Pre-release experience with a heterospecific competitor increases fitness of a translocated endangered species

Translocation is a key conservation strategy, with breeding programs increasingly providing animals for release. Despite its growing use, translocation efforts frequently encounter limited success, particularly when captive-born animals are involved. Ex-situ environments may lack cues needed to develop survival behaviors, such as predator avoidance, foraging, and social skills. Although practitioners typically address the lack of predator avoidance through training or predator removal, competitor naivety remains largely overlooked, despite its potential to hinder population establishment by compromising access to resources. We examined the impact of pre-release competitor experience on the Pacific pocket mouse ( Perognathus longimembris pacificus ), one of southern California's most critically endangered mammals. The Pacific pocket mouse is the smallest, and most behaviorally subordinate species in a community of native rodents that compete for resources. We conducted a controlled experiment to test if providing captive-born mice pre-release experience with kangaroo rats ( Dipodomys simulans ) would improve translocation outcomes. We compared fitness and body condition of pocket mice between the two treatments and examined if behavioral change was exhibited before release. Competitor-experienced females gained more weight and had higher survival and reproductive success than controls, with female survival surpassing that of males in both groups. These findings suggest that incorporating competitor experience into conservation breeding programs could improve translocation success. As competition is a common challenge in natural ecosystems, this behavioral intervention could be applicable for a wide range of species. • Conservation translocations often fail due to released animals' behavioral issues. • Captive bred animals are naïve to heterospecific competition, exposure may help. • We test first ever pre-release competitor training, in an endangered small mammal. • Competitor-experienced females had higher fitness than controls and males. • Pre-release competitor experience can substantially improve translocation outcomes.