March 29, 2021

The government of the United Kingdom (UK) recently approved a plan promoted by conservation and forestry groups to limit gray squirrel populations. The proposed method — delivering an oral contraceptive in hazelnut paste — is a welcome change of pace in the history of UK gray squirrel control. While further research on the welfare impacts of the contraceptive is needed, we are optimistic about the long-term welfare benefits of sustainable populations that wildlife fertility control may facilitate. Even if this new control strategy fails to improve welfare, it could still provide crucial data to inform future efforts.

History of squirrel relations in the UK

Gray squirrels (Sciurus carolinensis) were first introduced to the UK from the United States in the late 19th century. At that time, red squirrels (Sciurus vulgaris) were ubiquitous, sometimes even regarded as pests to be culled. Since their introduction, gray squirrels have multiplied to become the UK’s most common squirrel species. The numbers and range of red squirrels have declined dramatically over the same period (Rushton et al. 2006).

Food is one point of potential conflict between red and gray squirrels. Red squirrels are adapted to arboreal living, finding food in forest canopies. This may put them at a competitive disadvantage in urban environments shared with gray squirrels, who are more effective at ground foraging (Kenward and Holm 1993). Evidence is mixed on how and to what extent gray squirrels actually compete against red squirrels for resources, as opposed to simply coming to outnumber them due to innately higher birth rates or a greater ability to take advantage of novel resources and habitat. While gray squirrels do sometimes steal food from or harass red squirrels, adult red squirrels appear not to alter their behavior or food intake significantly in their presence (Wauters et al. 2002). Juvenile red squirrels, however, may disperse away from gray squirrels to claim territory (ibid.).

Disease adds another dimension to the interactions between gray and red squirrels. Squirrelpox (Poxviridae) appears to be a major factor in the decline of red squirrel populations (Rushton et al. 2006). Gray squirrels are partially resistant to the virus, making them effective vectors (Bruemmer 2010; Schuchert et al. 2014). Indeed, squirrelpox has reduced red squirrel numbers most dramatically in areas where they overlap with infected gray squirrels (Rushton et al. 2006).

The relative importance of these factors to the decline of red squirrels in the UK is not yet clear (Rushton et al. 2001). Still, there is an apparent conflict of interest between the squirrels of both species which is not straightforward to adjudicate. It is unfortunate that the success of gray squirrels contributes to suffering in another species. But the fact that many gray squirrels have managed to thrive in human-dominated landscapes is a good thing for those squirrels. Efforts to improve conditions for red squirrels should not be at the cost of gray squirrel welfare.

Until recently, the approach advocated by many red squirrel conservationists was to kill gray squirrels with guns and poison. Not only did these techniques fail to reduce the gray squirrel population, but the poison also caused untold suffering to non-target wildlife and even pets. Using contraception instead could make life easier for red squirrels and gray squirrels at the same time by reducing the pain of disease and the stress of competing for food and space.

Welfare effects of wildlife contraception

The contraceptive agent the UK government plans to use on gray squirrels is based on the same physiological principles as the immunocontraceptive known as GonaCon. Both target circulating gonadotropin-releasing hormone (GnRH) through antibodies in an animal’s bloodstream, but GonaCon is delivered by injection whereas the formulation intended for gray squirrels is edible.

GonaCon has been tested as a control method for other species, including prairie dogs, where it proved to be effective at reducing birth rates during a field trial. Fewer births could be good for the health of would-be mother gray squirrels. Lower birth rates should also cause the overall population size to begin to decline. This could improve the welfare of all squirrels, gray and red, by reducing competition and possibly disease spread within and between species. Few studies have looked at long-term effects of fertility control on small-bodied wild animals, like squirrels. However, Williams et al. (2007) found that sterilization increased the life expectancies of female European rabbits (Oryctolagus cuniculus) by approximately 67%. Non-sterilized rabbits of the same population showed a smaller positive effect on survival rates, presumably reflecting a release from competition. Fleas were also less abundant in the treated populations, though this did not actually lead to fewer cases of disease, possibly because the population remained large due to the improved survival rates compensating for lower fertility.

