Anyone who has ever set up a mist net in a lowland Neotropical forest is likely to have captured the short-tailed fruit bat (Carollia perspicillata), a member of the leaf-nosed bat family (Phyllostomidae). One of the most common mammals in the New World tropics, this bat ranges from Veracruz and Oxaca, Mexico, to southern Bolivia, Paraguay, and Brazil, and is also present on the Caribbean islands of Trinidad, Tobago, and Grenada. Throughout its range, the short-tailed fruit bat plays an extremely important role in the dynamics of tropical forest ecosystems, feeding on the fruits and dispersing the seeds of a variety of early successional plant species, the pioneer plants that are the first to colonize disturbed areas.
Weighing about two-thirds of an ounce (about 22 grams), this bat lives in colonies of a few dozen to several hundred individuals in a wide variety of roosts, most commonly in caves or hollow trees. At sunset, the bats leave their day roosts and begin searching for ripe fruit within a mile or so (about 1.5 km) in the forest understory. Using a combination of olfactory and visual cues, these sharp-eyed bats pluck fruits from shrubs and trees and carry them to safe feeding perches-- usually bower-like mats of vegetation that are safe from predators-- to eat. After quickly consuming a fruit (and its seeds), each bat returns to the same group of plants to search for additional fruits. After about 15 minutes of feeding, the bat takes a nap while digesting its meal of fruit pulp. Each one repeats this basic feeding rhythm all night.
The bulk of the short-tailed fruit bat's diet comes from fruits produced by shrubs. The most favored are those of the genus Piper, from the black pepper family, and such early successional trees as Cecropia peltata of the fig family. The number of seeds of fruits eaten in one night by a single bat may number as high as 60,000. Even if each bat averaged only 1,000 seeds nightly, just one colony of 400 would disperse 146 million seeds annually. If only one tenth of one percent of the seeds germinated, 146,000 new seedlings would result each year, helping replant the forest. If Carollia bats were to disappear from tropical ecosystems, rates of forest regeneration would likely slow dramatically.
Back in their day roosts, short-tailed fruit bats settle into well-defined social groups. Most males live separately from females in “bachelor” groups. Females live in small, harem-like groups of 10-20 individuals. Each harem contains one adult male, who defends his group against the intrusions of other males during the day. At night, “harem” males continue to defend their group's roosting site while females and bachelor males are out feeding. They minimize the time away from their territories by feeding at plants near the roost.
The payoff to “harem” males for assiduously guarding their harems is nearly exclusive mating rights with the females in their groups. Mating occurs twice a year, and after a four-month gestation period, females give birth to a single baby, once in the spring and again in the summer. Females carry their babies with them while foraging for the first couple of weeks after birth. When the pups become too large to carry, they are left in the roost at night for the remainder of the six-week nursing period.
Mothers form very strong bonds with their young soon after birth, as I once learned in dramatic fashion. We were netting bats in Costa Rica when we caught a female carrying a newborn. Mother and pup accidentally became separated as we removed them from the net, and the mother escaped, only to return an hour later. I took the audibly vocalizing baby from my shirt pocket and placed it on the rough bark of a nearby tree. After a couple of aborted attempts, mom got her baby to reattach to her nipple before flying off into the night.
Once they are weaned, most young bats leave their natal roosts and have no further contact with their mothers. At this time, young females join mixed-age harems and young males join bachelor groups. Females reach sexual maturity within a year after birth and are often pregnant by their first birthday. Although they probably reach sexual maturity in their second year, males cannot reproduce until they can gain access to a harem, a process that can take several years.
Most short-tailed fruit bats live less than five years, during which time females are likely to replace themselves with at least one daughter. Some individuals live longer than this, but rarely for more than six years. [Predators such as snakes in their day roosts, owls, and even the false vampire bat (Vampyrum spectrum), take their toll on these common bats.] Given their relatively short life expectancies, young males are faced with a real dilemma. They need to live long enough to acquire a harem so that they can reproduce, but many males probably fail to do this, meaning that only a few end up being the fathers of most babies.
Survival of this species is essential to the health of Neotropical forests. With their help dispersing seeds across scarred, clear-cut land, forest growth can begin anew.