Background
Poison frogs are known for the outstanding diversity of alkaloid-based chemical defences with promising therapeutic applications. However, current knowledge about chemical defences in Dendrobatoidea superfamily has two sources of bias. First, cryptic, brown-colored species have been neglected in comparison to those conspicuously colored, and second, there has been little interest in characterizing metabolites other than alkaloids mediating defensive functions. In an effort to contribute to fill the gap of knowledge about cryptic species and broadening the spectrum of compounds analyzed we have applied head-space solid phase microextraction coupled to gas chromatography and mass spectrometry (HS-SPME/GC-MS) for extracting amphibian alkaloids and volatile organic compounds (VOCs) from Silverstoneia punctiventris.
Results
Using the skin from 8 specimens in 4 biological replicates we have found 33 different compounds. Twenty of them were classified as VOCs into 15 chemical classes including alkanes, alcohols, carbonyl compounds, methylpyridines, benzothiazoles, N-alkylpyrrolidines, pyrazines, and sesquiterpenoids, some of which were previously reported as repellents, defence compounds or defence pheromones in other organisms, and as sex pheromones in a treefrog. Interestingly, six of the remaining compounds were identified as alkaloids previously reported in other toxic/unpalatable dendrobatid frogs.
Conclusions
This is the first report of alkaloids and VOCs found in the Silverstoneia genus, which has been assumed for decades as non-chemically defended. This study establishes HS-SPME/GC-MS as a new application for a simultaneous approach to amphibian alkaloids and VOCs in poison frogs while opens up new research questions to assess the co-occurrence of both type of compounds and to investigate the evolutionary significance of a defence gradient that includes olfactory avoidance, unpalatability, and toxicity in dendrobatids. In addition, our results show that amphibian alkaloids could have a dual function (olfactory at distance, taste by contact) never explored before neither in Silverstonaeia nor in any other dendrobatid species.
Sexual dichromatism has provided important information for understanding sexual selection, but its link to reproductive success has received little attention. Poison frogs and their relatives within the superfamily Dendrobatoidea present striking color variation. Despite this variability, evidence of sexual dichromatism in the over 330 species described is limited to the gular region of some cryptically colored species. Colostethus imbricolus is a cryptically colored dendrobatid with distinct orange and yellow spots at the axillar, inguinal and femoral regions. Here we show that these spots stand clearly out from the environment during behavioral displays and that they present marked sexual differences in their conspicuousness, with females having more conspicuous spots. Unlike most species of Dendrobatoidea that provide parental care in the form of male tadpole transport, we found that in C. imbricolus, females performed this behavior. By correlating color measurements and behavioral observations, we show that females with a higher number of transported tadpoles are also the most conspicuous. Our findings show a significant association between sexual dichromatism of the spots and female parental care while opening exciting perspectives for the occurrence of both traits. In addition, our results provide significant insights to address the function of dual-color patterns (i.e., cryptic from distance-aposematic from nearby) in amphibians.
Even so, calling behaviour in frogs in natural conditions may not be mirrored in captivity (Köhler et al., 2017). On the other hand, Erdtmann & Amézquita (2009) summarized the advertisement call of P. bicolor using the data of only one individual whose origin is not stated and retrieved from a public database which is currently not accessible. Recently, we visited two populations in Santa Cecilia (Risaralda, Colombia) that match the phenotypic description (
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