Morphological Divergence in an Insular Bat, Lasiurus cinereus semotus

Abstract

1. Bats are the only terrestrial mammals to have reached isolated oceanic islands without human assistance. Despite the fact that the greatest evolutionary divergence would be expected on such islands, very few studies on the divergence of oceanic island fauna from their mainland counterparts have focused on bats. This study investigated divergence of the insular Hawaiian Hoary Bat, Lasiurus cinereus semotus, from its North American subspecies, L. c. cinereus. 2. Divergence in the Hawaiian Hoary Bat involved characters related to flight and feeding. 3. The Hawaiian bat has undergone a 45% reduction in body size with allometric responses in the size of its wings. These changes resulted in a combination of low wing loading (ratio of weight to wing area) with relatively long, narrow wings. The low wing loading permits slower and more manoeuvrable flight near vegetation, while the relatively long, narrow wings enable enduring flight in open areas. This increased flexibility in flight behaviour has allowed the Hawaiian bat to expand its foraging habitat to include open habitats similar to those of L. c. cinereus, and closed habitats that are not used by L. c. cinereus. 4. Divergence in characters related to feeding involved relative increases in size of the gape, size of the masseter muscle and the height of the coronoid process. These changes give the jaw more crushing power for more efficient processing of large and hard-bodied prey. This has enabled the Hawaiian bat to include large, and hard-bodied insects (not taken by L. c. cinereus) in its diet despite a marked reduction in body size.