How did bats get underfoot?

You think snakes on a plane are crazy? Bats! On the ground!

Before humans arrived on New Zealand, the only mammals living there were bat species. One of only two remaining native Kiwi mammals is Mystacina tuberculata, the lesser short-tailed bat.

This bat’s second claim to fame is that it walks. Only one other bat, the vampire bat, does this, and vampire bats don’t spend anywhere near the same amount of time on the ground as M. tuberculata does. That there are no other land mammals in New Zealand has been suggested as a reason that these bats are such ground huggers. This was suggested by analogy with birds, which are often flightless on islands that don’t have large predators. Indeed, New Zealand provides an example here with the kakapo, which Douglas Adams famously described as the world’s largest and least able to fly parrot.

This is an appealing idea, but tricky to test. You can’t really test an evolutionary hypothesis with only one species, and there are no other species in this bat’s family.

This new paper by Hand and colleagues brings in a second species in this family by way of the fossil record. Importantly, the fossil, Icarops aenae, was not found in New Zealand, but in Australia. Australia famously has an abundant array of marsupials mammals, so bats and birds in this environment would be expected to remain mostly skyborne.

Now, even with the help of an online digital skeleton of this species, I’m way out of my depth with the anatomy described in this paper. (Hey, I’m an invertebrate biologist. This has fur and a backbone. Sue me.) But I appreciate the logic of the analysis.

Based on various molecular and fossil evidence, the fossil predates split that leads to the genus Mystacina in New Zealand. Thus, if walking was due to the predator=free environment provided by New Zealand, then you would expect limb morphology typical of other bats.

The fossil limb shows a lot of ground-dwelling adaptations.

It’s not a smoking gun killing the “lack of predators” hypothesis by any means. There are a raft of assumptions about distribution, timing of divergence, that are important in interpreting the results. Plus, it’s entirely possible that a lack of predators allowed an already ground-dwelling group of bats to go even further towards a land loving lifestyle. Nevertheless, it seems that this line of bats may have already touched down (in the ecological sense of their habitat) before they touched down (in the geographic sense) in New Zealand.

Also, these lesser short-tailed bats are good candidates to evolve into the terrifying predators that will outcompete humanity in the distance future, as seen on the television show Primeval. Primeval's predators are fast on their feet descendents of bats that don’t fly.

Which goes to show that New Zealand could be the source of the things that kill us all. ;)

Reference

Hand SJ, Weisbecker V, Beck RMD, Archer M, Godthelp H, Tennyson ADJ, Worthy TH (2009). Bats that walk: a new evolutionary hypothesis for the terrestrial behaviour of New Zealand's endemic mystacinids BMC Evolutionary Biology, 9 DOI: 10.1186/1471-2148-9-169

DOI not working yet; click here for abstract.