Lacking tooth and claw: Fighting frogs reveal their true colours

Our new research published in the journal Evolutionary Ecology aimed to unravel the ways male frogs identify other males as territorial threats, the results surprised us.

Deep in the Peruvian jungle, there’s a species of tiny frogs that are famous for two things: their huge array of colours and patterns, and their aggressiveness. The Mimic Poison Frog (Ranitomeya imitator), is a striking example of Müllerian mimicry. This adaptive phenomenon has led to a dazzling array of colour ‘morphs’ among these frogs, each mimicking another species’ colour and pattern for mutual benefit. As each species is poisonous, predators have evolved to avoid these warning colours and patterns.

The unique evolutionary journey of Mimic Poison Frogs doesn’t stop with their appearance. Mimic Poison Frogs exhibit a monogamous mating system that features complex biparental care, where both the male and female help to rear their offspring, an exceedingly rare behaviour in amphibians. These frogs aggressively guard their territories to defend from threats such as extra pair copulation (infidelity) and brood parasitism. Brood parasitism involves laying eggs in a neighbour's nest – or in the case of most poison frogs, in water-holding plants – with the expectation that the neighbour will raise their young.

Mimic Poison Frogs have been the subject of much research, which drew me to work with them at East Carolina University with Dr. Kyle Summers. Pretty quickly, I noticed that some of these territorial males enthusiastically fought (imagine mini sumo wrestlers!), while others ignored each other. I wondered, what cues are males using to identify each other as threats? Why are some males seemingly perceived as harmless while others raise the alarm? For many poison frog species, we know that colour is involved in mate selection, so as a species with so much colour and pattern diversity, I guessed that colour was somehow involved in regulating aggression as well. Because of their unique mating system and behavioural traits, answering this question would help us deepen our understanding of how behaviour influences evolutionary processes and how evolution influences animal behaviour.

To find answers, we traveled to the native range of the Mimic Poison Frog, in the remote Peruvian Amazon. It was here that I set up a makeshift wrestling arena. We traveled to different regions of the rainforest to find males of each colour morph. Frogs squared up against frogs of their own colour morph as well as frogs of other colours. From these ‘aggression trials’ we recorded the amount of time frogs spent in combat.

To our surprise, we found that they show no difference in aggression level based on colour, yet, using results from a genetic analysis, we determined they are more aggressive towards more closely related frogs. This raises the obvious question, if they aren’t using visual cues based on colour or pattern, how are they recognizing their genetically similar counterparts? We tested size differences, and still, that had no effect. Simply put, we still don’t know what cues they are using. Differences in territorial calls or pheromones may be key. More research could help us solve this puzzle.

These results are important in our understanding of evolutionary biology. As a species with such unique behavioural traits, studying the Mimic Poison Frog can teach us how their mating habits influence behaviours like aggression. This is crucial for predicting whether such behaviours will lead to the formation of new species or maintain diversity within the species. All in all, our research invites more questions about the intricate social behaviours of amphibians and urges us to look closer.

Eli Bieri, Student, Australian Museum

Acknowledgements

We thank Instituto de Investigación Biológica de Las Cordilleras Orientales (INIBICO) for their assistance with housing and caring for specimens. We are also appreciative of the support received from Manuel Panaifo in providing local knowledge, identifying field sites, and locating study specimens. The National Science Foundation Research Experience for Undergraduates (NSF REU) program and a NSF DEB grant (ID#: NSF-DEB1655336) provided funding for this project.