A greater mouse-eared bat, in a specially built box, takes in the (polarized) sunset rays before a flight to the home roost.
As is well known, bats use echolocation to do their short-range navigation; less familiar is that at least some bats have an internal magnetic compass that helps them find their way around over the long haul. Now, scientists from the U.K., Germany and Israel have discovered how one species, the greater mouse-eared bat (Myotis myotis), actually calibrates that internal compass—using the polarization patterns of sunlight at dusk (Nat. Commun., DOI: 10.1038/ncomms5488). According to the team, it’s the only known mammal that uses polarization cues in this way.
To suss out the bats’ use of polarization, the team built small, bat-sized boxes equipped with windows of polarizing filters, which allowed them to control the polarization signal that captured bats received. Individual bats were placed in the boxes and exposed to either the natural sunset polarization pattern, or to a 90° shifted polarization direction. The bats were then released, at about 1:00 a.m. local time and about 20 to 25 km away from their roosting sites, and tracked to see how well they were able to find their way home. The team found that the bats exposed to normal sunset polarization were significantly better oriented than those that had been presented with the differently polarized light.
One thing the research hasn’t revealed, however, is just how the bats actually detect the light’s polarization. A wide variety of animals, including some birds, anchovies, bees and even dung beetles, are known to use polarization patterns to help with long-distance wayfinding, and for some of these non-mammalian species scientists have identified retinal structures that may play a role in detecting polarized light. But for bats it remains a mystery—as it does for the only other mammal known to be able to perceive polarized light, humans.