The Rainforest Canopy


July 30, 2012

The most abundant mammals in the rainforest are not large ground-dwelling creatures, but bats. The tropics have the greatest variety of bats, and accordingly, the most diverse mammalian group of the tropical rainforest is bats, making up over 50 percent of mammal species. Bats range in size from the giant flying foxes, with wingspans of six feet (1.8 m), to the tiny bumblebee bat of Thailand, the world's smallest mammal, weighing less than an American penny. Equally diverse are the feeding habits of tropical bats, which include fruit, nectar, blood, and carnivorous feeders; and the places bats choose for shelter.

Although most bats of the world are insectivores, rainforests have a high percentage of fruit eaters. While insectivorous and other carnivorous bats rely on echolocation to find their prey, fruit-eating bats depend mostly on sight and a sophisticated sense of smell. True fruit-eating bats, the flying foxes of Madagascar, India, Sri Lanka, Southeast Asia, New Guinea, and Australia, did not reach the New World, so their niche was filled by spear-nosed fruit bats which evolved from insect-eating bats. Today, countless canopy plant species depend on bats for pollination or seed dispersal, making bats the best mammalian dispersal agents.

Flying foxes are the fruit bats of the Old World and are limited to the tropics by their need for fruit year round. Visitors to areas frequented by flying foxes are likely to notice trees with hundreds and even thousands of pod-like forms that shriek incessantly. These are likely a colony of flying foxes. The individual flying fox's place in the tree is determined by its stature in the colony which is established by fighting. The most powerful males occupy the highest, safest areas, while the weaker bats occupy lower, more exposed branches. There are places in the well-developed pecking order for sentries, which are posted to warn the colony when danger approaches.

Tent-making bats. Click image for more photos of bats. Photo by Rhett A. Butler

Nectar-feeding bats are important pollinators of tropical rainforest plants. Like fruit-eating bats, nectar-feeding bats rely on sight to locate their primary source of food: flower nectar. Thus flowers targeted by these nocturnal bats are night-blooming and are easily seen in relative darkness with large, white petals. Nectar-feeding bats are equipped with a long, thin tongue, like that of a hummingbird, to reach nectar deep inside the flower.

Vampire bats of the New World are well-known and quite often feared for their feeding on the blood of animals, even humans. Despite the stories of Dracula and Transylvania, vampire bats feed mostly on farm animals in tropical regions. In fact, man has inadvertently increased vampire bat populations by introduction of livestock, mostly cattle, into formerly forested lands. Vampires, which are only active in the darkest hours of the night in order to avoid predators, feed by using their chisel-like incisor teeth to make a small incision in the animal's skin. The bat drinks, not sucks, the blood which freely flows from the wound thanks to an anticoagulant, which incidentally, has been chemically isolated to create a drug for treating heart attack victims. Strangely, vampires prefer to land at a distance from their victim and approach by foot. Animals fed upon by vampires are rarely injured or killed by the feeding. Despite their cruel reputation, vampire bats have been known to adopt and feed orphaned bats.

Besides the best known insectivorous bats—each individual may eat over 3,000 insects a night—several bat species feed on vertebrate animals. One of the most notable bat groups are the fishing bats which have echolocation so sophisticated that they can detect minnows swimming at the water surface. Other bats feed on frogs and are able to distinguish poisonous species from edible species by listening to their calls. These bats associate the bad experience of eating one of these toxic frogs with their call.

Bats are best known for roosting in caves during the day when they are inactive. Where caves exist in the tropics, many bats do roost there and in some areas blacken the sky as they leave the caves at dusk. However, the majority of tropical rainforest regions lack caves, so bat species must look elsewhere for cover. Many species choose the hollows of trees, while flying foxes sleep out in the open. However, some species have adapted interesting, if not bizarre, retreats. The tiny woolly bats of West Africa live in the large webs of colonial spiders, while some bats of Central and South America construct shelters by cutting banana leaves into tent-like structures.

Madagascar Rousette (Rousettus madagascariensis), a fruit-eating bat. Photo by Rhett A. Butler

Because so many plant species, including kapok, eucalyptus, durian, mango, clove, banana, guava, avocado, breadfruit, ebony, mahogany, and cashew trees, depend exclusively on bats for pollination and seed dispersal, bats play a monumental role in the health of the rainforest. For example, bats are the dominant pollinators of forests on remote Pacific islands. Since many plant species on such islands co-evolved features to facilitate specific bat pollination, once bats are eliminated there are no other pollinators to fill the niche. Bats also play a crucial role in controlling insects. In several locations, municipal bat roosts have been proposed to stymie malaria-carrying mosquitos.

Bats are particularly subject to extermination by the destruction of their habitat. The reasons include their high concentrations in relatively few areas (especially in their caves), their specialized roles in filling feeding niches, and their relatively small number of young (since extra young add extra weight—a liability to flying). An infant bat weighs one-third of the mother's weight when it is born, essentially the same as a woman giving birth to a 40-pound (18 kg) baby (Bat Conservation International). In addition, bats can be highly sensitive to disturbances. For example, when there is a food shortage, bats may shut down their metabolism until more plentiful times. When a hibernating bat is disturbed, its body temperature spikes upward in preparation for escape, costing as much as a month of stored fat reserve. Baby bats are particularly sensitive to temperature and when disturbed, they frequently move to a slightly cooler area and die of exposure. More than 60 percent of bats do not survive infancy.

Insect-eating bat in Peru. Click image for more photos of bats. Photo by Rhett A. Butler

Review questions:

  • What is a flying fox and what does it eat?
  • Why are important for an ecosystem?
  • Are vampire bats real?

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