How sucker-winged bats hang on

In the tropical forests of Madagascar, there lives a very peculiar kind of bat. While most bats roost by hanging upside-down from cave ceilings or tree branches, the Madagascar sucker-footed bat (Myzopoda aurita) holds itself head-up thanks to a set of adhesive pads on its wings. Nor is it the only bat to do so. Thousands of miles away in the jungles of Central and South America, Spix's disk-winged bat (Thyroptera tricolor) does the same thing, but how do their sucker pads work, and why do they choose to roost in a different way from all other bats?

Previous studies have established that the disk-winged bats of the Neotropics hang on thanks to the suction produced by the suction-cup-like organs on their wrists and ankles, and while they can hang upside-down they probably hang head-up so that they can quickly escape if their roosts are disturbed. Almost nothing was known about how the two species of Madagascar "sucker-winged" bats adhered to their roosts, however, especially since they have flat pads instead of full-blown suction cups. To solve this mystery, researchers Daniel Riskin and Paul Racey studied Myzopoda aurita in the wild, and they published the results last year in the Biological Journal of the Linnean Society.

After observing 28 individual wild-caught bats and the forces their wrist pads exerted when placed on acrylic, Riskin and Racey found that the bats were using something called wet adhesion. The wrist pads of the bats seemed to glisten when they were caught, and even after the pads dried out they would suddenly become moist again (what the fluid was is unknown). Furthermore, one of the tests involved placing the bats on a plate with small holes which would have precluded them from successfully adhering themselves if they were using suction. Since the bats had no problem, it confirmed the suspicion of the scientists that they were using wet adhesion. As the authors stated in their report, "despite their common name, [Myzopoda aurita] do not actually suck at all.

The fact that these bats use wet adhesion may also explain why they roost head-up, the authors hypothesize. If the bats roosted head-down like other bats, the forces on the pads might be just enough to cause the pads to involuntarily detach, and so it appears that these bats have to roost head-up if they want to stay in the same place while they rest. In fact, when the authors placed a bat head-down on an acrylic plate to test this idea it fell off.

This finding may have important implications for how the more specialized sucker-disks of the Neotropical bats evolved, as well. Since we can only look at living species it is impossible to know for sure, but Riskin and Racey hypothesize that Spix's disk-winged bat evolved from an ancestor with wet-adhesion pads like Myzopoda aurita, especially since Spix's disk-winged bats secrete fluid which help them hang on when they cannot produce a tight seal with the suction cups alone. Since other species of bats have special pads to help them move over smooth surfaces, it is possible that such pads arose to help bats move around, were co-opted for a role in roosting thanks to wet adhesion, and then became more specialized into suction cups to allow the bats to hold on more securely, presenting a pattern of evolutionary co-option and specialization.

RISKIN, D., & RACEY, P. (2010). How do sucker-footed bats hold on, and why do they roost head-up? Biological Journal of the Linnean Society, 99 (2), 233-240 DOI: 10.1111/j.1095-8312.2009.01362.x