Dark Banquet - Bill Schutt [29]
Besides Diphylla’s fragile hind limb bones, the hairy-legged vampire has another anatomical characteristic not seen in its blood-feeding cousins. In fact, this feature is completely unique to all other animals.
Many bats have a structure called a calcar, which is a bony or cartilaginous extension of their heel bone (the calcaneus). Since bat hind limbs are rotated up to 180 degrees from the typical mammalian condition (picture your knees facing backward), the calcar generally points toward the midline of the body. Its function is to strengthen and straighten the trailing edge of the tail membrane (uropatagium) that spans the space between a bat’s hind limbs. Basically, the calcar increases aerodynamic efficiency by preventing this extra lift surface from flapping around during flight.
As one would expect, the calcar varies in size and shape among the eleven hundred bat species. In Noctilio, the fishing bat, for example, the calcar is a huge, bladelike affair. Noctilio uses it to get its substantial tail membrane out of the way while the gafflike hind limb claws skim the water’s surface for prey.
It’s also no surprise that the calcar is absent in bats that do not have a tail membrane. At least that’s what I thought until I started examining preserved specimens of Diphylla at the American Museum of Natural History where I was working as a postdoctoral research fellow.
After determining that differences in performance existed between Desmodus and Diaemus (“Diaemus doesn’t jump!”), I had started looking to see if these behavioral differences might be reflected by variations in their anatomy. While comparing the hind limbs of the three vampires, I noticed that the calcar was absent in Diaemus and reduced to a flaplike tab in Desmodus. Like I said, no big deal, when you consider that all three vampires lacked a functional tail membrane.*42
The calcar of Diphylla was a completely different story. Not only was it present but it also stood out like a tiny finger. I immediately pulled out several additional specimens—just to make sure I wasn’t just looking at one extremely weird individual. But in each instance, I saw the same digitiform structure. Next, I hit the literature, looking for any mention of Diphylla’s calcar. “Small but well developed” ran the typical description—but nothing more. Finally, I called the vampire bat expert, Scott Altenbach, recalling that he had once maintained a colony of Diaphylla ecaudata in New Mexico. Scott had done the original work on quadrupedal locomotion in the common vampire bat in the 1970s and he’d joined us in Ithaca during our force platform project in 1993.
I remember a conversation something like what follows taking place over another crackling long distance phone line.
“Hey, Scott, did you ever take any photos of Diphylla climbing around the branches?”
“Yeah, but they weren’t branches. We used wooden dowels.”
“Well, check it out and let me know if your bats were using their calcars to grip the dowels.”
“What?”
“I think Diphylla is using its calcar as a sixth digit during arboreal locomotion.”
(Long pause)
“Scott?”
“I’ll go get the photos.”
(Sound of phone dropping)
Basically, what I’d proposed was similar to the story of the panda’s thumb (popularized in an essay by Stephen J. Gould). The giant panda (Ailuropoda) feeds on bamboo leaves that it strips off branches, apparently with the aid of its opposable