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Winter Hummingbird Expedition to Costa Rica
in February 2006

HANGING BY A TOENAIL

On a chilly April morning several years ago we were walking the trails at Hilton Pond Center and happened upon a little cluster of feathers lodged in the crotch of a tree. On closer inspection, we discovered the feather ball was actually a fluffed-up Ruby-throated Hummingbird, and as we reached up and touched the bird it felt cool--not the usual 105 degrees transmitted to our fingers when we band hot-blooded hummers. At first we suspected the hummingbird--which undoubtedly had just returned from its wintering grounds--had succumbed to a combination of migration exhaustion and less-than-tropical spring temperatures in South Carolina's Piedmont. However, when we removed the bird from the branch and cupped it in our hand it began to vibrate, indicating it was still very much alive. The vibrations gradually increased over the span of about a minute, when the hummer opened its eyes, spread out its wings, and was gone like a shot. This was our first experience with "torpor," a short-term state in which hummers lower their body temperatures 30 degrees or more. Torpor reduces the hummingbird's metabolic output and radiates less heat overnight--a remarkable way to save energy for a tiny animal with a risky surface-area-to-body mass ratio.

We were reminded of hummingbird torpor this week at Hilton Pond when we came across another object in a tree, this time a 1" by 2" tuft of light brown fur. However, instead of a hummingbird it was a tiny bat, its body pressed tightly against a Pecan twig. As we had done with the hummer, we reached to feel the bat; not only was it cold to the touch but the bat also seemed a bit stiff. When we caught a faint whiff of decomposition, we knew the final verdict: The bat, alas, was dead, hanging by a toenail.

All text & photos © Hilton Pond Center

Actually, the bat was hanging by FIVE toenails--the total number of digits on its right hind leg (below right); the left leg was tucked up close to the bat's body. In most bats, the hind toes are equal in length and tipped with long decurved claws--very different from their greatly elongated fingers that provide a framework for the broad expanse of skin that comprises the wing. Unfortunately, our new-found bat had been dead long enough that its wings were set firmly into a collapsed position, and attempts to unfold either wing for closer examination resulted in the membranes shredding. The same was true of the uropatagium, the membrane that stretches between the hind legs. However, since we could still see that the uropatagium completely enveloped the bat's tail, we had at least one character useful in identifying what species our bat might be. (There are, by contrast, bats that are "free-tailed.") In addition, the uropatagium was mostly naked, with fur occurring only close to the bat's body.

After examining the dead bat as it hung on the branch, we decided to bring the carcass back to our lab for closer looks and more photos. By way of caution, we should mention that handling dead bats is not recommended. Our bat likely died of natural causes such as old age, starvation, or failure of some body organ, but there's a remote possibility it could have contracted rabies--although bats seldom succumb to this disease. As a precaution, we handled the animal minimally and washed our hands thoroughly after photographing it. That said, public (and media) fear of rabies in bats is vastly overblown. Even the National Center for Infectious Diseases admits that rabid bats are uncommon and almost always transmit the virus by biting a victim; in other words, "people cannot get rabies from having contact with bat guano, blood, or urine, or from touching a bat on its fur."

After welcoming the dead bat into to our lab at Hilton Pond Center, we set up our lights, desktop tripod, and Canon 20D camera with macro lens and took a series of photos, including the nose-to-nose view above. This shot shows the bat's dog-like snout, its relatively narrow external ears, folded wings, and long, curved thumbs tipped with claws similar to those on the hind toes. Bats use their thumbs to maneuver on and hold fast to a perch--be it a tree branch, attic screen, or cave wall--although most species hang primarily by their hind limbs. This behavior is facilitated by a 90-degree rotation of the hip joint that allows the legs to project sideways and the knee to face nearly backwards. Similar to birds, bats have a leg tendon that tightens as the animal relaxes, causing the toes to close tightly around a perch during sleep.

From the side (above), the elongated nature of the bat's snout is more easily observed. In a live bat, we would have been able to see the animal's beady black eyes, but these seem to have receded, perhaps dried out after death. All our North American bats are microchiropterans--literally translated as "little hand wings"--and most have very small eyes, but these organs are still functional. The macrochiropterans--Old World Fruit Bats--have very large eyes and presumably sharper vision. The photo above also provides a different view of our bat's ear--dark-tipped, somewhat pointed, and with a narrow but prominent tragus at its base.

The bat's tragus (above) may seem insignificant, but it has great value for the bat, especially microchiropterans that echolocate in complete darkness. As they fly, these bats emit ultrasonic waves from their mouths or nostrils, a "biosonar" that bounces off an object and returns the echo to the bat's ear. This signal is transmitted to the brain where in microseconds it is processed to tell the bat how far away it is from on object such as a stationary tree or a moving insect and potential food item. The tragus apparently helps the bat triangulate the vertical position of an object, while its right and left ear are far enough apart to pinpoint an object in the horizontal plane.

The tragus is also useful to naturalists because--in conjunction with the bat's overall size, ear shape, fur and wing membrane color, presence or absence of a tail membrane, and other external anatomy--it's a useful character in helping identify backyard bats, which can be rather puzzling. Ours happened to be an Eastern Pipistrelle, Pipistrellus subflavus, one of the smallest and most widely distributed bats in the eastern U.S., and a species we have seen before at Hilton Pond Center. Eastern Pipistrelles can live for 15 years or so and are essentially nonmigratory, hibernating in small groups in caves and rock crevices. In summer they are more solitary and tend to shun buildings, roosting instead in trees and clusters of leaves. That helps explain the presence of our dead bat in a Hilton Pond Pecan, but we guess we'll never know just how it came to die where we found it, hanging by a toenail.

All text & photos © Hilton Pond Center

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