Oligosoma polychroma

Northern grass skink

Oligosoma polychroma
(Patterson & Daugherty, 1990)

Northern grass skinks. <a href="https://www.instagram.com/nickharker.nz/">© Nick Harker</a> (above), <a href="https://www.instagram.com/samanimalman/">© Samuel Purdie</a> (below).
Image attribution
Northern grass skinks. © Nick Harker (above), © Samuel Purdie (below).
Herpetofaunal category
NZ Skinks
Species complex
Conservation Status
Not Threatened
Previous scientific names
Oligosoma nigriplantare polychroma
Common names
Northern grass skink,
Common skink.

Length: SVL up to 80mm, with the tail being equal to or slightly longer than the body length

Weight: Up to at least 5 grams

Description

Formerly known as the 'common skink', northern grass skinks are an abundant species that is commonly encountered in both native and modified habitats such as suburban gardens.

Head has a short, blunt snout. Pattern can be highly variable. The dorsal surfaces are often various shades of brown with cream or pale-brown dorsolateral stripes which may be smooth-edged or serrated, a mid dorsal stripe (when present) usually extends down the tail. Flanks have a broad dark brown lateral stripe, often edged with black and bordered below by a cream-coloured stripe which may be smooth or crenulated. A pale stripe runs down the anterior surface of the forelimb. The ventral surface is variously coloured shades of grey, brown, or yellow, without speckling. 

Life expectancy

Northern grass skinks have been reported living up to four years in the wild, and up to six years in captivity.

Distribution

Occurs from the central North Island to Wellington in the North Island, and from Nelson along the West Coast to Hokitika in the South Island. In the northern South Island, the species eastern extent is Takapourewa / Stephen's Island in the western Marlborough Sounds, further east it is replaced by a related species - the Waiharakeke grass skink (Oligosoma aff. polychroma Clade 2). Further south it is replaced by the South Marlborough grass skink (Oligosoma aff. polychroma Clade 3) and Canterbury grass skink (Oligosoma aff. polychroma Clade 4) to the east and south of it's range.

Ecology and habitat

Northern grass skinks are diurnal, bold and strongly heliothermic. They can often be seen out basking fully exposed in sunny areas.

In common with other grass skinks the species occupies a wide range of habitat types preferring open areas including coastal vegetation, rock piles, grassland, flaxland, shrubland, screes, forest margins tussock and modified urban / suburban habitats. Often takes refuge in dense vegetation or under rocks and logs when not active.

Social structure

Thought to be largely solitary, but often lives in high-density colonies where multiple individuals can be observed basking in close proximity.

Breeding biology

Northern grass skinks reach maturity in around two years, mating has been reported to occur in March, and females reproduce annually giving birth to litters of up to six juveniles in mid-to-late summer (January- February).

Diet

Northern grass skinks are active hunters and consume a wide range of small invertebrates such as spiders, insects, isopods and molluscs. They also consume the berries / fruit from native plants.

Disease

Unknown.

Conservation strategy

Northern grass skinks are regarded as "Not Threatened" having a wide distribution and generally occurring in abundance at sites throughout its range.

No specific conservation strategy has been developed for Northern grass skinks although they do occur on several pest-free islands.

Interesting notes

Northern grass skinks are members of a cryptic species complex which includes the Waiharakeke grass skink (Oligosoma aff. polychroma Clade 2), south Marlborough grass skink (Oligosoma aff. polychroma Clade 3), Canterbury grass skink (Oligosoma aff. polychroma Clade 4), and southern grass skink (Oligosoma aff. polychroma Clade 5). The various species are regionally distributed, similar in both appearance and habit, and were once regarded as a single highly variable species - the 'common skink'. There are currently no known morphological features to distinguish northern grass skinks from the other species within the complex.

References

Chapple, D. G., Ritchie, P. A. & Daugherty, C. H. (2009). Origin, diversification, and systematics of the New Zealand skink fauna (Reptilia: Scincidae). Molecular Phylogenetics and Evolution, 52(2), 470-487.

Hitchmough, R.A., Barr, B., Lettink, M., Monks, J., Reardon, J., Tocher, M., van Winkel, D., Rolfe, J. (2016). Conservation status of New Zealand reptiles, 2015; New Zealand threat classification series 17. Wellington: New Zealand Department of Conservation.

Jewell, T. (2011). A photographic guide to reptiles and amphibians of New Zealand. Auckland: New Holland Publishing.

Robb, J. (1986). New Zealand Amphibians & Reptiles (Revised). Auckland: Collins, 128 pp.

van Winkel, D., Baling, M. & Hitchmough, R. (2018). Reptiles and Amphibians of New Zealand: A field guide. Auckland: Auckland University Press, 376 pp.