Length: SVL up to 116mm, with the tail being equal to or longer than the body length

Weight: unknown

Description

A large, and enigmatic skink species endemic to the isolated Poor Knights Islands off the coast of Northland. Although a close relative of the Hauraki (Oligosoma townsi), Coromandel (Oligosoma pachysomaticum), and Whitaker's skinks (Oligosoma whitakeri), they have remained isolated from them for around 2 million years, becoming larger, and filling the niche that larger skinks (e.g., robust skink (Oligosoma alani), and Falla's skink (Oligosoma fallai)) do on other small island groups. 

They are characterised by their light brown dorsal (upper) surfaces, which on occasion are flecked with washed-out patches of dark brown/black, and lighter brown pigments. The lateral surfaces (sides) have a marbled appearance, comprising a patchwork of dark grey to black markings and irregular white patches. This pattern is concentrated on the front half, particularly around the face and neck, and often becomes more washed out towards the rear (particularly in older animals). A ‘tear-drop’ marking is present under each eye (white with black edging), with the remainder of the ‘lips’ being completely black, apart from occasional small white markings. The ventral surface (stomach) is cream with broad dark blotches, whilst the throat is white with heavy black speckling or streaks. 

Marbled skinks can easily be differentiated from the four other species of skink on the Poor Knights Islands based on size and pattern.  Shore skinks (Oligosoma smithi - ≤80mm SVL), Aorangi skinks (Oligosoma roimata; ≤65 mm SVL), and Hardy’s skinks (Oligosoma hardyi; ≤62 mm SVL) are much smaller than adult Marbled skinks, whilst the similarly-sized egg-laying skink (Oligosoma suteri) significantly differs in colouration, and patterning, lacking the marbled (black and white patchwork) appearance seen in marbled skinks. 
Young marbled skinks may be confused with the related Aorangi skink (Oligosoma roimata), but as with the other skinks on the Poor Knights Islands, Aorangi skinks lack the marbled (black and white patchwork) pattern seen on the lateral surfaces of marbled skinks. 

Life expectancy

Largely unknown, however, other Oligosoma species have been recorded reaching ages of over ten years in the wild, whilst the closely-related Whitaker's skink (Oligosoma whitakeri) has been reported to live for 55+ years in captivity (D. Keall, personal communication, May 22, 2021).

Distribution

Marbled skinks are restricted to several islands and rock stacks in the Poor Knights group, off the coast of Northland.

Ecology and habitat

The marbled skink is nocturnal in nature, with crepuscular (dawn and dusk) tendencies, although gravid females are known to bask during the day. As with most Oligosoma, they are terrestrial (ground-dwelling), spending the majority of their time foraging on the forest floor.

Being endemic to the Poor Knights Islands, this species is associated with the coastal forest and scrub that dominates this group. In particular, this species is associated with areas of deep leaf litter, as well as the honeycomb-like systems of burrows that occur within the large petrel colonies on the islands.

Social structure

The marbled skink is generally considered to be solitary in nature, although they may be found inhabiting the same refugia as congeners. Males are likely to show aggressive behaviour towards other males, especially during the breeding season. Neonates (babies) are independent at birth.

Breeding biology

Like the majority of Aotearoa's skink species, the marbled skink is viviparous, giving birth to 2-4 live young around March/April. As is the case with many lizard species, mating in Oligosoma may seem rather violent with the male repeatedly biting the female around the neck and head area. Sexual maturity is probably reached between 2-4 years.

Diet

Marbled skinks are omnivores. They are primarily insectivorous in nature, but are known to feed on the nectar, and small fruits of several plant species when they are seasonally available. Being terrestrial in nature, their invertebrate prey tends to be predominantly composed of ground beetles, spiders, weta, and other ground-dwelling invertebrates. They have been recorded feeding on the fruit and berries of Coprosma species, kawakawa (Macropiper excelsum), and small-flowered nightshade (Solanum nodiflorum).

Disease

The diseases and parasites of Aotearoa's reptile fauna have been left largely undocumented, and as such, it is hard to give a precise determination of the full spectrum of these for many species.

