Factors contributing to the evolution of reproductive strategies have been of great interest to evolutionary biologists. In tropical amphibians predation and competition have been suggested to play a major role. Poison frogs of the family Dendrobatidae display a trend towards the use of very small pools and increased parental care, particularly in the genus Dendrobates. Some species with female parental care, asymmetrical biparental care and biparental care, have evolved novel behaviors in association with the use very small phytotelmata. It has been hypothesized that selection pressure imposed by predation and competition favored the use of small phytotelmata, and this, in turn, produced selection for trophic egg provisioning to ameliorate the lack of available nutrients. To elucidate the ecological factors associated with the transition from uniparental male care to biparental care and associated changes in social behaviors, we evaluated key behavioral and ecological differences between Dendrobates imitator and D. variabilis. Dendrobates imitator used significantly smaller phytotelmata in different plant species than D. variabilis for tadpole and embryo deposition. The parental strategy of D. variabilis was limited to male parental care, whereas D. imitator exhibited biparental care. Males and females of D. variabilis were observed to have a promiscuous mating system with little mate fidelity. This contrasted with D. imitator, where paired males and females were observed interacting daily and were never observed courting additional mates. Our results are consistent with the hypothesis that a key ecological difference between these species, involving the size of pools typically used for reproduction, is strongly associated with the evolution of biparental care and monogamy in D. imitator. © 2008 Brill Academic Publishers.