The timid invasion: Behavioural adjustments and range expansion in a non-native rodent

Animal behaviour can moderate or favour biological invasion processes, and the native fauna’s ability to adapt. The importance of behavioural traits for colonization success remains debated, as well as the nature of behavioural traits that facilitate colonization. We investigated behavioural responses associated with risk-taking and exploration, in both non-native bank voles (Myodes glareolus, N = 225) accidentally introduced to Ireland a century ago, and in native wood mice (Apodemus sylvaticus, N = 189), that decline in numbers with vole expansion. We repeatedly sampled behavioural responses in three colonization zones: source (2 sites), expansion edge (4) and pre-arrival(2). Only source and edge zones are occupied by bank voles, for more than 80 years and 1-4 years, respectively. Individuals of both species varied consistently in risk-taking and exploration, and mice had not adjusted their behaviour to the presence of non-native voles, as their behaviour did not differ between the zones. Voles at the expansion edge were initially more risk-averse but habituated to the repeated testing situation, compared to voles that were consistently less risk-averse in the source population, indicating spatial sorting for risk-taking propensity along the expansion front We suggest that in non-native species under high predation pressure, the ability to develop timid phenotypes may thus represent a fundamental component for range expansions.