Evolution of Monogamy in Poison Frogs

Genetic tests have revealed the secret sex life of a tiny poison dart frog species that lives in the Peruvian rain forests: remarkably, it turns out that these frogs are monogamous. But the reason this species is monogamous is surprising: it's all about the size of the pools that their tadpoles mature in. This is the best evidence yet that just a single cause can affect evolution of a major life history trait, such as a species' mating system.

The forces that underlie the evolution of biparental care have been the subject of lively debate within the scientific community, and in turn, how different forms of parental care affect the evolution of each species' mating system is also not well understood.

According to one long-standing hypothesis proposed by mammologist Devra Kleiman, monogamy may arise as a life history strategy when biparental care becomes critical to offspring survival. In this situation, both parents may experience higher reproductive success by investing in their mutual offspring instead of seeking extrapair reproductive opportunities [DOI: 10.1086/409721]. Life history studies of a variety of avian and mammalian taxa support this hypothesis.

But which ecological factors drive the evolution of biparental care and monogamy? Is just one ecological factor enough, or is more than one necessary? One way of answering these questions is to compare the life histories and ecologies of closely related species where one species is monogamous and provides biparental care while the other is promiscuous and does not provide biparental care. But which species fulfill these criteria?

Jason L. Brown, now a postdoctoral researcher at Duke University, spent six years studying Peruvian poison frogs while a graduate student at East Carolina University. In those studies, he evaluated key ecological and behavioral differences between Peruvian poison frogs and framed those key differences in the context of differing parental care strategies. Building upon his dissertation work, Dr Brown and his colleagues, Victor Morales, at the Ricardo Palma University's Natural History Museum in Lima, Peru, and his dissertation advisor, Kyle Summers, Associate Professor in the Department of Biology at East Carolina University, took a closer look at two closely-related poison frog species: the mimic poison frog, R. imitator, which exhibits biparental care (where both parents care for their tadpoles for a period of months) and the variable poison frog, R. variabilis, which exhibits male parental care (where only the males transport tadpoles to a pool of water and leaves them to fend for themselves).

Like all amphibians, poison frogs are intimately bound to water because that is where they lay their eggs and where their tadpoles mature. But bodies of water are filled with numerous hungry mouths, so the pressures of predation pushed the tiny poison frogs out of the larger ponds and into the considerably smaller (but safer) pools contained in the cup-shaped leaf axils of a variety of plant species that grow in trees. These tree-top plants are not often visited by hungry predators, but their small pools of rainwater, known as phytotelms, lack sufficient nutrients to successfully grow a tadpole (unlike larger ponds of water).

To ensure that their tiny offspring grew up into mature frogs, the mimic poison frogs adopted a new tactic. Merely transporting their tadpoles to individual phytotelms and abandoning them to grow up on their own was no longer enough: the males needed help. Female mimic poison frogs evolved a behavioral strategy known as trophic egg feeding, where they lay unfertilized nutrient-rich eggs into each tadpole's phytotelm for it to eat.

But this strategy requires teamwork: while the female spends her days eating voraciously so her body can manufacture these special trophic eggs, the male visits each tadpole every day or two, for only he knows where they are hidden. Whenever a tadpole nibbles on its father, the male calls to his mate. The female responds to the male's calls by following him to their offspring's phytotelm, jumps into the tiny pool and produces a trophic egg, which the tadpole consumes. The female produces at least one trophic egg for each tadpole every week for a period of months