A Bat Man in the Tropics: Chasing El Duende

5. Three Hundred Nights of Solitude

Insectivory has always been the most common feeding mode in bats. Fully sixteen of the eighteen families and about three-quarters of all species of bats have evolved a diverse array of foraging styles for exploiting nocturnal insects. Among the insect eaters that occur in a tropical forest, such as Santa Rosa,are species that pursue insects far above the forest canopy (freetailed bats), species that pursue insects in forest clearings and along streambeds (sac-winged bats and some vesper bats),species that hunt for their prey in cluttered vegetation (mustached,funnel-eared,and some vesper bats),and species that pluck insects off vegetation (certain phyllostomid bats). Given their vast numbers and protein-rich nutritional value, it is not surprising that insects are the food of choice for most kinds of bats. Fruits, on the other hand, are routinely consumed by members of only two families of tropical bats—the New World leaf-nosed bats and the Old World pteropodids, or flying foxes. Worldwide, only about 250 species of bats are dedicated fruit eaters (or flower visitors). Why are there so few species of fruit-eating bats, and what are the consequences of being a fru101 5 Three Hundred Nights of Solitude Short-tailed fruit bat eating a Piper fruit. Redrawn by Ted Fleming, with permission , from a photo by Merlin D. Tuttle, Bat Conservation International. givore rather than an insectivore? Over three decades ago the British ornithologist David Snow pointed out that there are both advantages and disadvantages for birds in being a frugivore. I’m sure the same advantages and disadvantages also hold for frugivorous bats. One of the advantages is that fruits “want to be found” by their consumers . Whereas insects are continually trying to escape from their predators , the raison d’être of fruits is to be eaten so that their seeds will be dispersed , optimally to a good germination site. To maximize their chances of being found, most fruits advertise themselves with visual and/or olfactory cues. As we have already seen, bat fruits usually rely on olfactory, rather than visual, cues to attract bats. As a result, some bat fruits (and bat flowers ) can be downright stinky. Another advantage to fruit eating is that fruits are relatively large compared with the average nocturnal insect and often occur in large patches. Patches of fruit may be some distance apart, but once a frugivore finds one, it often has a large mass of food at its disposal. Compared with insectivores, therefore, frugivores spend relatively little time searching for and pursuing their “prey.” From his studies of fruit-eating manakins inTrinidad,Snow found that these colorful understory birds spend a small fraction of each day feeding and have plenty of time left over to concentrate on other biological imperatives, such as reproduction. As a result, lek mating systems, in which males spend long periods each day advertising for females with vocal or visual signals, are especially common in fruiteating birds. Countering these advantages are at least two major disadvantages to eating fruit. The first one is the low average nutritional value of fruit compared with insects.Gram for gram,fruits are poorer in proteins and fats than insects, although they are usually richer in water and carbohydrates. They can also be high in fiber, a nutritionally useless plant material. All in all, then, fruit eaters need to consume larger amounts of food to obtain a given amount of protein or fat than insectivores. A second disadvantage is that there are far fewer ways of pursuing fruit than there are of pursuing insects . As a result, there are fewer food niches for fruit eaters in a tropical forest than there are for insect eaters.This means that most tropical faunas and communities contain many fewer species of frugivores than insectivores. This generalization holds true for both birds and bats. So how does a species such as the short-tailed fruit bat make its living? How much time does it spend searching for and consuming food each night? How far does it have to travel from its day roost to find food? How does it balance its need to feed with its need to mate and reproduce? And what are the botanical consequences of its food choices and foraging be102 / Chapter 5 havior? Are Carollia and its frugivorous relatives effective seed dispersers ? Our fieldwork at Santa Rosa National Park was designed to answer all of these questions...