We performed two experiments to address the questions of whether toad tadpoles (Bufo a. americanus): (1) gain an advantage from consuming conspecifics and (2) engage in active cannibalism as opposed to scavenging. Our results show that tadpoles fed algal mats (and associated debris found within them) from their natal pond and supplemented on the bodies of dead tadpoles for 28 d attained an average of 5 more stages of development than tadpoles fed only on algal mats, which suggests a strong advantage to eating conspecifics. No tadpoles fed algal mats died, which suggests that this is an adequate food source. In a series of trials used to address the second question, we found no indication that tadpoles engage in active cannibalism. We tested this under conditions of hunger and using injured tadpoles as potential prey and progressively older tadpoles as potential cannibals. In no cases did we observe any tadpole attacking living tadpoles (even if injured) within 1-h after being placed in experimental tanks. Although some tadpoles were missing from tanks after 3-d periods, our results suggest that some (e.g., injured) tadpoles may succumb and are eaten thereafter. Test tadpoles began consuming dead crushed tadpoles significantly more quickly than dead intact tadpoles, suggesting that cues used to induce feeding are chemical and perhaps a result of some bacterial decomposition. We suggest that toad tadpoles have not evolved active cannibalism because of energetic and other costs and because mortality rates of tadpoles are naturally high in shallow ephemeral breeding ponds. Living tadpoles thus have access to dead tadpoles as a food source in such circumstances without incurring some costs of cannibalism.
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Vol. 153 • No. 2