Diel pattern of migration in a poisonous toad from Brazil and the evolution of chemical defenses in diurnal amphibians

Abstract

Most amphibians with biphasic life cycles have aquatic eggs and larvae and terrestrial adults that migrate between terrestrial habitats and aquatic breeding sites. Migration usually occurs at night in order to avoid desiccation and predation. However, some amphibians also migrate during the day, and it has been proposed that this may have evolved as a result of poisonous skin secretions and aposematic coloration that release individuals from visually oriented diurnal predators. Based on this hypothesis and recent observations of 24 h breeding activity in the poisonous, aposematic toad Melanophryniscus cambaraensis, we predicted that migration in this species would occur equally during the day and night. To test our prediction we documented the diel pattern of migratory activity over multiple explosive breeding events from October 2008 to February 2009 (127 nights) at a temporary stream in Rio Grande do Sul state, southern Brazil. We also obtained environmental data to determine if diel activity could be attributed to variation in rainfall, barometric pressure, temperature, and/or relative air humidity. Contrary to our prediction, migratory activity in M. cambaraensis is strongly diurnal. Although temperature and humidity varied significantly between day and night intervals, this variation does not account for the diurnal-only migratory activity of M. cambaraensis. We suggest that the diurnal-only migratory activity of M. cambaraensis is best explained by phylogeny, not contemporary functions or selective pressures. Diurnal activity is primitive for this species and evolved in the common ancestor of Agastorophrynia, prior to the chemical defenses found in toads (Bufonidae) and poison frogs (Dendrobatidae). This suggests that chemical defenses in these groups may have evolved as a result of the diurnal activity that brought them into contact with visually oriented diurnal predators, and not the other way around. © 2010 Springer Science+Business Media B.V.