Don’t be fooled: brown poison frogs also have chemical defenses on their skin

Gonzalez et al. take us through their study showing that the frog genus Silverstoneia should no longer be assumed as a non-chemically defended genus.

The golden poison frog (Phyllobates terribilis). Photo by Adolfo Amézquita.

Poison frogs within the superfamily Dendrobatoidea (the superfamily of poison frogs) contains numerous species that use bright coloration to visually advertise their toxicity or unpalatability to predators. The presumably most toxic vertebrate on earth, the frog Phyllobates terribilis, with its bright yellow coloration also uses this strategy.

 

Silverstoneia punctiventris carrying two tadpoles. Photo by Pablo Palacios-Rodríguez.

However, a recent publication in Frontiers in Zoology demonstrated the presence of toxic alkaloids and possibly repellent smells in a brown little frog from the same superfamily, which was thought by decades to lack chemical defenses. This finding shows how little we know about chemical defenses in cryptically colored species (e. g. brownish) and increases the chemodiversity of compounds found in poison frogs.

Have you ever wondered what is the smell of a poison frog? Well, we can tell you that it’s like nothing any of us had ever smelled before—so we were determined to find out what made up that very unique smell.

We used a headspace technique (HS-SPME/GC-MS) to characterize the smell of Silverstoneia punctiventris, an endemic poison frog from Colombia. The smell is composed of a mixture of 6 toxic alkaloids and 20 volatile organic compounds (VOC) reported for the very first time in this species and the complete superfamily. This technique allowed a simultaneous analysis of alkaloids and VOCs, in contrast with previous analysis in other species exclusively focused on alkaloid profiles. The alkaloids detected are known to be harmful to predators, thanks to their toxic effects and bitter taste.

Compounds extracted from Silverstoneia punctiventris

 

In the same way as bright coloration could be a useful visual signal to advertise toxicity, a bad smell could be a useful olfactory signal to advertise toxicity.

The characterization of the smell of this less colorful frog incorporates a new olfactory dimension of chemical defense in dendrobatids, where frogs’ smells could be airborne transported and perceived by distance (without contact). In the same way, as bright coloration could be a useful visual signal to advertise toxicity, a bad smell (whatever that means to a predator) could be a useful olfactory signal to advertise toxicity. Possible behavioral functions proposed in this article need to be verified in the future. In addition, surveys of the volatilome in other dendrobatids will be essential to examine our hypotheses. Right now, we know that humans (as potential predators) can detect, at least, some poison frogs’ volatiles. Indeed, the motivation of the authors to characterize this cryptic frog volatilome came from observations, or better, smells from field work after manipulating the frogs.

In the meantime, the undeniable truth is that, in opposition to what was thought some decades before, not always the most colorful poison frogs are more toxic, and we need to gain more knowledge about the biology and the chemistry about brownish dendrobatid frogs. Fighting against this color bias of how science is performed within this field will lead to discovering more secrets, as those who have been recently uncovered from S. punctiventris.

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