Rigor Vitae: Life Unyielding

Wednesday, June 18, 2008


A good chunk of the past year's blog posts have covered the current global frog decline; still, I make no apologies. At the moment, anurans are crashing harder than any other animal group. But lest anyone infer that all is well in the rest of the animal kingdom, let's take a glimpse at another taxon, and what better direction to glimpse than toward the order Chiroptera, the bats? Few of us take notice of bats, but they're all around us. Aside from rodents, they are the biggest, most diverse mammal order, with over a thousand species and a nearly global distribution. They're only absent from a few small, remote islands and the polar regions, and they often occur in huge numbers. At over 100 million, the Mexican Free-tailed Bat (Tadarida brasiliensis) is one of the US' most plentiful mammals, despite being restricted to the southern third of the lower 48. Even so, bats' nocturnal habits and mostly small size and secretive nature keep them out of sight and, for most of us, mind. In temperate latitudes, bats are small, insectivorous creatures that devour many tons of insects every night, but as one moves toward the equator, they become far more diverse in form and behavior. In the tropics, they have evolved to feed on fish, birds, and other vertebrates—even on blood, as well as fruits and nectar. Many tropical plants, like the Merinthipodium neuranthum feeding a pair of Lonchophylla robustum in the painting above, are dependent on bat pollinators. Many fruits, including Africa and Madagascar's iconic baobabs (Adansonia spp.) rely on fruit-eating bats to disperse their seeds.

Bats' dainty structure makes them exquisitely adapted for flight, but poorly so for leaving fossil records, and bat evolution is not well understood. The oldest known bat fossils, from early Eocene deposits in both Europe and North America, are quite similar to modern forms and don't likely represent the order's roots. Modern bats fall into two large suborders. The Megachiroptera comprises the family Pteropididae, the fruit bats, a group restricted to the Old World, including the biggest bats, the flying foxes (Pteropus spp. - above), whose wings can span a meter and a half. In all, there are 42 megachiropteran genera with around 173 species. The remaining 17 bat families reside in the suborder Microchiroptera. It was long assumed that both groups were derived from a common ancestor, but a recent and controversial theory, based on similarities between megachiropteran and primate brains, proposes that the two bat groups evolved flight independently of each other, spurring a lively and continuing debate—and that's about as far as I care to wade into those shark-infested waters. Whatever their phylogenic trajectory, the first proto-bats probably evolved from tree-dwelling gliders similar to the modern Colugo (Cynocephalus volans - below) of Southeast Asia, probably bats' closest living relative.

The largest microchiropteran family is Vespertilionidae, the vesper bats. This family of 42 genera and about 355 species is distributed globally. Typically small insectivores, vesper bats include most temperate zone species. The genus Myotis, with around 100 species belongs to this family. Aside from Homo, it's the most widely-distributed terrestrial mammal genus, with representatives on every continent save Antarctica, and as far afield as Samoa.

The most diverse family, Phyllostomidae, is restricted to the New World. This group includes the tongue-feeding bats of the subfamily Glossophaginae, like our friends Lonchophylla up top. These nectar eaters are important pollination vectors for many plants, including the crucial Blue Agave (Agave tequilana), from which tequila is manufactured. Other notable phyllostomids include the three vampire bats (subfamily Desmodontidae), with three monotypic genera, and the carnivorous Vampyrum spectrum (below), the largest New World bat, indeed the largest microchiropteran.

The fabulous-looking Old World horseshoe bats (Rhinolophidae) include two of the biggest bat genera, Hipposideros and Rhinolophus (below), with 51 and 69 species, respectively.

The afore-mentioned Mexican Free-tailed Bat belongs to the Mastiff family, Mollosidae, with 16 genera and 86 species. The family, whose tails extend well beyond the interfemoral membrane or patagium, includes some of the most social mammals with colonies that can number in the millions.
The emballonurid, or sac-winged bats comprise 12 genera and 48 species, with pantropical distribution. Most members sport a glandular wing sac near each shoulder that secretes a strong-smelling, reddish fluid. The remaining twelve bat families are small ones, with less than ten species apiece. They include such favorites as the monotypic Craseonycteridae from Thailand, whose sole species, Craseonycteris thonlongyai, is possibly the world's smallest mammal, and the ditypic Noctilionidae, whose two Neotropical species (Noctilio leporinus is pictured above) are common throughout the American tropics, including the islands of the Caribbean, wherever there is still water. Remarkably specialized for catching small fish swimming near the water's surface, they eat little else and show no capacity to forage in any other way. These creatures' sensitive hearing can pinpoint the tiny ripples made by fish swimming near the surface surface by echolocation, then, dragging their long claws beneath them, they rake up their prey, eating it on the wing. The sight of one of these large bats fishing on a foot-and-a-half wingspan, its talons raking the glassy expanse of a moonlit blackwater lagoon, is not soon forgotten. There's our glimpse of general bat ecology and biogeography. The next post will address their current decline.
upper: MARKEA NEURANTHA (1995) acrylic on illustration board 30" x 15" (note--this plant was re-designated as Merinthopodium neuranthum a couple of years after I painted and named this piece.
All photographs by CPBvK; Locations (in order): Maraonsetra, Madagascar; Nusa Tengara, Indonesia; Sarawak, Malaysia; Fortuna, Costa Rica; Flores, Indonesia
lower: FISHING BULLDOG BAT (1997) acrylic on illustration board 15" x 20"


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