Dark Banquet - Bill Schutt [28]
There is evidence that both Diphylla ecaudata and Diaemus youngi also share blood (as I mentioned, we saw this behavior once in two captive specimens of Diaemus).*38 Unlike Wilkinson’s in-depth study of Desmodus, however, this behavior in Diphylla and Diaemus has yet to be studied in detail.
This brings up an important point regarding original research—and one that I found quite helpful when I was just starting out in the field. I often advise students who are looking for research projects to seek out classic studies (like Gerry Wilkinson’s) and then think about applying similar techniques to other organisms that have yet to be studied. Likewise, if the original research was done years earlier, new studies on the topic may warrant publication if the new researcher employs technology or methods that weren’t around in the past (or asks questions that wouldn’t have been asked in “the old days”).†39
Before leaving vampire bats to their bloody business, it’s only fair that I mention the third genus, Diphylla ecaudata, the hairy-legged vampire bat. So named for the frill of hair that borders the back margin of its hind legs, Diphylla is thought to exhibit the most primitive anatomical characteristics for its group.*40 In other words, scientists believe that Diphylla has undergone the least amount of evolutionary change from ancestral vampire bats—whatever they were.
One such primitive characteristic is that most bats (including Diphylla) have extremely thin hind limb bones (i.e., the femur, tibia, and fibula), and by thin, I mean that their diameter is quite small compared to their length. Scientists believe that this is an evolutionary trade-off related to flight. By having thinner, lightweight limb bones, bats have reduced their weight—an important factor for any flier. The downside of the trade-off becomes apparent if you watch a bat moving around on the ground (which is something you generally don’t see very often). In this regard, most of the eleven hundred species of bats can do little more than a clumsy shuffle when grounded, and even those that can walk are anything but graceful. Engineering models have shown that most bat hind limb bones did not evolve to withstand the compressive loads associated with walking. To demonstrate this for yourself, take a two-inch length of uncooked spaghetti and hold the ends between your thumb and index finger. Then bring your fingers together. You’ve just applied a compressive load to a model of a bat hind limb bone. Neat, huh? Now go pick up those pieces of spaghetti before someone steps on them.
As you have already learned, an inability to move about terrestrially is not a problem for Desmodus and Diaemus. These blood feeders are quite adept (and, in the case of Desmodus, even spectacular) as they walk, run, and hop about on the ground.
If you examine the hind limb bones of these two bats, it’s not surprising that compared with Diphylla, they’re thicker in Diaemus (i.e., they have greater diameter to length ratios) and much thicker in Desmodus—where they more closely resemble those of a small terrestrial mammal than they do typical bat hind limb bones. Apparently, stronger limb bones evolved in some vampire bats as they became adapted for current feeding strategies, namely, preying on large quadrupeds like pigs and cows. The evidence that Diaemus was once a terrestrial hunter lies in their robust limb bones, which seem overdesigned for their current arboreal roles. Additionally, Diaemus can scoot along quite well on the ground when it needs to, and it is quite capable of feeding while doing so.
The fragile hind limb bones of Diphylla, on the other hand, are clues to this bat’s arboreal feeding habits; in other words, form reflects function. Unlike the requirements for walking and hopping, you don’t need thick limb bones to hang