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Early detection of predators is fundamental for the survival of the prey. Predator detection can occur by perception of the chemical substances released by predators or injured conspecifics, granting prey the opportunity to avoid the attack. However, overreacting to cues can generate false alarm reactions that result in fruitless energetic costs. Given that chemical cues in natural habitats rarely occur singly, individuals can benefit from obtaining information from multiple sources, such as the combined chemical cues from predators and conspecifics. We experimentally tested the movement reaction of cane toad (Rhinella marina) tadpoles when in contact with isolated chemical cues from a conspecific, a predator (dragonfly larvae), and the combination of these two substances. We quantified duration of swimming activity before and after the exposure to the chemical cues and compared it to a control group. A significant reduction in swimming activity was only observed when the tadpoles were exposed to the combination of both cue types. Our results suggest that the tadpoles can recognise the chemical cues from conspecifics and predators, but a behavioural response is only elicited when both stimuli are detected simultaneously. We suggest that behavioural change in response to predation threat in R. marina is a compromise between early detection of predators and avoidance of unnecessary energetic expenditures.

Due to the importance of camouflage to avoid detection by predators, predation pressure can cause coupled evolution of skin colour and preference for substrate colour. Individuals can choose regions where the background makes their skin colour less noticeable (crypsis) or where it accentuates warning coloration (aposematism). In such scenario, individuals should optimally choose substrate according to their skin colour and mechanism of predation avoidance: toxic species are expected to choose highly lit places and non-toxic species should avoid such places. We studied substrate choice on five species of tadpoles that differ in body colour and toxicity. The results of the present study did not confirm our prediction that non-toxic and cryptically coloured species would prefer a lower contrast substrate that maximizes camouflage. We show that individuals preferred highly lit areas that accentuated their contrast with the substrate. The general preference for lighter substrate might be related to the tadpole’s limited vision on a dark substrate, which hampers their ability in detecting predators. This study demonstrates that tadpoles can distinguish the substrate colour and that their choice of habitat might be linked to both their defence mechanism in the case of aposematic species and recognition of habitat elements in the case of cryptically-coloured species.