This statement is supported by the following organisations:
The Mammal Society
The Wildlife Trusts
British & Irish Association of Zoos and Aquariums
Summary
Reintroducing a native species to a landscape where it has disappeared can be one of the most powerful and rewarding tools in nature conservation. However, captive breeding and releases of mammals into the wild can do more harm than good, if the implications of such releases are not researched and addressed responsibly. Release into unsuitable habitat, or into a location that is already at carrying capacity, will cause welfare issues for the released individuals as well as introducing risks to wildlife already present, and finding suitable habitat that has sufficient vacancy is difficult. It is essential that any release of captive-bred mammals is backed with appropriate scientific expertise and careful consideration of the human social landscape into which the species may be reintroduced, to ensure human and wildlife benefits. Captive breeding should be undertaken only where there is a rigorous scientific rationale and careful oversight by trained and qualified professionals to ensure that risks are mitigated as much as possible.
As has been seen in public and media responses to recent stories about projects breeding and releasing harvest mice, captive breeding of animals for release can be an attractive concept for many nature enthusiasts due to the opportunities it gives for direct human-wildlife interaction and the sense of making a tangible impact in the landscape when releasing animals into the environment. This is true of other taxa, but especially the case with mammals due to the popularity of some mammal species as domestic pets. Instances where animals are released into the wild, whether to establish or to strengthen a population of that species, are significantly more likely to garner media coverage than other conservation actions. Releases are almost invariably covered as a good news story, with potential issues or risks unreported. This can lead to a misalignment between the realities of conservation, and public perception of what protecting and restoring species really means. It can also lead to captive breeding becoming an aspirational form of conservation volunteering, and devalue more common and urgent volunteering commitments, such as monitoring wildlife in situ, maintaining or improving habitats, and control of invasive species. Caution is needed to avoid conservation funding, attention or public and political urgency being diverted away from the protection of wild mammals and their habitats.
This is particularly pertinent for small rodents such as harvest mice. Owing in part to their small size, it is sometimes assumed that harvest mice are straightforward to rear in captivity without causing the animals any distress. This has contributed to an increasing trend of non-professional breeding programmes that are well-intentioned, but not designed and managed with sufficient understanding of the potential risks, in particular to compromised genetics, behaviour, disease, cognition and animal welfare.
Detail
There is currently no legal obstacle to the release of captive-bred native mammal species in England, Wales and Northern Ireland that are considered ‘ordinarily resident’ (i.e. currently present species, meaning this would exclude extinct native species such as Lynx) or are not listed under Schedule 9 of the Wildlife & Countryside Act – currently, the only mammals listed on this schedule are the Beaver and Wild Boar
In the case of species without protected status - such as harvest mice where they are native and present in Wales and England - animals can be captured, bred in captivity and released into the landscape without any requirement for licences, provided their welfare needs are met as defined in the Animal Welfare Act 2006. This allows those who are attracted to the idea of captive breeding and release as a personal or group project to do so legally without there being any guarantee of appropriate oversight. In Scotland a licence is required for any animals to be released into the wild, even if native.
In 2016 the IUCN's Species Survival Commission (IUCN SSC) revised existing guidelines originally drafted in 2002 to assist in identifying when ex situ management (including captive breeding) may contribute to species recovery. The IUCN SSC proposes a five-step process: (1) compile a status review; (2) define the role(s) that ex situ management might play; (3) determine the precise nature of the ex situ population in order to meet identified role(s); (4) define resources and expertise and appraise the feasibility and risks; and (5) make a decision that is informed based on the above analysis, and is transparent.
Even threatened species may not always require or even benefit from ex situ management (Snyder et al. 1996), nor do all ex situ populations provide direct conservation benefits (Lacy 2013). Factors that influence whether or not a species is managed in captivity or if ex situ management plays a role in conservation can be complex (Bowkett 2014; Martin et al. 2014), and careful evaluation of the benefits and potential risks is required to determine realistic conservation value (Balmford et al. 1996; Snyder et al. 1996).
Risks from captive breeding
Captive breeding of any wild mammal can introduce significant risks that can be difficult and costly to adequately evaluate, monitor and mitigate. Captive environments impose substantially different selection pressures on animals relative to the wild environments in which they evolved. Negative genetic fitness effects in a captive-bred population are difficult to avoid, due to the limited subset of wild gene diversity typically represented in a selection of animals used for the breeding programme (Frankham, 2008). Given that genetic adaptation to captivity can occur in only a few generations (Christie et al., 2012), avoiding deleterious changes is challenging (Ford, 2002; Frankham et al., 2017), especially in the case of species with fast breeding and maturing cycles such as harvest mice, water voles and other small mammals.
The following risks apply to all captive breeding programmes, and require expertise to mitigate:
Genetic risks
Captive-bred populations starting with a small number of founder animals may suffer a loss of genetic diversity, which can lead to the fixation of non-favourable traits, lower reproductive rates, and reduced overall fitness. Within this limited population, selection may favour traits suited to captivity rather than living in the wild, so that the offspring are less well adapted to survive in the wild.
