Mariana Fruit Bat

Pteropus mariannus mariannus

Description

Seven identified subspecies of Pteropus mariannus inhabit the Micronesian Pacific region, including Belau, Yap, Kosrae, Guam, and the Northern Marianas, as well as the Ryukyu Islands, Japan; the relationships between some of these subspecies remain in dispute. Perhaps the best known of these subspecies is the Mariana fruit bat, P. m. mariannus. This bat, with a forearm length averaging 5.3 in (135.5 mm), has a dark brown back, sides and belly with a small number of silvery hairs. The back and rump are somewhat darker in color than the sides of its belly, while its neck and mantle are a much paler earthy yellow-buff. The teeth of P. m. mariannus are slightly larger and heavier than those of its close kin such as P. m. yapensis.

Behavior

Usually gathering in large colonies, especially on cliffs or other isolated terrain, the Mariana fruit bat consumes the fruits and flowers of a large number of plant species; even the twigs and leaves of a few species are eaten. While they normally roost in large numbers, bats on occasion may be seen roosting alone or in small groups. The Mariana fruit bat mates throughout the year; bats assemble in harems containing two to 15 females and a single male. Non-mating males gather in "bachelor groups" or roost singly near a harem. At a given time, 7-20% of females in harems can be found with recently-born young. The bat's movements are poorly understood. Colonies have been observed ranging as far as 6.2-7.4 mi (10-12 km) from their roosting location while foraging for food; in addition, individuals or small groups have been observed flying between islands. Large groups of 50-300 bats have occasionally been observed flying between Rota and Guam or Anguijan, a distance of 37-50 mi (60-80 km). It is thought that hunting may be the cause of these migrations.

Habitat

The Mariana fruit bat inhabits mangroves, forests, and coastal areas. Roosts are usually located on cliffs or at other locations protected from strong winds and human interference.

Distribution

This bat is found on Guam and the adjoining Northern Mariana Islands of Aguijan, Rota, Saipan, and Tinian. In addition, groups of P. mariannus of unknown subspecies have been recorded from a number of other islands in the group; these may also be P. m. mariannus. The population of Mariana fruit bats can be estimated as follows: roughly 1,400 bats on Rota, a single colony of 600-650 bats on northern Guam (fluctuating from year to year with migrations from Rota); fewer than 100 individuals on Saipan, roughly 50 bats on Aguijan, and fewer than 25 on Tinian. Considered common before the 1920s, the Mariana fruit bat became increasingly rare on Guam after the introduction of guns to the island. The population steadily dropped; in 1958 the island supported a maximum of 3,000 individuals, but by 1978 perhaps only 50 bats remained. In 1980, a new colony appeared at Pati Point, numbering over 500 individuals; this colony may have arrived from Roti. It grew to around 850 bats in 1982 before declining to some 600 the following year. Meanwhile, the island-wide population reached nearly 775 bats the same year, though nearly all the bats inhabited sites owned by the U. S. Air Force (USAF) in northern Guam. Since 1984, the population has fluctuated from year to year; as of 1988 it stood at about 650.

Threats

The island of Rota supported as many as 2,500 bats prior to 1988, but a violent typhoon in January of that year, as well as an increase in poaching, roughly halved the island's population by mid-1989. The Mariana fruit bat is currently subject to several threats, including predation, hunting for food, and typhoons. Deforestation is not considered to be a major danger at present. Overall, hunting has probably been the major cause of decline. Fruit bats are a traditional food source over much of the Pacific including the Marianas, where they are savored as a delicacy. In parts of Southeast Asia, the meat of fruit bats is considered medicinal. Guam has served as a major crossroads for trade in fruit bats; it is estimated that more than 16,000 bats were exported to Guam from neighboring islands between 1975 and 1989. Although hunting these bats on Guam was banned in 1973, it was listed as endangered by the U.S. Fish and Wildlife Service (FWS) in 1981; enforcement has not been effective and illegal hunting remains a problem. In the 1980s, for example, only one person was arrested and fined on Guam for hunting the bat, even though many incidents of poaching were recorded. Hunting has also been implicated as a serious cause of decline on other Northern Marianas Islands. Through the 1960s and 1970s, bat populations on Aguijan, Saipan, and Tinian were extensively hunted for local food or export. In recent years, predation by the brown tree snake (Boiga irregularis ) on young bats has exceeded the threat of hunting as the most serious threat to P. m. mariannus on Guam. This snake, a nocturnal tree-climber, has also devastated the bird population of Guam's forests. Surveys of bat colonies made since the 1980s have demonstrated that juvenile fruit bats rarely reach adulthood due to predation. Strong typhoons strike the region about every 10-15 years, though they have probably not been responsible for serious declines in bats in the past. However, Guam's single clony of bats could be devastated should a typhoon strike the northern part of the island directly.

Conservation and Recovery

A number of conservation steps have been recommended for the Mariana fruit bat. These include: implementing methods to control brown tree snakes in bat roosting areas (as a precursor to a wider snake control program which would take years to put in place); coordinating the efforts of military, civilian, and conservation authorities to enforce sanctions against poaching; expanding conservation education programs; translocating some bats from the colony on USAF property in northern Guam to previously inhabited parts of the island; and upgrading the listing of the P. m. mariannus bat population in the Northern Mariana Islands to fully endangered, thereby granting U. S. federal authorities law enforcement status to protect the bats found there. In late 1993, the FWS established the Guam National Wildlife Refuge on 370 acres (148 hectares) of forest it was granted by the federal government. Containing the single colony of Mariana fruit bats remaining on Guam, this tract of land may eventually be expanded to more than 28,000 acres (11,200 hectares) if cooperative agreements can be worked out between the FWS, the U. S. Navy, the USAF, the government of Guam and other parties now administering land proposed for the park.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 1990. "Guam Mariana Fruit Bat and Little Mariana Fruit Bat Recovery Plan." Portland, Oregon. 63pp.