The unique species with an upside down (or is it right-side up?) view of the world.
02.06.18
The first thing you may notice, when peering into the rolled-up tropical leaves where they like to roost, is that Spixs disk-winged bats look right back at you, face-up.
The second thing youd notice, quickly after, is that each of these little bats is clinging to the inside of the leaf via tiny little suction cups at the bases of their thumbs and ankles.
This is perhaps the only bat that roosts head up, and one of just a handful of species with the unique ability to cling to slick surfaces through suction. Spixs disk-winged bats (Thyroptera tricolor) are common throughout their range from southern Mexico to northern Brazil. Small groups of five or six roost together in the emerging, unfurled leaves of trees in the genus Heliconia, so they move around daily to find new resting spots.
Theyre genial, friendly bats, says Gloriana Chaverri, a researcher at the University of Costa Rica who specializes in the species. Shes interested in deciphering what seems to be a highly organized method of communication between group members, an observation made by chance after she released one of a group captured during a study.
After I released one, it kept flying around me while I still had the other bats in bags, and thats when I started getting curious about whether they were using some calling system to find each other, Chaverri said. I wanted to know how theyre able to stay together when theyre flying, and how they manage to stay together after one member of the group finds a leaf to use as a roost.
Their sounds differ depending on their activities. But Chaverri has identified at least two distinct vocalizations that help groups stick together: a contact call that helps group members maintain proximity to each other, and a homing call for when a suitable roost has been found.
When one member finds a leaf, it enters and starts making a different kind of call, Chaverri said. Her collaborator at North Dakota State University, Erin Gillam, described it like the game Marco Polo, where the rest of the group changes their vocalizations in response until they are all reunited in the new roost.
Roosting groups seem to be made up of related individuals, though nothing is known about where or how far away they go to find mates, or even when their mating season is. Females give birth to a single pup that can weigh up to a quarter of its mothers body weight. But because suction cups dont work so well on fur, instead of clinging to mom with its claws like most other bat babies, they must employ a somewhat more unconventional method to hang on. They hitch a ride on moms nipple.
They attach very strongly, so when the mother starts flying, the pup just hangs from its mouth, dangling from its mothers nipple, Chaverri said.
Preferred foods include jumping spiders and leafhoppers, prey that Chaverri said is fairly unique as a bat food item. Scientists still arent sure why or how the Spixs bats go after these arthropods, but Chaverri hopes her studies of their echolocation may help answer some of those questions. Shes also observed that they often echolocate very, very faintly–so quietly that when shes recording them in a flight cage, the microphone doesnt register any sound until theyre about three feet away.
Lets not forget about their sticky-fingered wings. Unlike your typical kitchen suction cup, which you might have to moisten to create a vacuum seal, the cups on a Spixs bat work primarily through specialized muscles that change the shape of the disk to cling and release.
Courtesy of Christian Ziegler
Juveniles often have a stronger hold than adults, lacking the fine motor control of their elders. Though theres no formal measurement of cling strength of youngsters versus adults, Chaverri likes to demonstrate the strength of these bats grip by putting the suction cup on a persons fingernail. The cup cant be pried off (without injuring the bat), and must be pushed down to the edge of the nail to break the seal.
And the Spixs bat isnt the only bat with this ability. Though the Old World sucker-footed bat (Myzopoda aurita) also features suction cups, the two groups are in no way relatedits a prime example of convergent evolution, where two unrelated groups evolve similar features independent of one another.
However this unique adaptation came about, suction cups are a highly useful quirk that allow these bats to make the best use of the abundance of broad, leafy greens that surround them in their forest homes.