Serpent Research: Crocodile Skinks of the South Pacific

Skinks come in a plethora of shapes and sizes, with about 1500 species they have invaded a huge variety of habitats and evolved a great diversity of life styles. They are found in almost all landscapes that support squamates and are perhaps the most successful lineage of living reptiles – if the measure is by the number of species. Among the most bizarre skinks are the South Pacific Crocodile Skinks of the genus Tribolonotus. Currently 8 species are recognized and they inhabit northern New Guinea and the Admiralty, Bismarck and Solomon Archipelagoes. They are semi-fossorial lizards, often found under vegetation and in the vicinity of water. At least two species are known to vocalize, and they demonstrate parental care. Hartdegen et al. (2001) described defensive vocalizations and parental care in captive specimens. They observed females curled around their egg and when eggs were gently handled by the observer the female exhibited defensive open-mouth lunges. When eggs were left uncovered by human observers they were reburied by the female. Hatchlings stayed near their mother (within 2 cm) and on occasion they were observed resting on the female's dorsum for two weeks after hatching. Their overall appearance is distinctive; they have unusually heavily keeled or spinose scales and two unique characters, abdominal glands and volar pores (pores on the plantar and palmer surfaces).

Recently, Austin et al. (2010) used molecular techniques and found evidence that Tribolonotus originated on either Greater Bougainville Island or in New Guinea, and subsequently dispersed to surrounding islands multiple times. Maximum body size ranged from 40 mm in T. blanchardi (and T. schmidti was a close second at 41 mm) to 125 mm in T. ponceleti. The authors did not find a phylogenetic explanation for differences in body size, and suggest that it evolved as the result of character displacement and ecological factors.

Exactly what the sister of Tribolonotus is remains a point of contention. They have been considered lygosomine skinks, allied with the genera Sphenomorphus, Mabuya, and Egernia by various authors. However, Donnellan (1991) found Tribolonotus gracilis, has 32 chromosomes, a similar karyotype, to Egerina but there were differences that did not allowed a firm conclusion.

Literature

Austin, C. C., E. N. Rittmeyer, S. J. Richards and G. R. Zug. 2010. Phylogeny, historical biogeography and body size evolution in Pacific Island Crocodile skinks Tribolonotus (Squamata; Scincidae). Molecular Phylogenetics and Evolution, 57:227-236.

Donnellan, S. C. 1991. Chromosomes of Australian lygosomine skinks (Lacertilia: Scincidae).Genetica 83:207-222.

Hartdegen, R. W., M. J. Russell, B. Young, and R. D. Reams. 2001. Vocalization of the Crocodile Skink, Tribolonotus gracilis (De Rooy, 1909), and evidence of parental care. Current Herpetology 2001(2). On-line.

McCoy, M., 2006. Reptiles of the Solomon Islands. Pensoft Publishing, Sofia-Moscow.