Disease and parasite risks
In a confined environment pathogens and parasites can spread quickly and be difficult to recognise and treat before significant damage or welfare implications have occurred. Without stringent biosecurity and quarantine processes these can then be carried by released animals and cause serious harm to wild populations. Breeding in environments where other animals such as domestic animals or exotic pets are present can create the potential for the introduction of novel pathogens with the potential for especially severe consequences if released into wild populations.
Behavioural risks
Over time in a captive environment mammals may lose essential survival skills, such as anti-predator aversion, hunting/foraging instincts, or proper social and mating behaviours. Relaxed selection in captive environments can result in trait variance over generations, with small populations being particularly vulnerable (McPhee & McPhee 2012). This can lower their survival prospects when released into the wild. Sterile environments may also be unsuitable for eliciting natural behaviours, resulting in stereotypies and an absence of opportunities to learn behaviours through social learning. The maintenance of cognitive and behavioural skills required for survival requires the provision of complex ex-situ environments and tailored enrichment opportunities (Clarke et al. 2023) that would be challenging to provide in a domestic setting.
Developmental risks
Where a sterile environment is maintained to lower the risk of introducing diseases and parasites, captive animals may not get the opportunity to build up immunity to common wild pathogens as they mature and will be especially vulnerable to these upon release.
Captive diets can be different or more restricted than wild diets, leading to animals being unable to recognise food items in the wild environment upon release, or their jaws and/or gut becoming adapted to the captive diet and therefore unable to process essential food in the wild. The gut microbiome composition of a small mammal has been demonstrated to shift to a captivity-associated state within two-three generations, and were associated with changes in the host species weight and reproductive performance (Williams et al. 2026).
Additional considerations for harvest mice and other small mammals
While harvest mice may survive and breed in captivity in a relatively small space and with little hands-on intervention, this does not mean that by ensuring their welfare and mitigating potential risks that captive breeding is straightforward. The following considerations apply to harvest mice and should also be considered with other species.
Harvest mice mature rapidly, making it likely that in a confined space fathers may mate with offspring, creating an increased risk of genetic defects through inbreeding. Sexing individual animals and separating them into different enclosures is difficult and time-consuming, and requires expertise.
The short generation times of animals such as harvest mice also exacerbate risks of detrimental behavioural, cognitive and physiological changes due to the lack of complexity and stimulus in non-specialised captive living conditions .
Harvest mice can and do fight, and in a restricted environment such as a cage, dispersal is impossible, and this can result in conflict, stress, injury and death of individuals. Juveniles are especially at risk from adult males.
As shown by the Mammal Society’s National Harvest Mouse Survey, harvest mice are widely present where the habitat is suitable, with previous concerns about the species arising largely from a data deficiency that is now being addressed. It is likely that in many sites where absence can be confirmed, there will be a reason why it is unsuitable for harvest mice, or if it is suitable, that harvest mice will find and populate the habitat naturally if it remains so. Releases of this and other prevalent small mammal species are therefore unlikely to be warranted on scientific grounds in the majority of cases, either for the benefit of a local ecosystem or for the health of the national population.
Risks from releases
Conservation translocations carry ecological, genetic, socio-economic, animal welfare and biosecurity risks, which is why following best practice is essential. That is why the IUCN, and the UK and Irish Governments’ current guidance emphasises the need for evidence-based planning, risk assessment, appropriate sourcing, legal permissions, monitoring and long-term evaluation.
Undertaken without forethought and professional expertise, releases of captive-bred animals can lead to a range of issues including (but not limited to) the following:
Harm to existing animals of target species or other species due to competition or conflict with introduced animals.
Welfare issues for released animals if the site(s) does not provide suitable habitat, or if they are vulnerable to threats such as predation, road collision, flood or drought.
Introduction of parasites, diseases or genetic abnormalities into the wild due to captive breeding in unsuitable conditions or from a restricted gene pool.
Artificially inflated prey availability in a focused area of habitat caused by a release, leading to a rise in predators impacting both the introduced population and animals already present.
Disruption of natural, sustainable expansion of the target species into new areas.
Obfuscation of the true state of wild populations and any issues impacting the range, population density or health of the target species, due to records of recently released animals being interpreted as natural range expansion or sustained presence.
