects are, if any.

Genetic Effects

Fertility control may have complex genetic effects (Ransom et al., 2014, p. 263). Managers may purposely or accidentally select animals with particular traits, such as ease of capture, for contraception, which over time will reduce the presence of that trait in the population (ibid: 263). Contraception may also favor animals who make certain behavioral decisions, such as reproducing with fertile females or increasing or decreasing aggression (ibid: 263-265). If dominant males are targeted for sterilization, most offspring will be fathered by males who would not have been the primary breeders or who females do not prefer, potentially causing severe genetic consequences (Asa & Porton, 2005, p. 91). If only certain females breed, there may be a risk of inbreeding in future generations (Jewgenow, 2017, p. 273). However, in one study of wild horses, over the course of thirteen years of treatment with PZP, there was no loss of genetic diversity (Jay F. Kirkpatrick & Turner, 2008, pp. 517–518). I believe this is the only long-term study of the effects of contraception on genetic diversity.

Immunocontraceptive vaccines could result in natural selection for individuals that remain fertile because of no or low response to vaccination or compromised immune function (Asa & Porton, 2005, p. 75; Cooper & Larsen, 2006, pp. 823–825; Giovanna Massei & Cowan, 2014, pp. 12–13). If most reproduction happens among nonresponders, it may compound the effects of inbreeding discussed above, particularly if there is a very small population of nonresponders or if nonresponse is heritable (Cooper & Larsen, 2006, p. 826; Magiafoglou, Schiffer, Hoffmann, & McKechnie, 2003, p. 155). Depending on how heritable response to immunocontraceptive vaccines is, it may be possible to select for decreased immune function within only a few generations, potentially placing animals at risk of illness (Ransom et al., 2014, p. 265). Conversely, if variation in immunocontraceptive effectiveness is primarily due to the environment, even huge amounts of immunocontraceptive use will have little effect on future generations (Ransom et al., 2014, p. 265).

Evidence suggests immune function is heritable (Ransom et al., 2014, p. 265). However, immune function is only one component of fertility effects, and underlying components of traits typically have higher heritability than traits themselves (Magiafoglou et al., 2003, p. 153). Natural situations are more stressful and more variable than laboratory situations, in which immune response is most often studied, so nonresponse will be less heritable (ibid: 153). Whether nonresponse to immunocontraception will evolve depends on selection intensity, the frequency of genetic variants, and how genetic variants affect the phenotype: for example, low selection intensity means the spread of a particular gene is not favored very strongly, while very high selection intensity may mean the species is driven extinct before resistance evolves (ibid: 154). Immigration from non-immunized sources may lower the frequency of resistant individuals and limit adaptation (ibid: 154). While immunocontraception resistance probably trades off against something, it is currently uncertain what the fitness costs of immunocontraception resistance are and whether they would prevent it evolving (ibid: 155). Nonresponse appears to be heritable in brushtail possums (Holland, Cowan, Gleeson, Duckworth, & Chamley, 2009).

It is important to monitor the heritability of nonresponse to immunocontraception in both the laboratory and the field (Giovanna Massei & Cowan, 2014, p. 13; Ransom et al., 2014, p. 265). We may also reduce the selective pressure on animals, perhaps by allowing each female to have one child before using contraception or only contracepting a percentage of the population (Ransom et al., 2014, pp. 265–266). Immunocontraception may work best on species with long generation times, where adaptation will take decades or longer (Cooper & Larsen, 2006, p. 825). A multi-vaccine approach may slow the rate of resistance evolving (Magiafoglou et al., 2003, p. 156).

Crucial Considerations for Wildlife Contraception

Right now, wildlife contraception is speculative yet promising. Further research has positive expected value; however, because so little research has been done already, it is very possible that wildlife contraception will turn out to be net-negative or not cost-effective.

Many potential positive effects of wildlife contraception have not been proven through studies. In part, this is because few people are interested in maintaining wildlife populations permanently below the carrying capacity in order to promote their welfare. Of the theoretical possible benefits of wildlife contraception, one (reduction in disease) does not seem to have been borne out in studies; because of this, we might expect other theoretical benefits to not occur.

