Mobility decisions and the predation risks of reintroduction

Predation is a common problem limiting the success of efforts to reintroduce vertebrates. Naı̈ve animals typically suffer high predation rates soon after release despite strong site fidelity in the first few days that would normally reduce predator encounter rates. However, recent theory predicts that low mobility can be associated with high concentrations of odour wastes that are attractive to predators, leading to low survival for individuals that move little. In this paper we test this model for captive-raised Microtus voles reintroduced into suitable habitat in western Finland where they are mainly hunted by chemo-sensing predators. Patterns in vole movement behaviour showed a humped relationship with time: animals moved little soon after release, then made exploratory movements from 3 to 6 days, which then subsided. Predation rates were highest in the first 3 days, with no vole mortalities occurring after voles began substantially moving beyond their release sites. Moreover, voles that survived the 3-week study period were more mobile than voles that were killed by scent-hunting predators, during the initial 3 days and during the final 3 days when voles also had lower mobility. These results suggest that the innate fear response of naı̈ve animals, which limit their movements in an unfamiliar environment, may be counterproductive when reduced mobility leads to waste accumulation that is attractive to potential predators. Consequently, pre-release conditioning of captive-raised individuals may need to consider methods to overcome initial release-site fidelity in order to enhance long-term survival prospects after reintroduction.