Dark Banquet (10 page)

Another way that
Diaemus
differs from
Desmodus
and
Diphylla
is by the presence of a pair of cup-shaped oral glands located at the rear of the mouth. When
Diaemus
gets upset (or, as we observed, during dominance hierarchy behavior), the glands are projected forward and they can be seen quite easily when the bat opens its mouth. As it does,
Diaemus
produces a strange hissing vocalization that is accompanied by the emission of a fine spray of musky-smelling liquid from the oral glands. Although a detailed study remains to be performed, the oral glands of
Diaemus
appear to function in self-defense (like the scent glands of skunks) and as a means of communicating information like status, mood, and territorial boundaries to others of its kind.

Besides their actual ability to feed on blood, perhaps the most fascinating of all vampire bat adaptations is one that we observed only once in our colony of
Diaemus.

In 1984, zoologist Gerry Wilkinson reported that vampire bats in the wild commonly share food by regurgitating blood. Wilkinson, who made his initial observations on
Desmodus rotundus,
determined that about 75 percent of the time blood sharing occurred between a mother and her dependent offspring (until about the age of one). In other instances, sharing took place between related or unrelated bats.

Gerry's results indicated that there were several reasons why this behavior occurred. Blood sharing between mothers and newborn pups presumably transfers nutrients and bacteria to the infant's digestive tract. In humans there are normally over two hundred species of bacteria living somewhere on or in our bodies (it's rumored that in some college dorms this number can hit five million species). In any event, these essential microbes (termed
bacterial flora
) are vital components of several physiological processes, most notably digestion.

In that regard, the mammalian small and large intestines (the terms refer to diameter, not length) are home to billions of bacteria that have evolved a number of mutualistic relationships with their warm-blooded hosts. Often referred to as endosymbionts, these bacteria get food and a warm, moist environment in which to live. The mammals reap a number of benefits from the relationship, including the absorption of vitamins B
₁₂
and K, which are secreted by the bacteria as part of their day-to-day functioning.
^*37
Additionally, indigenous bacterial flora inhibit or kill nonindigenous forms, and they also prevent infection by stimulating the immune system to produce antibodies that can cross-react with potentially harmful nonindigenous bacteria, should they appear. In hooved mammals (i.e., ungulates), as well as wood munchers like termites, the presence of certain endosymbiotic bacteria enables their digestive tracts to break down cellulose—the structural protein that makes up the plant cell wall. These bacteria are the prime reason herbivorous creatures are able to digest plant structures like leaves, stems, and wood. Since we don't have these specific endosymbionts, it's also the main reason why this type of “fiber” goes through humans like the vegan version of Roto-Rooter. Young herbivores aren't born with their bacterial flora either but instead obtain them from adults (like their mothers) through regurgitation or by consuming their feces (coprophagia). For this reason, termite “babies” denied their fecal formula are unable to digest wood and quickly starve to death.

Other related studies, by researchers such as Long Island University geneticist Ted Brummel, have shown that symbiotic bacteria increase the life span of fruit flies, even though the bacteria are apparently not involved in the digestion of plant matter.

Blood sharing between related and unrelated vampire bats also occurs on a reciprocal basis; that is, bats that were experimentally starved for one night before receiving blood from another nonrelated individual were more likely to donate blood to that individual when it was starved. This behavior is almost certainly related to the fact that the bats need to acquire a blood meal every night (and will starve to death in two or three days if they don't obtain one). So, over the course of their long lives (up to twenty years), there will presumably be numerous opportunities to receive and share food. The implication here is that
Desmodus
can remember past donors and can also recognize cheaters—those individuals who try to beat the system by rarely sharing blood. It's also interesting to note that although adult males share blood with females and young bats, they do not share with other adult males—which makes perfect sense. Why share food with someone you may be competing with for a mate?

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 under a branch when you feed since bones like the tibia and femur would be loaded under tension rather than compression. Researchers in the 1970s cited engineering models to hypothesize that hanging behavior in bats actually evolved because of thin hind limb bones, and you can demonstrate this concept with another short piece of pasta. Using the thumb and index finger (of both hands this time), gently pull on the ends of your two-inch length of experimental noodle. Unless you've twisted or bent the pasta by accident, you should be holding a piece of fracture-free fettuccine.
^*41
There—you've just modeled the tensile forces encountered by the hind limb bones of a hanging bat.

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.