Juvenile mortality contributes a great deal to wild animal suffering. However, no studies have explored the effects of wildlife contraception on juvenile mortality. If the use of contraception decreases juvenile mortality through a density-dependent compensation process, it may improve welfare.

The effects of wildlife contraception on stress and other biomarkers of wellbeing have not been explored, and may represent one of the most important factors in whether wildlife contraception is worthwhile.

The rate of injection site reactions varies wildly between species, from less than one percent to almost ninety percent. It is unknown how much suffering an injection site reaction causes or which factors predict whether a species will have a high or low rate of injection site reactions. Since injection site reactions are the single most significant side effect of immunocontraception use, it should be studied in more detail.

By far the most striking effect of immunocontraception is its effect on longevity. Immunocontracepted mares may live past 25 years, an age never before seen in wild horses. Unfortunately, few species have been managed with contraception long-term, so we do not know whether the increased lifespan is true only in horses or in other species as well.

There is a high level of uncertainty about the welfare effects of ContraPest. While VCD is fairly well-studied, because it is used as a menopause model, triplotide has barely been studied. There have been no studies on the welfare effects of combining VCD and triplotide. Several side effects of VCD are highly concerning, such as anxiety, inflammation, and loss of renal function. While we encourage further study of ContraPest, at this time we cannot recommend its use by people concerned about wild animal suffering.

Strategy for Expanding the Use of Wildlife Contraception

A Vision for Wildlife Contraception Activism

Wildlife contraception is rarely used. By 2012, management attempts to control free-ranging populations with contraception barely numbered in the double digits, including six wild horse populations, three white-tailed deer populations, and two African elephant populations (Rutberg, 2013, p. S38). Today, fertility management is still not a tool in most wildlife managers’ arsenals (Jewgenow, 2017, p. 268). However, while wildlife contraceptive use is unpopular in some circles, nonlethal animal control in general and fertility control in specific are becoming more popular ((Fagerstone, Miller, Killian, & Yoder, 2010, p. 24)). Conversely, lethal control is becoming gradually more unpopular (ibid: 24). A popular yet rarely implemented program that has the potential to benefit wild animals is an opportunity for activism.

Ideally, wild-animal advocates would begin by advocating for the use of fertility control as an alternative to lethal control of wildlife populations that have conflict with humans. There are several benefits to this approach. If wildlife contraception leads to less suffering than lethal control, it will benefit animals directly. The use of wildlife contraception on more populations will allow us to better understand the effects of contraception in the wild. Studies can be conducted that will resolve areas of uncertainty, such as the effects of wildlife contraception on stress, juvenile mortality, longevity, genetic diversity, and other species. Wildlife contraception to prevent human-wildlife conflict avoids many common criticisms of wild-animal welfare interventions, because it’s an alternate way of doing something people were going to do anyway.

Wildlife contraception is an especially useful tool for managing long-lived, low-fecundity species (Giovanna Massei & Cowan, 2014, p. 12) and small, closed populations of species which are not promiscuous breeders (Ransom et al., 2014, p. 262).

Expanding wildlife contraceptive use is likely to promote the development of better contraceptives. Drug companies generally require sales of millions of doses a year for research to be profitable, and few wildlife contraceptives will reach that level of sales (Asa & Porton, 2005, p. 215). For that reason, wildlife contraceptives are generally developed by nonprofits or the government, or are developed for use in domestic or companion animals (ibid: 215). By creating a market for wildlife contraceptives, we can encourage the development of drugs which meet the specific needs of free-living wildlife. Expanding the use of wildlife contraception may encourage the development of more cost-effective means of delivery, such as genetically modified self-sustaining infectious vectors (Giovanna Massei & Cowan, 2014, p. 6).

