Frogs Of A Different Colour
Suggest ‘colour changes’ and one immediately thinks of a chameleon or an octopus or even fall foliage. But nature has more than one ‘trick’ up her sleeve. The above, after all, are temporary changes in response to environmental cues. Some species, however, show non-reversible (permanent) colour and pattern changes as they develop progressively from juvenile to adult. Scientists refer to this as ontogenetic colour change (OCC). Though OCC has been used in evolutionary and behavioural biology studies in reptiles, it has rarely been used in the science of naming, describing and classifying (taxonomy) amphibians.
Photograph by Dr. S.D. Biju.
Changing that scenario is a new and detailed study by a team of scientists at the Systematics Lab, Department of Environmental Studies, University of Delhi. Scientists S.D. Biju, Rachunliu G. Kamei, Stephen Mahony, Ashish Thomas, Gargi Sircar and Robin Suyesh reviewed four known species of Rhacophorus endemic to the Western Ghats – their current taxonomic status, their geographic distribution on the basis of morphological and molecular data, and shed light on the ontogenetic colour changes in these species. Based on this, they also suggested a taxonomic revision of the species. Their paper, published in Zootaxa (3636 (2): 257–289), also provides observations on the nesting behaviour of the four species and reports multiple male participation (polyandry) during amplexus, oviposition and foam nest construction in two of them – R. lateralis and R. malabaricus.
The Rhacophoridae (Asian treefrog) family has some 350 species in 16 genera of which 14 species are found in India. Four of these – Rhacophorus calcadensis, R. lateralis, R. malabaricus and R. pseudomalabaricus are found only in the Western Ghats.
It was seen that R. calcadensis (facing page, first row) changes from a uniformly green dorsum (upper side) to a light brown in the adult. The red eye colour of the juvenile becomes grey. In R. lateralis (facing page, second row), the light green or greenish-orange dorsum shows heavy pigmentation (black spots) in the juvenile. These are lost or become lighter in the adult. The bright yellow ring bordering the pupil also vanishes in the adult. In R. malabaricus (facing page, third row), the greenish black, pale yellowish-green or bluish green spotted juvenile transforms into a uniform bright green adult. The black spots may or may not persist to adulthood. The juvenile also has longitudinal black bands on the fore and hind limbs that are not seen in the adult. The webbing between fingers and toes is yellowish in juveniles and turns to light or dark red in adults. In R. pseudomalabaricus (facing page, fourth row), the light green dorsum including the fore and hind limbs of metamorphs have distinctive thick zebra-like black lines that become fainter like the venation of leaves on a vibrant green dorsum in adults.
Says Dr. Biju, “The study provides a framework for a more comprehensive study on ontogenetic changes in Rhacophorus that may help in improving the current poor taxonomic definition of the genus. A phylogenetic study on OCC in rhacophorids, that is currently lacking, might yield interesting results. This taxonomic review primarily functions to provide a modern description of the type specimens of this relatively well-known group of large, conspicuous tree frogs, and also clarifies the longstanding confusion regarding the holotype of R. malabaricus."
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First appeared in: Sanctuary Asia, Vol. XXXIII, No. 3, June 2013.