Pteropus mariannus, commonly known as the Mariana fruit bat or fanihi in the indigenous Chamorro language, is a medium-sized megabat belonging to the megachiropteran family Pteropodidae. It is native to the Mariana Archipelago and the island of Ulithi in Micronesia. The genus name Pteropus derives from Greek roots meaning "wing-footed," a reference to the expansive flight membranes attached to their limbs, while the species epithet mariannus identifies its geographical origin in the Mariana Islands. Culturally and historically, the fanihi holds immense traditional importance to the indigenous Chamorro people, for whom it was historically prized as a central delicacy reserved for special occasions and cultural celebrations.

The Mariana fruit bat is a medium-sized flying fox weighing between 300 to 500 grams, with a wingspan stretching up to 1 meter (3.3 feet). It features dark brown to blackish fur on its body, accented by a distinct, bright golden-yellow or amber mantle across its neck and shoulders. Like other megabats, Pteropus mariannus lacks echolocation; instead, it possesses large, dark eyes adapted for low-light nocturnal vision and a well-developed sense of smell to locate ripe fruit and flowering trees across island forest canopies. The species roosts colonially in high canopy trees, predominantly within native limestone forests and volcanic ravines across islands like Guam, Rota, Tinian, and Saipan.

Ecologically, the fanihi functions as a critical keystone species within its island ecosystem. As a specialist frugivore and nectarivore, its diet consists of tropical fruits, nectar, and pollen from native forest trees such as Artocarpus (breadfruit), Ficus (fig), Pandanus, and Ceiba. Through its foraging flights across island landscapes, Pteropus mariannus serves as a primary seed disperser and pollinator for key canopy species, maintaining native forest biodiversity and forest regeneration across Micronesian islands.

In modern ecological history, Pteropus mariannus has faced severe population declines, leading to its classification as an endangered species under the United States Endangered Species Act and the IUCN Red List. Historically, heavy overhunting for traditional consumption—often prepared by boiling the whole bat in coconut milk broth—severely depleted populations. This threat was compounded by severe habitat loss from human development, frequent tropical typhoons, and, critically, predation by the invasive brown tree snake (Boiga irregularis) on Guam, which preys heavily on roosting bats and juveniles. Consequently, strict conservation regulations, habitat protection, and invasive species control programs now guard remaining populations across the archipelago.