Guidelines for translocations and reintroductions, developed and endorsed by a wider range of conservation organisations, can be found here:
Good practice for responsible reintroductions and other conservation translocations — Mammal Society
Recommendations
The Mammal Society strongly recommends that captive breeding for release of any species is only undertaken where extensive research has shown that a release will achieve conservation objectives for that species, where translocation of wild-caught animals (avoiding the need for captive breeding) is not viable, and where extensive groundwork is done to ensure all risks are reasonably mitigated against through careful sourcing of animals, biosecurity during captivity, expert monitoring and management of captive breeding to avoid genetic defects, enrichment to promote normal behavioural development, extensive pre-release tests, and thorough ecological feasibility research to assess potential impacton any release site. Since a release is likely to be aiming to establish a population where none is present, or to strengthen a population that is so low that it is in danger of disappearing, it is vital that the causes of this absence are investigated before a release is considered, as part of the feasibility research phase of planning. Threats to the survival of the species should be understood and addressed before a release is undertaken.
Furthermore, any captive breeding should only be undertaken where there is a specific pre-determined site or sites where sufficient monitoring and other ecological feasibility work have already been undertaken to confirm suitability and value of a release. This allows for a timeline to be in place that can be used to properly plan the captive breeding in a responsible way to mitigate risks outlined above. The Mammal Society strongly advises against captive breeding of harvest mice or other mammal species without a specific timed release at a suitable site already confirmed as the goal for this breeding.
The Mammal Society is strongly opposed to captive breeding in domestic situations of harvest mice and other wild mammals by people who are not relevantly experienced and qualified, due to the impracticability of ensuring the best practice conditions outlined above are met, and the danger that this practice promotes captive breeding of such wild animals as a popular hobbyist activity rather than a focused and justified conservation intervention.
The Mammal Society recommends a change in wildlife licensing regulations in England, Wales and Northern Ireland to match Scotland in allowing wild release of captive-bred native mammals only under licence, with strict and enforced licence conditions that recognise the good practice points above.
Any licensed release should require thorough Disease Risk Analyses to be undertaken, alongside disease screening of animals prior to release, with quarantine and biosecurity measures in place to mitigate risk of introducing pathogens to wild populations.
References
McGowan, P.J.K., Traylor-Holzer, K. and Leus, K. (2017), IUCN Guidelines for Determining When and How Ex Situ Management Should Be Used in Species Conservation. CONSERVATION LETTERS, 10: 361-366. https://doi.org/10.1111/conl.12285
Clark FE, Greggor AL, Montgomery SH, Plotnik JM. 2023 The endangered brain: actively preserving ex-situ animal behaviour and cognition will benefit in-situ conservation. R. Soc. Open Sci. 10: 230707. https://doi.org/10.1098/rsos.230707
Martin, T.E., Lurbiecki, H. and Mooers, A.O. (2014), Economic geography ex situ conservation. Anim Conserv, 17: 104-105. https://doi.org/10.1111/acv.12123
Lacy RC. Lessons from 30 years of population viability analysis of wildlife populations. Zoo Biol. 2019 Jan;38(1):67-77. doi: 10.1002/zoo.21468. Epub 2018 Dec 26. PMID: 30585658.
Crates, R., Stojanovic, D. and Heinsohn, R. (2023), The phenotypic costs of captivity. Biol Rev, 98: 434-449. https://doi.org/10.1111/brv.12913
McPhee, M.E. & McPhee, N.F. 2012. Relaxed selection and environmental change decrease reintroduction success in simulated populations. Animal Conservation 15: 274-282. doi:10.1111/j.1469-1795.2011.00512.x
Snyder, N., Derrickson, S., Beissenger, S., Wiley, J.W., Smith, T., Toone, W., and Miller, B., 1996, Limitations of captive breeding in endangered species recovery: Conservation Biology, v. 10, no. 2, p. 338-348, https://doi.org/10.1046/j.1523-1739.1996.10020338.x.
Weeks AR, Sgro CM, Young AG, Frankham R, Mitchell NJ, Miller KA, Byrne M, Coates DJ, Eldridge MD, Sunnucks P, Breed MF, James EA, Hoffmann AA. Assessing the benefits and risks of translocations in changing environments: a genetic perspective. Evol Appl. 2011 Nov;4(6):709-725. doi: 10.1111/j.1752-4571.2011.00192.x. Epub 2011 Jun 18. PMID: 22287981; PMCID: PMC3265713.
IUCN/SSC (2013). Guidelines for Reintroductions and Other Conservation Translocations. Version 1.0. Gland, Switzerland: IUCN Species Survival Commission, viiii + 57 pp.
Guidelines for reintroductions and other conservation translocations | IUCN Library System
Kim E. Chandler, Eloy Gálvez-López, Stuart Black, Andrew C. Kitchener, Kat Hamill, Bethan Roberts, Sean Doherty, Philip G. Cox; Morphological change in an isolated population of red squirrels (Sciurus vulgaris) in Britain. R. Soc. Open Sci. 1 January 2025; 12 (1): 240555. https://doi.org/10.1098/rsos.240555
Williams, C.L.; Williams, C.E.; King, S.N.D.; Shier, D.M. Captivity Drives Multi-Generational Shifts in the Gut Microbiome That Mirror Changing Animal Fitness. mBio 2026, 17, e03516-25.