While fertility control is very likely superior to lethal control in terms of individual welfare, some questions still remain about potential side effects on animals’ health and behavior. For example, a study of injected GonaCon in North American fox squirrels (Sciurus niger) found that treated individuals had lower body mass and abscesses on their bodies near the injection site (Krause et al. 2013). Several other studies (reviewed in Gray and Cameron 2010) have reported similar effects using injected contraceptives, though these seem to be related to the adjuvant used with the drug, rather than the immunocontraceptive itself. The contraceptive planned for treatment of UK gray squirrels differs from these in its precise formulation, and is delivered orally rather than injected. Potential adverse side effects of the contraceptive must also be weighed against evidence for benefits from reduced competition and avoiding reproductive costs (Massei and Cowan 2014).

While the results of this existing research are promising, much more information is needed about the overall effectiveness of contraceptives as a welfare intervention for abundant small-bodied animals. To help fill this research gap and to set a standard for future research, Wild Animal Initiative has developed a project to validate the efficacy and test the welfare impacts of an oral contraceptive called OvoControl for urban pigeons. Beyond the direct health effects of the treatment, this project will involve monitoring likely proxies for welfare — including multiple indicators of stress and health — over a two-year period. We are currently seeking funding for this project.

References

Bruemmer, C. M., Rushton, S. P., Gurnell, J., Lurz, P. W. W., Nettleton, P., Sainsbury, A. W., Duff, J. P., Gilray, J., & McInnes, C. J. (2010). Epidemiology of squirrelpox virus in grey squirrels in the UK. Epidemiology and Infection, 138(7), 941-950. https://doi.org/10.1017/S0950268810000816.

Gray, M. E. & Cameron, E. Z. (2010). Does contraceptive treatment in wildlife result in side effects? A review of quantitative and anecdotal evidence. Reproduction, 139(1), 45-55. https://doi.org/10.1530/REP-08-0456.

Kenward, R. E. & Holm, J. L. (1993). On the replacement of the red squirrel in Britain: a phytotoxic explanation. Proceedings: Biological Sciences, 251(1332), 187-194. https://www.jstor.org/stable/49850.

Krause, S. K., Kelt, D. A., Gionfriddo, J. P., & Van Vuran, D. H. (2013). Efficacy and health effects of a wildlife immunocontraceptive vaccine on fox squirrels. Wildlife Management, 78, 12-23. https://doi.org/10.1002/jwmg.635.

Massei, G. & Cowan, D. (2014). Fertility control to mitigate human-wildlife conflicts: a review. Wildlife Research, 41, 1-21. https://doi.org/10.1071/WR13141.

Rushton, S. P., Lurz, P. W. W., Gurnell, J., & Fuller, R. (2001). Modelling the spatial dynamics of parapoxvirus disease in red and grey squirrels: a possible cause of the decline in the red squirrel in the UK? Journal of Applied Ecology, 37, 997-1012. https://doi.org/10.1046/j.1365-2664.2000.00553.x.

Rushton, S. P., Lurz, P. W. W., Gurnell, J., Nettleton, P., Bruemmer, C., Shirley, M. D. F., & Sainsbury, A. W. (2006). Disease threats posed by alien species: the role of a poxvirus in the decline of the native red squirrel in Britain. Epidemiology and Infection, 134(3), 521-533. https://doi.org/10.1017/S0950268805005303.

Schuchert, P., Shuttleworth, C. M., McInnes, C. J., Everest, D. J., & Rushton, S. P. (2014). Landscape scale impacts of culling upon a European grey squirrel population: can trapping reduce population size and decrease the threat of squirrelpox virus infection for the native red squirrel? Biological Invasions, 16, 2381-2391. https://doi.org/10.1007/s10530-014-0671-8.

Wauters, L. A., Tosi, G., & Gurnell, J. (2002). Interspecific competition in tree squirrels: do introduced grey squirrels (Sciurus carolinensis) deplete tree seeds hoarded by red squirrels (S. vulgaris)? Behavioral Ecology and Sociobiology, 51, 360-367. https://doi.org/10.1007/s00265-001-0446-y.

Williams, C. K., Davey, C. C., Moore, R. J., Hinds, L. A., Silvers, L. E., Kerr, P. J., French, N., Hood, G. M., Pech, R. P., & Krebs, C. J. (2007). Journal of Applied Ecology, 44(2), 291-301. https://www.jstor.org/stable/4539244.