Poison Dart Frogs - Evolution of Skin Coloration and Toxicity

Evolution of Skin Coloration and Toxicity

An earlier study on the evolution of skin coloration and toxicity in the Dendrobatidae family indicated that evolution of skin toxicity was significantly correlated with the evolution of bright coloration. Supporting this research, another study went on to explain that conspicuous coloration was also significantly correlated with a poison frog's diet specialization, its body mass and its chemical defense. This study was of importance as it strongly correlated diet specialization and chemical defense with aposematism.

Santos et al. found that aposematic species have greater aerobic capacity; this was also significantly related to diet specialization. The authors offer up two scenarios for how this unfolded evolutionarily and how this could be the origin of aposematism. One possibility is that aposematism and aerobic capacity preceded greater resource gathering, making it easier for frogs to go out and gather the ants and mites required for diet specialization. This is counter to classical aposematic theory that assumes toxicity from diet arises before signaling. Their second hypothesis suggests that diet specialization preceded higher aerobic capacity and that aposematism evolved in order to allow Dendrobatidae to gather resources without predation.

The most recent study to come out on this matter, however, finds itself in disagreement with earlier research. This study found that polymorphic poison dart frogs who are less conspicuous are more toxic in comparison to the brightest and most conspicuous species; sequestration by the less conspicuous frogs of alkaloids considered to be strong convulsants was proposed as the explanation for this finding.

On the other hand, conspicuousness and toxicity may be inversely related because the energetic costs of producing toxins and bright color pigments lead to potential trade-offs between toxicity and bright coloration. Prey with strong secondary defenses have less to gain from costly signaling. Therefore, prey populations that are more toxic are predicted to manifest less bright signals. Prey mobility could also explain the initial development of aposematic signaling. If prey have characteristics that make them more exposed to predators, such as size or habitat, then they have ample reason to develop aposematism – more resource exploitation through greater exposure. In 2003, Santos et al. alluded to this mechanism in the switch that dendrobatids made from nocturnal to diurnal activity. Dendrobatids now had greater ecological opportunities in daylight and dietary specialization arose. Thus, aposematism is not merely a signaling system, but a way for organisms to gain greater access to resources and increase their reproductive success.

Marples et al., however, showed that dietary conservationism (long-term neophobia) in predators could facilitate the evolution of warning coloration if predators avoid novel morphs for a long enough period of time. Another rarely acknowledged avenue of the evolution of aposematism is the gradual-change hypothesis. Lindström et al. found that the gradual-change hypothesis did not provide an easy solution to the beginnings of aposematism, but that “cost-free stepwise mutations over the range of weak signals could accumulate under neutral selection to produce effective strong signals”.

Maan and Cummings suggested that sexual selection is another reason why toxicity is evolving. With female preferences in play, the coloration of males would change more rapidly. Sexual selection is influenced by many things. The parental investment may shed some light on the evolution of coloration in relation to female choice. In the species O. pumilio (a member of the dendrobate genus) parental investment is not equal. The female makes the eggs and provides care for the offspring for several weeks whereas the males makes the sperm (less energy) and only provides care for a few days. This differential in parental investment indicates that there will be a strong female preference. Sexual selection makes the phenotypic variation in a species increase drastically. In a study by Tazzyman in 2010, there were populations that participated in sexual selection and populations that did not. In the populations that did use sexual selection, the phenotypic polymorphism was evident.

The lack of sexual dimorphism found in some dendrobate populations has also been used to suggest that sexual selection is not a valid hypothesis of the changing coloration. This could be explained because the main protective measure in aposematic frog populations is their ability to warn predators of their toxicity. This bright coloration should be consistent across the sexes, and the female preference for brighter males will increase the coloration in both sexes and thus increase the fitness of both of the sexes. If there were a sexual dimorphism between males and females within an aposematic population there would be a difference in the predation of males and females that would unbalance the number of males and females and, in turn, the mating strategies and ultimately effect the mating behavior of the species.