Character release in the endangered Hawaiʻian hoary bat, Lasiurus cinereus semotus

Abstract

The insectivorous Hawaiian hoary bat, Lasiurus cinereus semotus, is an endemic subspecies of the North American hoary bat, L. c. cinereus. This study investigated morphological and ecological divergence of the island population, with emphasis on the potential for character release. Divergence in the Hawaiian hoary bat involved characters related to flight and feeding. The Hawaiian bat has undergone a 45% reduction in body size with allometric responses in size of its wings. These changes decreased wing loading (ratio of mass to wing area), without altering the high aspect ratio (wingspan^2/wing area) of the ancestral species, thereby permitting slower and more maneuverable flight near vegetation. High aspect ratio enables the bat to retain the fast and efficient flight of its ancestor in open areas. This increased flexibility in flight behavior has allowed the Hawaiian bat to expand its foraging habitat to include the open habitats of the North American hoary bat and the closed habitats of putative competitors of the North American hoary bat. There has been a relative increase in skull length of the Hawaiian bat, with non-allometric increases in the gape of the jaws, height of the coronoid process, and size of the masseter muscle. This gives the jaw more crushing power for more efficient processing of large and hard-bodied prey. In the open habitat the Hawaiian bat fed predominantly on moths as does the North American hoary bat. In the closed habitat the Hawaiian bat fed predominantly on beetles as do putative competitors of the North American hoary bat. The morphological and ecological changes are consistent with the assumptions and prediction of the character release hypothesis. However, uncertainties remain about the historical aspects of the assumptions. In addition, the interaction between flight speed, echolocation, and insect prey was investigated. Apparent prey selection by fast-flying bats in open habitats was the result of a decrease in detectability of smaller prey. Bats responded to changes in prey density by altering the pursuit component of handling time.