The marbled skink, as with other Oligosoma species, is a likely host for at least one species of endoparasitic nematode in the Skrjabinodon genus (Skrjabinodon trimorphi), as well as at least one strain of Salmonella. In addition to this, it is known to be a host for at least one species of ectoparasitic mite in the Ophionyssus genus.

Conservation strategy

DOC classify this species as 'At Risk - Naturally Uncommon' (Hitchmough et al. 2021), due to their small natural range. Marbled skinks are not being actively managed and are present in moderate to high numbers on islands within the Poor Knights group. Accordingly, they are at no immediate risk of extinction.

Interesting notes

Named after the former Director of the Dominion Museum, Dr W.R.B. Oliver. Accordingly, marbled skinks are sometimes referred to as Oliver’s skink.

The marbled skink species-complex has proven especially difficult to resolve because genetic traits have often suggested a different evolutionary scenario than what other (morphological, ecological and biogeographical) traits imply, and as a result the taxonomy of the complex has been much debated and somewhat unstable for the past thirty years. The definition of the marbled skink has thus changed from its original concept of a Poor Knights Islands endemic (McCann 1955), to a species widespread among various islands from the Poor Knights south to the Coromandel Peninsula area (Hardy 1977), to one found on the Poor Knights and Coromandel areas but not in between (Chapple et al. 2008), and eventually back to its original Poor Knights Islands only concept (Jewell 2019).

The marbled skink, along with its sister taxa (the Coromandel skink and Whitaker's skink) sit within clade 4 (the teardrop skink complex) of the Oligosoma genus, with the Hauraki skink being their closest relative within the group.

References

Chapple, D. G., Daugherty, C. H., & Ritchie, P. A. (2008). Comparative phylogeography reveals pre-decline population structure of New Zealand Cyclodina (Reptilia: Scincidae) species. Biological Journal of the Linnean Society, 95(2), 388-408.

Chapple, D. G., Patterson, G. B., Gleeson, D. M., Daugherty, C. H., & Ritchie, P. A. (2008). Taxonomic revision of the marbled skink (Cyclodina oliveri, Reptilia: Scincidae) species complex, with a description of a new species. New Zealand Journal of Zoology, 35(2), 129-146.

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.

Hardy, G. S. (1977). The New Zealand Scincidae (Reptilia: Lacertilia); a taxonomic and zoogeographic study. New Zealand journal of zoology, 4(3), 221-325.

Hardy, G. S. (1979). The karyotypes of two scincid lizards, and their bearing on relationships in genus Leiolopisma and its relatives (Scincidae: Lygosominae). New Zealand Journal of Zoology, 6(4), 609-612.

Hitchmough, R., Barr, B., Knox, C., Lettink, M., Monks, J. M., Patterson, G. B., Reardon, J. T., van Winkel, D., Rolfe, J., & Michel, P. (2021). Conservation status of New Zealand reptiles, 2021. New Zealand threat classification series 35. Wellington: New Zealand Department of Conversation.

Jewell, T. R. (2019). New Zealand forest-dwelling skinks of the Oligosoma oliveri (McCann) species-complex (Reptilia: Scincidae): reinstatement of O. pachysomaticum (Robb) and an assessment of historical distribution ranges. Zootaxa, 4688(3), 382-398.

McCallum, J. (1981). Reptiles of Tawhiti Rahi Island, Poor Knights Islands, New Zealand. Tane, 27, 55-58.

McCann, C. (1955) The lizards of New Zealand. Gekkonidae and Scincidae. Dominion Museum Bulletin, 17, 1–127.

Towns, D. R. (1999). Cyclodina spp. skink recovery plan 1999–2004: Threatened species recovery plan 27. Wellington: Department of Conservation.

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

Whitaker, A. H. (1968). The lizards of the Poor Knights Islands, New Zealand. New Zealand journal of science, 11, 623-651.

Worthy, T. H. (1987). Osteological observations on the larger species of the skink Cyclodina and the subfossil occurrence of these and the gecko Hoplodactylus duvaucelii in the North Island, New Zealand. New Zealand journal of zoology, 14(2), 219-229.

Wotton, D. M., Drake, D. R., Powlesland, R. G., & Ladley, J. J. (2016). The role of lizards as seed dispersers in New Zealand. Journal of the Royal Society of New Zealand, 46(1), 40-65.