East Asian Forest Frogs

Black-striped frog Sylvirana nigrovittata, from here.

One group of animals that has somewhtat flown (or at least hopped) under the radar here at Catalogue of Organisms is the frogs. Frogs are perhaps one of the most instantly recognisable of all terrestrial animal groups, with a combination of features that is truly unique (see this post at an older iteration of Tetrapod Zoology for a list of some of their eccentricities—I mean, the things don't have a rib-cage. Maybe fish can get away with those sorts of shenannigans, but I expect any vertebrate crawling around on land to be fully skeletoned up, thank you.) Frogs come in a wide range of shapes and sizes, but perhaps the group most often thought of as the classic 'frogs' are the members of the family Ranidae. A large proportion of these mostly smooth-skinned, long-legged frogs were classified until recently in a single genus Rana. This was always seen as something of a generalised group, characterised as much by the absence of the derived features of other ranid genera such as the torrent-dwelling Amolops as by anything else. As such, it was long expected that more detailed studies of ranid relationships would lead to the Rana monolith being broken down somehow. In 1992, Alain Dubois presented a classification of the Ranidae in which he divided Rana between a number of subgenera, some of which were further divided into sections and species groups. This classification was presented by Dubois as explicitly provisional: the arrangement of taxa was based on overall similarities rather than any explicit analysis, and was largely intended to provide some sort of starting point for future analyses.

One of the new taxa recognised by Dubois (1992) was Sylvirana, which he erected as a new subgenus of Rana containing an assortment of species found in southern and eastern Asia. Members of this group had a foot with an external metatarsal tubercle, suction pads on digit III of the fore foot and digit IV of the hind foot but often not on fore digit I, and males with a humeral gland and internal or external subgular vocal sacs. Their tadpoles had long papillae along the edge of the lower lip, and often had dermal glands. As indicated by the name, species of Sylvirana were mostly found in forests.

Günther's frog Sylvirana guentheri, copyright Thomas Brown.

When the broad genus Rana was later carved up by Frost et al. (2006), they recognised Sylvirana as a separate genus (albeit without quite the same composition as Dubois' version). Since then, the status of Sylvirana has shifted around a bit; some authors have sunk it into a broader Old World tropical genus Hylarana on the grounds of non-monophyly. Oliver et al. (2015) conducted a molecular phylogenetic analysis of the Hylarana group that lead them to propose Sylvirana as the name for a clade of southeast Asian frogs that they recovered. A number of Indian species previously assigned to Sylvirana formed a separate clade that they recognised as a distinct genus Indosylvirana. Morphological differences between Sylvirana and Indosylvirana are slight, but males of the former have a larger humeral gland: three-quarters the length of the humerus vs two-thirds the length in Sylvirana. It's worth noting that, although Dubois (1992) recognised a number of ranid taxa as lacking a humeral gland, most if not all of them do indeed possess this gland, just not raised and readily visible externally as in Sylvirana.

The species of Sylvirana sensu Oliver et al. (2015) are generally medium-sized, robust frogs with a shagreenate back and smooth or slightly warty sides. They generally have a dark stripe along the side of the body, becoming broken into dark spots lower down. Widespread species include Sylvirana nigrovittata, commonly known as the black-striped frog (a completely non-distinct name, I have to say, considering that it could apply to any one of dozens of ranid species; Wikipedia calls it the sapgreen stream frog, which on the one hand is a much more distinctive name, but on the other hand suffers from the point that all the individuals I've seen photographed of this species look more brown than green). This species is found over pretty much the entire continental range of the genus, from eastern India and Nepal to Vietnam and Malaysia. Also widespread is Günther's frog S. guentheri, which is found in southern China, Taiwan and Indochina. This species is also found in Guam where it was first recorded in 2001 and has since become well-established (Christy et al. 2007). It is believed to have made its way there as a stowaway in shipments of aquaculture stock though, as it is itself captured for food in its native range, it is not impossible that it may have been introduced deliberately.

REFERENCES

Christy, M. T., J. A. Savidge & G. H. Rodda. 2007. Multiple pathways for invasion of anurans on a Pacific island. Diversity and Distributions 13: 598–607.

Dubois, A. 1992 Notes sur la classification des Ranidae (Amphibiens Anoures). Bulletin Mensuel de la Société Linnéenne de Lyon 61 (10): 305–352.

Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. de Sá, A. Channing, M. Wilkinson, S. C. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green & W. C. Wheeler. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297: 1–370.

Oliver, L. A., E. Prendini, F. Kraus & C. J. Raxworthy. 2015. Systematics and biogeography of the Hylarana frog (Anura: Ranidae) radiation across tropical Australasia, Southeast Asia, and Africa. Molecular Phylogenetics and Evolution 90: 176–192.