Convergence in Reflectance Spectra Among Treefrogs

Abstract

Infrared reflectance occurs in a wide diversity of vertebrates, but its biological significance remains largely unknown. Biological roles of visual crypticity, thermal crypticity, and reduction in heat load were explored in frogs with quantitative data on reflectance capability. Reflectance spectra between 450 and 1000nm were measured on two categories of green treefrogs, one of which was thought to reflect near infrared radiation, the other of which was not. In a paired comparison, leaf-sitting frog species from five genera (Litoria infrafrenata Gunther, Agalychnis callidryas Cope, Phyllomedusa sauvagii Boulenger, Pachymedusa dacnicolor Cope, and Rhacophorus malabaricus Jerdon) were selected on the basis of previous infrared photographic observations. Analyses of reflectance spectra revealed that in all cases, the leaf-sitting, near infrared reflecting frogs had a reflectance spectrum that contrasted significantly from that of the confamilial non near infrared reflecting frog species (Osteopilus septentrionalis Dumeril & Bibron, Hyla cinerea Schneider, and Polypedates leucomystax Gravenhorst). In all cases, there was a strong convergence in the reflectance spectra of these distantly related near infrared reflecting frog species. Additionally, for all five near infrared reflecting frogs, their reflectance spectra was very similar over the 450-1000nm waveband to that of leaves, their typical resting sites. An energy budget analysis of the reflectance spectra suggests that thermoregulation by itself is not the primary biological role of high near infrared reflectance. Rather, this capacity appears to be most likely associated with predator avoidance, functioning to make frogs visually and, perhaps, thermally cryptic to their predators. The convergence of similar patterns of spectral reflectance between distantly related species within the same family and between unrelated species in different families further suggests that reflectance in the near infrared wavelengths has adaptive signi-