Over time, assuming that wildlife contraception proves promising, we could expand the use of wildlife contraception. Most forms of wildlife contraception are not species-specific. If you put out an oral immunocontraceptive vaccine (for example), pretty much any mammal who eats it will have lower reproductive potential. This is highly speculative. It is unknown what the ecological effects of maintaining wildlife populations below the carrying capacity are. Since contraceptives are more effective in some species than others, the widespread use of contraceptives may change relative abundances, species interactions and ecosystem function in unknown ways.

Nevertheless, the widespread use of contraceptives has two major advantages. First, it is likely more cost-effective than other ways of managing wild-animal populations, since you don’t have to treat each animal individually. This is a particular advantage for managing large populations of small mammals. Second, contraceptive use preserves much of the “naturalness” of nature. Many people have moral intuitions that rule out heavy management of wild animals that turns all of nature into a sort of zoo. Wildlife contraception allows animals to live ordinary animal lives, but perhaps happier ones.

The Regulatory Environment

Unfortunately, the very popularity of wildlife contraception may make research more difficult. At present, wildlife management is generally considered to be synonymous with hunting (Rutberg, 2013, p. S40). The general public generally supports hunting as a management tool that prevents greater harms such as starvation, but is leery of purely recreational hunting (ibid: S40). Wildlife management agencies have control over scientific research conducted on wildlife (ibid: S40). There have been several cases of wildlife management agencies delaying or denying wildlife contraception research applications or placing so many limitations on them that research is impossible (ibid: S40). They may also spread misinformation to the media (ibid: S40-S41). (It is perhaps not a coincidence that one of the most successful uses of wildlife contraception is on wild horses in America, who are not hunted.)

As of 2010, only the United States had registered wildlife contraceptives (that is, permitted them to be used for purposes other than research) (Fagerstone, Miller, Killian, et al., 2010, p. 20). Wildlife contraceptives are currently regulated by the EPA (ibid: 20). To register a product, a company must submit a series of studies on product chemistry, toxicity, efficacy, harms to non-target species and environmental fate (ibid: 20).

The registration process makes wildlife contraceptive use more difficult in several different ways. It’s expensive: registering a GnRH vaccine cost between $200,000 and $500,000 (National Research Council, Division on Earth and Life Studies, Board on Agriculture and Natural Resources, & Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program, 2013, p. 130). The registration process sometimes takes several years (Fagerstone, Miller, Killian, et al., 2010, p. 20). Currently approved wildlife contraceptives may only be used by certified pesticide applicators, because of worries about animal welfare, hazards to the applicators, and inappropriate use or use on nontarget species (ibid: 20). This likely makes contraceptives more expensive to use.

Even a registered contraceptive faces regulatory hurdles before it can be put into use. Seventeen American states have specifically banned administering fertility control without a permit from the state wildlife department (Eisemann, O’Hare, & Fagerstone, 2013, p. S50). Many others ban fertility control without a permit without a specific law allowing them to do so (ibid: S50).In all states, research on contraceptives in free-ranging wildlife is legal with the permission of the wildlife department (ibid: S50). Some states, under public pressure to expand the use of fertility control, have begun to create a permitting process for this research, which often requires an explanation for why hunting and other traditional management processes aren’t appropriate (ibid: S50). While this paper is from 2013, I contacted one of the authors and she says it continues to be an accurate description of the state of wildlife contraception regulation (O’Hare, 2018).

While wildlife contraception is popular, as far as I can tell there is no advocacy group pushing specifically for the use of wildlife contraception. For this reason, hunters and wildlife managers can put up barriers to wildlife contraceptive use without meaningful challenge. I believe an advocacy group supporting the use of wildlife contraceptives has the potential to meaningfully improve the conditions of wild animals.

Popularity of Wildlife Contraceptives

Proponents of wildlife contraception include (J. F. Kirkpatrick, 2007, p. 548):

  • scientists who are intellectually interested in wildlife contraceptives;
  • wildlife managers who wish to expand their options;
  • animal welfare advocates who prefer nonlethal control;
  • habitat managers and public health officials who wish to control the environmental and health effects of wild animal overpopulation;
  • zoo managers who face unlimited population growth and limited space; and
  • politicians who believe the public supports wildlife contraception.

