The annual migrations of three species of West African fruit bats (Chiroptera: Pteropodidae)

Abstract

The seasonal movements of Eidolon helvum, Myonycteris torquata, and Nanonycteris veldkampi were studied over a 2-year period in West Africa. During the dry season, E. helvum roosts in at least one large colony (ca. 500 000 individuals) in the southern forest zone of Ivory Coast. Following the birth of young in February, males and females move into the savanna zones, and the progressive establishment and decline of colonies along a south–north axis indicates that E. helvum migrates at least to the Niger River basin by the middle of the wet season (July). During the dry season, both M. torquata and N. veldkampi are absent from savanna sites, but are common in the forest zone. With the onset of the rainy season in March, catch rates of both species increase first at a southern Guinea savanna site and subsequently at a savanna site 400 km to the north, indicating that both species migrate at least to the southern Sudanese savanna zone during the wet season. Both sexes of N. veldkampi migrate, but the migration of M. torquata is restricted to the immature male cohorts. I argue that the high amplitude of the seasonal fluctuations in fruit abundance at savanna sites creates a wet-season surplus of food which results in low intra- and inter-specific competition levels at these sites relative to the forest zone. This may provide the conditions leading to the observed annual migrations.