We can visualize ecological communities as organized chains of interacting carnivores, herbivores and plants (Fretwell, 1987). In this food web context, prey have the ability to discriminate among predator-specific threats (Schmitz, Krivan & Ovadia, 2004), the predator–prey interaction being the basic direct interaction link between two species. The comprehension of this basic relationship is necessary for understanding other community properties (Werner & Peacor, 2003) and to know whether behavioral responses toward predators can generate predictable patterns of species distribution (Binckley & Resetarits, 2003; Steffan & Snyder, 2010).
Anuran tadpoles present a suitable model for studying predator–prey interactions because they represent a food source for a number of different vertebrates (birds, turtles, amphibians and fish) and invertebrates (beetle larvae, water bugs, dragonfly larvae and spiders) that show different foraging strategies (sit-and-wait or active foraging) and several levels of sensitivity to unpalatability (Heyer & Muedeking, 1976; Morin, 1987; Wellborn, Skelly & Werner, 1996; Alford, 1999; Hero et al., 2001). Generally, tadpoles present two types of defense mechanisms (sensuBrodie Jr, Formanowicz & Brodie, 1991): those that reduce the chance of encounters with predators (predator avoidance mechanisms), and those that reduce the predators' capture success (antipredator mechanisms). Predator avoidance mechanisms are generally behavioral (e.g. changes in the time of activity or in the foraging micro-habitat), whereas antipredator mechanisms can be behavioral, physiological or morphological (e.g. immobility or unpalatability) (Brodie Jr et al., 1991). Several studies have shown the importance of predator–prey interactions in tadpole distribution patterns among different bodies of water (e.g. Hero, Gascon & Magnusson, 1998; Azevedo-Ramos & Magnusson, 1999; Azevedo-Ramos, Magnusson & Bayliss, 1999), and they have suggested that antipredator mechanisms are fundamental for explaining the coexistence of tadpoles with their predators (Hero et al., 2001).
Several sources show that a tadpole's coloration is related to its antipredator mechanism. Unpalatable tadpoles present black coloration, which is generally associated to aposematism (Heyer, McDiarmid & Weigmann, 1975; Crossland & Alford, 1998; Crossland & Azevedo-Ramos, 1999; Hero et al., 2001). Additionally, unpalatable black tadpoles do not show strong reductions in foraging activity upon perceiving predation risk (D'Heursel & Haddad, 1999; Jara & Perotti, 2009, 2010). In contrast, tadpoles with brown coloration usually exhibit cryptic behaviors, staying motionless in the presence of a predator and moving from one point to another at high speeds if the predator attacks (Heyer et al., 1975; Azevedo-Ramos et al., 1992; Nomura, Rossa-Feres & Prado, 2006). Unpalatability mechanisms are the main defensive trait that makes the coexistence of tadpoles and fish possible (Hero et al., 2001) because fish are considered to be the main predators of tadpoles in permanent water bodies, such as pools and lakes (Heyer et al., 1975). Nevertheless, the conspicuous coloration of unpalatable tadpoles increase their chances of encountering a predator (Azevedo-Ramos et al., 1992; Chovanec, 1992; Hero et al., 2001). Thus, because palatability does not restrict the consumption of tadpoles by many kinds of dragonfly larvae as it does for fish species (Crossland & Alford, 1998; Crossland & Azevedo-Ramos, 1999), dragonfly larvae are one of the most important predators of tadpoles among invertebrates (Gascon, 1992; Hero et al., 2001; Gunzburger & Travis, 2004) and they can restrict the presence of unpalatable tadpoles in bodies of water (Hero et al., 2001). However, tadpoles' cryptic behaviors are efficient to these invertebrate predators because the dragonfly larvae are sit-and-wait predators, and they are guided by a mixture of tactile and visual clues generated by the prey's movements (Pritchard, 1965; Azevedo-Ramos et al., 1992). Owing to these differences, the efficiency of each strategy (unpalatability or crypsis) should vary according to the type of predator (vertebrate or invertebrate) (Hero et al., 2001).
In this study, we tested whether the tadpoles of Eupemphix nattereri (crypsis) and Rhinella schneideri (unpalatability), which present different antipredator mechanisms, have different mortality rates depending on the predator type, the fish Oreochromis niloticus and the dragonfly larvae of Aeshna sp. Our hypothesis is that the efficiency of the antipredation strategy will be affected by the predator types: cryptic behavior will have higher success rates against the invertebrate predator, whereas unpalatability will have better success against the vertebrate predator. As suggested by Gunzburger & Travis (2005), once it has been established that a prey species is unpalatable to a predator, an experiment should be conducted to evaluate whether predators are capable of distinguishing palatable from unpalatable prey and are able to learn to avoid unpalatable prey once they have encountered it. Thus, we evaluated the ability of the fish predator to distinguish palatable from unpalatable prey but we also hypothesized that the experience of the predator and the antipredator mechanisms should interact and that the outcome of this interaction is dependent on the efficiency of the mechanism used to avoid predation. Thus, we designed two simple experiments to answer the following questions: (1) are tadpole antipredator behaviors designed for encounters with a specific predator, thus representing differential survival strategies?; (2) is there any difference in tadpole mortality rates between experienced and inexperienced predators according to the type of antipredator mechanism exhibited by the tadpole?