Anurans that breed in plant-held water bodies (phytotelmata) often have larvae with unique modifications for surviving in micro-aquatic environments. These larvae are exposed to severe abiotic conditions (e.g., food limitation, low dissolved oxygen levels, risk of desiccation), and multiple cohorts are frequently found in the same container. To test for cannibalism and interactions among cohorts in two phytotelm-dwelling anuran larvae (Mantidactylus bicalcaratus and M. punctatus) from Madagascar, I reared larvae in experimental microcosms. Larvae of both species had significantly faster growth rates in a high detritus treatment compared to the control (P < 0.0001). The experimental evidence also demonstrated that the larvae of these two species display no cannibalistic behavior, regardless of multiple cohorts, sibling relationships, density, or food and water levels. These data indicate that larvae of both species obtain most or all of their nutrition from eating detritus and the organisms associated with detritus. In M. punctatus, experiments with multiple cohorts indicate that competition for food is strongest within, not among, cohorts, and I found no evidence for a priority effect. Relatively low stage-specific mortality, along with a prolonged breeding season, philopatry, and a lack of predatory behavior, may explain why cannibalistic behavior has not evolved in these species.