YOU could call it a batty idea, but bats seem to be more closely related to horses than cows are.

Once thought to belong to the same group as primates, bats actually belong to the super-order Pegasoferae, which contains horses, cats and dogs, cows, whales and hedgehogs. Within this group, bats were thought to be only distant cousins to horses, but DNA analysis suggests that only cats and dogs are more closely related to horses than bats are (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0603797103).

“I think this will be a surprise for many scientists,” says Norihiro Okada at the Tokyo Institute of Technology, Japan. “No one expected this.”

Okada and his colleagues looked at genetic mutations caused by retroposons, lengths of DNA that can copy themselves into RNA and then reverse-copy themselves back into DNA at a different location on a chromosome. Closely related species share more of these mutations than more distant relatives. The analysis by Okada’s team forces a rethink of the relationships of many mammalian orders, which are currently classified by morphological and nuclear DNA sequence data.

“We need to look at fossils from a new point of view, because there must have been a common ancestor of bats, horses and dogs,” Okada says.