Behavioural mechanisms underlying the evolution of cooperative burrowing in Peromyscus mice

While some behaviours are largely fixed and invariant, others can respond flexibly to different social contexts. Here, we leverage the unique burrowing behaviour of deer mice (genus Peromyscus) to investigate if and how individuals of three species adapt their behaviour when digging individually versus with partners. First, we find that pairs of mice from monogamous (P. polionotus) but not promiscuous (P. maniculatus, P. leucopus) species cooperatively construct burrows that are approximately twice as long as those dug by individuals and similar in size to burrows found in the wild. However, the length of burrows built by P. polionotus pairs differs: opposite-sex pairs construct longer burrows than same-sex pairs. By designing a novel behavioural assay in which we can observe and measure burrowing behaviour directly, we find that longer burrows are achieved not by changing individual behaviour, but instead because opposite-sex pairs are more socially cohesive and thus more likely to dig simultaneously, which is a more efficient mode of burrow elongation. Thus, across social contexts, individual burrowing behaviour appears largely invariant, even when the resultant burrow from pairs of mice differs from expectation based on individual behaviour, underscoring the fixed nature of burrowing behaviour in Peromyscus mice.