Preliminary evaluation of skin toxins and vocalizations in taxonomic and evolutionary studies of poison-dart frogs (Dendrobatidae). Bulletin of the AMNH ; V. 157, Article 3. Authors: Myers, Charles W.; Daly, John W.

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Studies in progress reveal at least three novel classes of toxic alkaloids in skin secretions of Neotropical dendrobatid frogs. Batrachotoxins are characterized by a steroidal ring structure; those batrachotoxins having a pyrrole carboxylate substituent are among the most toxic of nonprotein poisons. Pumiliotoxins are less toxic and poorly known, but pumiliotoxins A and B appear to have a bicyclic ring system with differing side chains; pumiliotoxin C is a simple cis-decahydroquinoline. Histrionicotoxins have an unusual spiro-ring system and both 4-carbon and 5-carbon side chains, on the cyclohexane and piperidine rings, respectively. Structures are uncertain for other alkaloids of lower molecular weight, but some appear structurally related to pumiliotoxins and others to histrionicotoxins. Methods of study include thin-layer chromatography, gas chromatography, electron impact and chemical ionization mass spectrometry, and quantitative mass spectrometry. Combined gas chromatography-mass spectrometry gives reproducible results for small-sample taxonomic comparisons of frogs containing pumiliotoxins, histrionicotoxins, and similar alkaloids. Limitations of molecular data in taxonomic and evolutionary studies are considered. Biochemical and other variation are analyzed in Dendrobates histrionicus, a rain-forest frog of western Colombia and northwestern Ecuador. Sexual dimorphism is slight, and geographic variation in body size appears correlated with climate. There are geographic differences in relative tibia length and in escape behavior. Interpopulational differences in color and color pattern are extreme, and intrapopulational variation also may be considerable. Most colorations are thought to be aposematic, but highly variable frogs of one population seem cryptically colored. Dendrobates histrionicus elaborates histrionicotoxins and lower molecular-weight alkaloids; one population sample had trace amounts of a higher molecular-weight compound, tentatively identified as pumiliotoxin B. Southern populations produce large amounts of histrionicotoxins and lesser amounts of lower weight alkaloids, a situation reversed in most northern populations. Individual populations have 8-10 of 19 alkaloids detected by gas chromatography-mass spectrometry. Alkaloid similarity comparisons show that most populations share more compounds with near than with distant populations; a geographically intermediate population shares as many or more alkaloids with distant as with neighboring populations. Analysis of geographic variation in skin toxins supports the notion of conspecificity of most histrionicus-like frogs, but different spectra of toxins revealed two additional sibling species. These new species resemble D. histrionicus in morphology (including absence of omosternum), vocalizations, and type of male aggressiveness; and their color patterns, although distinctive, are approached in the variation of histrionicus. Dendrobates lehmanni, new species, lacks histrionicotoxins and produces pumiliotoxins and other alkaloids not detected in histrionicus; it is a black frog with crossbands of vivid orange or orange-red. Dendrobates lehmanni was known for years only from specimens sold in the animal trade, but its habitat is traced to a restricted area of montane forest in the Río Anchicayá drainage of the western Andes, Department of Valle, Colombia. Dendrobates occultator, new species, shows greater biochemical resemblance to histrionicus, but shared alkaloid similarity values are relatively low, and it produces significant amounts of pumiliotoxins and only two kinds of histrionicotoxins (5-7 kinds in populations of histrionicus). Dendrobates occultator is a red-backed frog with yellow lateral spots; it occurs in the Pacific lowlands, in the upper Río Saija drainage, Department of Cauca, Colombia, where it is sympatric with a population of histrionicus. Both lehmanni and occultator may have speciated from geographical isolates of histrionicus; it is suggested that some character divergence might have occurred after the range of occultator was rejoined with the parent species. Dendrobates viridis, new species, is also described, as it occurs sympatrically with the other two new species and evidently has an extensive range in Pacific-side Colombia, along the western flank of the Andes. It is a miniature, uniformly green frog, whose skin secretions include pumiliotoxins. Species of Dendrobates in Central America and northwestern South America seem characterized by either of two kinds of vocalizations, which correlate with calling behavior and type of aggressiveness. Buzz calls are a nearly uniform series of pulses, which are produced too fast for resolution by the human ear but which can be directly visualized on sound spectrograms made with a wide-band filter. Buzz calls are given by D. auratus and D. minutus (and ‘Phyllobates’ espinosai), species that call relatively infrequently and which seem relatively unaggressive. Chirp calls are trains of harsh, poorly modulated notes, in which pulses are produced too fast for resolution on wide-band sound spectrograms. Chirp calls are given by D. granuliferus, D. histrionicus, D. lehmanni, D. occultator, D. pumilio, and D. speciosus — all of which are characterized by nearly incessant calling from perches, and by pronounced male aggressiveness related to territorial defense. Geographic variation in normal and aggressive calls suggests that D. pumilio may actually be a composite of two species, but biochemical and other variation have yet to be investigated.


Authors:  Myers, Charles W.; Daly, John W

Date Published: 1976

Original Publication:  Bulletin of the American Museum of Natural History (AMNH), Volume 157, Article 3.

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