Many people, particularly animal rights advocates, have ethical objections to lethal control of wild animals, which means an effective method of nonlethal control is likely to be far more popular (Asa & Porton, 2005, pp. xii–xv). Many people believe that contraception is ethical when it prevents suffering of a sentient being (ibid: 5).

Fertility control supporters often believe that animals have a right to life and either should not be killed or should only be killed as a last resort (Bruce Lauber, Knuth, Tantillo, & Curtis, 2007, p. 126). Fertility control supporters typically prioritize the animal’s quality of life, although they may be willing to preserve an animal’s life even if the animal is suffering (ibid: 126-127). Fertility control supporters prioritize individual animals over ecosystems (ibid: 127-128). Fertility control supporters care about humaneness, protecting other wildlife and pets, political acceptability, and avoiding violence (Fagerstone, 2002, p. 5). They also tend to believe contraception is effective (ibid: 5). Proponents of wildlife contraception care about the suffering of deer (Kreeger, 1997, p. 252).

Opponents of wildlife contraception include (J. F. Kirkpatrick, 2007, p. 548):

  • hunters who believe it may replace hunting as a means of population control;
  • state wildlife agencies who are aligned with and rely economically on hunters; and
  • some animal welfare organizations for various reasons.

Some animal rights advocates argue that contraception is unethical because it is interfering with the animal’s control over its own reproductive life (Asa & Porton, 2005, pp. 4–5). Some people argue that hunting is the only ethical method of population control, because hunting is a natural biological process which controls excess populations and because it connects humans to nature (ibid: 5).

Fertility control opponents support killing animals as a solution for human-wildlife conflict even if other options are available (Bruce Lauber et al., 2007, p. 126). Fertility control opponents often believe that contraceptives reduce the animal’s quality of life or that highly abundant animals have low quality of life to begin with (ibid: 127). Fertility control opponents typically prioritize biodiversity and the balance of nature over the welfare of the individual animal (ibid: 127-128). Some fertility control opponents may object to fertility control because it is perceived as getting rid of the animal’s ‘wildness’ or preserving a non-native species that should be eliminated (ibid: 128-129). Opponents of fertility control care about preserving hunting opportunities, low management costs, and quickly reducing population size (Kreeger, 1997, p. 252).

Wild-animal advocates should build alliances with potential supporters of wildlife contraception and prepare arguments to counter opponents.


Depending on other factors, the cost to render a single animal infertile ranges from $25 to $1000 (Giovanna Massei & Cowan, 2014, p. 13). For example, wildlife contraceptives have been used on wild horses on Assateague Island National Seashore since 1994 (Asa & Porton, 2005, pp. 202–203). Here, maintaining zero population growth for a 166-horse herd costs $1500 a year plus the cost of labor (ibid: 203). Other factors may make contraception more expensive: for example, remote delivery of contraception is more expensive in low-density areas (Ransom et al., 2014, p. 262) and becomes more expensive over time as animals become warier (Jewgenow, 2017, p. 271; Ransom et al., 2014, p. 262).

By comparison, the cost-per-life-saved equivalent for GiveWell’s top charity, the Against Malaria Foundation, is about $1,993 (“2018 GiveWell Cost-Effectiveness Analysis — Version 3,” n.d.). Wildlife contraception is not an effective intervention if we consider all costs incurred to any stakeholder. However, it may be an effective intervention if we consider only the cost to advocates (in time, money, etc.) to convince a wildlife manager to use contraception rather than another means of managing wildlife. (It may also be an ineffective intervention; we are highly uncertain about the cost of a political advocacy campaign.)

For this reason, cost-effectiveness estimates for wildlife contraception are highly sensitive to one’s model of costs incurred to people other than the funder. Wild-Animal Suffering Research does not, at this time, have a consensus about how best to analyze those costs.

We expect most of the positive effect of wildlife contraception advocacy at the present time to come from positive flowthrough effects, such as normalizing wildlife contraception and concern for wild animals, increasing funding for wildlife contraception, and encouraging research into wildlife contraception.


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