Social composition of soft-release groups is correlated with survival of translocated gopher tortoises

Abstract

The social structure of translocated animal populations can have important effects on the survival and reproduction of translocated individuals for both solitary and social species. The gopher tortoise (Gopherus polyphemus) is a reptile of conservation concern that is currently experiencing high levels of mitigation translocation in Florida, USA. Individuals live in aggregations of burrows with frequent agonistic, courtship, and burrow-sharing interactions between residents. Given that exposure to many unfamiliar individuals may increase the frequency of aggressive interactions and social stress following translocation, we predicted that tortoises with greater numbers of familiar individuals co-translocated from the same origin site would have higher survival after translocation. To test this, we updated a recently published survival analysis of 2,822 translocated tortoises and 502 identified carcasses from a translocation site in the western Florida panhandle from 2006–2022. After controlling for simultaneous effects of soft-release enclosure identity, release season, release density, region of origin, sex, and size, adding the number of potentially familiar individuals improved model fit and showed increasing the number of familiars reduced the probability of being found dead. This effect was modulated by release density, being apparent only when density was high, suggesting a role for social interactions. This effect was also present only in the first few years after release, prior to the removal of soft-release enclosures preventing dispersal, and was similar in magnitude to previously identified effects of density, release season, and region of origin. We suggest that this effect may result from reduced aggressive interactions or social stress for tortoises with a greater number of familiar individuals in their release enclosures but cannot rule out the possibility of reduced novel pathogen exposure for individuals released with a greater number of individuals from the same source site or other factors that may be confounded with the size of translocated groups. Designing and implementing mitigation translocations to account for social composition of gopher tortoise groups could improve survival in release enclosures.