Samoa Guide, 22. Flying Foxes (fruit bats)

At least 42 plant species provide food for the pe'a vao and pe'a fanua in American Samoa. These consist mostly of forest tree species, such as aoa (Ficus prolixa and Ficus obliqua), asi (Syzigium inophylloides), a’amati’e (Elaeocarpus ulianus), ifi (Inocarpus fagifer), fetau (Callophyllum inophyllum), mamalava (Planchonella samoensis), and tava (Pometia pinnata). The bats also eat fruits grown by man such as mango (Mangifera indica) and ulu (Artocarpus altilis), but damage to crops is limited if fruits are harvested before they ripen on the tree. Also, when you hear the bats squealing in banana patches at night, it’s usually because they are fighting over the banana flowers, not the fruits. Despite the broad similarity in their diets, pe'a fanua exploit flower resources and feed on trees (especially domestic ones) closer to human habitations more often than do pe'a vao.

But the pe'a vao does not do this. Instead, these bats usually roost singly on branches, or as pairs of males and females (suggesting a monogamous mating system), or as a female with its young. When roosting, pe'a vao males tend to hang from exposed or dead branches of trees on ridge tops while females roost in more covered positions on forest slopes.

There is little information on how long flying foxes live, although some held in captivity have lived 20 years. We assume that they live shorter lives in the wild, because natural catastrophes like tropical cyclones may periodically reduce their numbers, as occurred in the early 1990s. With proper management and protection from hunting, the populations will generally bounce back but this requires a number of years, because females of both species probably have only one young per year and we do not know how many of their young survive to adulthood.

The rewards of the interaction between flying foxes and plants are certainly not one-sided, because the plants and forest also benefit from being visited by bats. When the pe'a fanua visit patches of vavae flowers (Ceiba pentandra), for example, they help transfer pollen from one tree to another and that, in turn, enhances fruit formation and production. Flying foxes are also important for seed dispersal. When they consume fruits with small seeds (such as aoa), some seeds that get swallowed do not get digested but are carried and deposited away from the tree source. In American Samoa, flying foxes are the only animals that can carry fruits with large seeds, such as ifi, over distances and uphill. Studies of bat movements on Tutuila reveal that bats may commute between feeding areas up to 40 km apart during a single night. Seeds that are transported can colonize naturally bare or cleared areas and aid in the revegetation of gaps in forests where trees may have fallen from disease, age, or bad weather.

It may bother us that flying foxes eat some of the fruit that we grow for ourselves, but these bats are tireless workers that help maintain the health of our rainforest, and they are fun to watch.

The two flying foxes are especially distinctive: they are renowned for being large (with a wing span up to 3 feet wide) and active both day and night. Pteropus samoensis (pe'a vao) is commonly called the Samoan flying fox. It is presently found only in the Samoan Archipelago and Fiji. It once occurred in Tonga but is now extinct there. The other flying fox, Pteropus tonganus (pe'a fanua), has several common names such as the Insular, White-naped, White-necked or Tongan fruit bat. It has a wider distribution in the Pacific, ranging from islands off Papua New Guinea to the Cook Islands.

In American Samoa, flying foxes can be seen flying, soaring, feeding, or just hanging in trees. Although individuals of the two species overlap in size (adults weigh 300-600 grams), there are ways to differentiate them from a distance. When silhouetted against the sky, the pe'a vao has a more triangular shape, with wings that are slightly scalloped and relatively dark and opaque. Their flight appears more relaxed, usually with slower wing beats and deeper wing strokes. It is not unusual to observe them soaring in the air in the day, taking advantage of rising currents of warm air (thermals) to seemingly float up and about without flapping their wings.

In contrast, pe'a fanua has a more cross-like appearance: the neck and head appear more pronounced, the wings are narrower and more scalloped, and the hind legs stretch out like a tail. In flight, pe'a fanua tend to have faster wing beats and shallower wing strokes. They are less likely to soar in thermals and generally take a directional route to and from roosting sites at dawn or dusk.

Despite these differences, it takes keen eye to distinguish the two species from a distance. Close up, the pe'a vao may sport a white to yellowish triangular patch that starts from the forehead and extends to the back of its head, or it may simply exhibit a generally grayish head with or without flecks of white hair (much like a graying man). Its neck and shoulder areas are a beautiful russet brown, while the rest of the body has a dusty black appearance. The pe'a fanua, on the other hand, has a basically black head and body. These black areas serve to set off a distinctive band of creamy yellow on the back of the neck and which extends slightly below its shoulders as if in a cape. This explains why they are called whitenaped fruit bats although the color is not really white.

The two species have quite different social behaviors. During the daytime, pe'a fanua form large roosting groups or colonies of hundreds to thousands of bats. These colonies are generally organized according to their reproductive status and may be composed of bachelor males, clusters of females defended by an adult male (suggesting a harem mating system), or groups of females and their young. In any case, individuals appear to be relatively “faithful” to their roosts, usually returning to their respective colonies following foraging flights.

The care and energy that both bat species put into their young is remarkable. Pregnancy lasts approximately 5 months in both species, and once the young are born, it takes at least another 3 months before they are weaned. Even after they are capable of flight, the young continue to receive parental care, perhaps until they reach adult size or become reproductively active themselves. We know this from observations of pairs of individuals seen to alight independently on the same tree and subsequently come together with one individual (presumably the juvenile) being wrapped up in the other's wings as they settle down to roost. Sightings of pregnant females and individuals carrying young in flight indicate that pe'a vao give birth mostly between April and June. Pe'a fanua births appear to occur yearround but are more common in January and June to August.

Although their name indicates that they are fruit-eaters, both species also eat nectar, pollen, leaves, and sap. They tend to consume only the “juice” of fruits and leaves. To do this, a bat will carefully chew on food (usually eating around large seeds), press the pulp against the roof of its mouth with its tongue, squeeze and suck in the juice, then spit out most of the pulp in pellets called “ejecta.” These ejecta are especially abundant under breadfruit trees (ulu) where the bats have been feeding overnight. Among the splatter of mushy bits of the fruit, you can find these pellets of drier material that sometimes show tooth and palatal (roof of the mouth) impressions, much like a dental cast produced at a dentist’s clinic. Ejecta pellets help us to identify food items that bats eat even when we do not directly observe them feeding. In addition to direct observations and analysis of ejecta, their diets are also determined through pollen present in facial hair and in feces. These data help scientists determine which plants are important because they are preferred, nutritious and/or available year-round.