On the Advantages of Putative Cannibalism in American Toad Tadpoles (Bufo A. Americanus): Is It Active or Passive and Why?
Heinen, Joel T., Abdella, Jennifer A., The American Midland Naturalist
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.
Cannibalism is recognized across a wide variety of animal taxa under various conditions (Elgar and Crespi, 1992). Benefits of this behavior are a ready food source that contains proper lipid, protein and mineral components (e.g., Christie, 1982; Wildy et al., 1998; Arts and Wainman, 1999). Studies demonstrate growth and developmental advantages to cannibalism for many taxa (e.g., Meffe and Crump, 1987; Crump, 1990), but costs can be numerous. They include risk of injury or death, parasite or disease transmission and the possibility of eating close relatives (Crump, 1992; Pfennig et al., 1998). Larval amphibians are highly variable with respect to the known incidence and types of cannibalistic behavior (Polis, 1981; Crump, 1986; Caldwell and Arujo, 1998). Both obligate and facultative cannibalism are known across various amphibian taxa (Crump, 1983; Pfennig and Frankino, 1997) and morphological and physiological adaptations related to cannibalism have been studied across many taxa under different ecological circumstances (e.g., Newman, 1987; Petranka and Thomas, 1995; Kam et al., 1997; Grassland, 1998a; Petranka et al., 1998).
Many generalized anuran larvae from temperate habitats such as toads (Bufo) feed on conspecifics (below), but it is not known whether this is active cannibalism or if the behavior consists only of feeding on dead animals. This is more-appropriately defined as scavenging (Elgar and Crespi, 1992). Such species are predominantly grazers and filter feeders that lack the mouth structures of actively predatory tadpoles, but this may not preclude active facultative cannibalism if conspecifics are injured or significantly smaller than those feeding on them (e.g., Kusano et al., 1985). Most such temperate species breed in large wetland complexes containing thousands of conspecific tadpoles that may represent an important food source. Some (e.g., Bufo) school in sibling groups, metamorphose synchronously and recognize kin (Arnold and Wassersug, 1978; Waldman, 1985, 1991; Heinen, 1993a) although chemical mechanisms for kin recognition are poorly known in most taxa (e.g., Cornell et al., 1989; Gamboa et al., 1991; Wakahara, 1997). Toxicity of both eggs and tadpoles promote predator avoidance and have been proposed as reasons for sibling aggregations in toads (Crossland, 1998b). …