Up until now the prevailing view has been that evolution of terrestrial locomotion in our bats was the result of our long period of isolation and lack of mammalian predators – much the same reasons why flightlessness evolved in many of our native birds. However a new study published in BMC Evolutionary Biology has challenged this hypothesis by comparing the anatomy of the short-tailed bat with the fossil remains of a related extinct species from Australia. This study suggests that terrestrial locomotion evolved in this family of bats before their arrival in New Zealand, and in the presence of mammalian predators.
The lesser short-tailed bat is the only surviving member of the Mystacinidae family of bats, which diverged from other bats between 41 and 51 million years ago. It is likely that New Zealand’s Mystacinidae bats (which also include the now extinct greater short-tailed bat Mystacina robusta) arrived from Australia sometime after this. Fossil remains dating to 26 million years ago of Icarops aenae, a now-extinct species from this family, were recently found at the Riversleigh World Heritage Area in northwestern Queensland.
These fossil remains share several distinctive features of their elbow joint with the lesser short-tailed bat. These features are functionally correlated with walking on the ground, suggesting that Icarops was also adapted for terrestrial locomotion. The presence of shared derived traits in two closely related species, which are absent from more distantly related species, suggests that those traits were present in the common ancestor of the two species. This suggests terrestrial locomotion evolved between 51 and 26 million years ago, and that the ancestral Mystacinidae bats in Australia were already adapted to terrestrial locomotion prior to their dispersal to New Zealand. The New Zealand bats do show a greater development of these specialisations, suggesting that further evolution of terrestrial habits occurred in New Zealand.
These ancestral walking Australian bats would have co-existed with a raft of terrestrial nocturnal predators, putting a dampener on the theory that terrestrial locomotion in bats evolves in response to a lack of predators, much like flightlessness in birds. The authors propose that instead, terrestrial behaviour evolved because it conferred a selective advantage in these species, enabling them to take an opportunistic approach to feeding and have a more diverse diet.
Suzanne J Hand, Vera Weisbecker, Robin MD Beck, Michael Archer, Henk Godthelp, Alan JD Tennyson, & Trevor H Worthy (2009). Bats that walk: a new evolutionary hypothesis for the terrestrial behaviour of New Zealand’s endemic mystacinids BMC Evolutionary Biology, 9 (169) : 10.1186/1471-2148-9-169