Why be diurnal? Shifts in activity time enable young cane toads to evade cannibalistic conspecifics

Why are some animals active by day and others by night? The selective forces that favor diurnal versus nocturnal activity may be evaluated by comparing age classes within a species that exhibits intraspecific (ontogenetic) variation in activity times. In many species of toads, adults are nocturnally active but postmetamorphic animals are primarily diurnal. The small body sizes of these animals render them vulnerable to desiccation and overheating - so why are they active by day? To answer this question, we studied an invasive population of cane toads (Bufo marinus) in tropical Australia. In the field, these small toads often encounter cannibalistic conspecifics because desiccation risk concentrates toads around the moist margins of the natal pond. We manipulated factors that differ between day and night (time of day, illumination, presence of cannibalistic conspecifics, scent, or visual cues from cannibalistic conspecifics) to identify the proximate cues and fitness advantages associated with diurnal versus nocturnal activity. Activity levels, response to disturbance, and feeding rates of metamorph toads were enhanced by light but suppressed by the presence of a larger conspecific. Metamorphs used both visual and scent cues to detect larger toads. An endogenous diel rhythm in activity was present also but weaker in metamorph toads than in larger (cannibal sized) individuals. The risk of cannibalism was high only at night and only in dark conditions. Thus, the diurnal activity of metamorph toads enables these vulnerable animals to avoid conspecific predators.