''Saiphos equalis'', commonly known as the yellow-bellied three-toed skink or simply three-toed skink, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of burrowing

skink Skinks are lizards belonging to the family Scincidae, a family in the infraorder Scincomorpha. With more than 1,500 described species across 100 different taxonomic genera, the family Scincidae is one of the most diverse families of lizards. Ski ...

found in eastern

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. It is the only species classified under the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

''Saiphos''. The lizard has attracted scientific attention due to its dual reproduction habits of producing young via eggs in coastal populations, or via live young in colder mountain regions.Marshall,M., Live birth, evolving before our eyes, ''New Scientist'', 25 August 2010

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Description

''Saiphos equalis'' grows to a length of including the tail. It has a brown back and an orange belly. The skink is active at night, and feeds on insects. Along the coastal lowlands of Australia, the skink has been observed laying eggs and giving birth to live young for reproduction. Individuals of the species in the lowlands lay eggs (oviparous), while its neighbors to the north in the mountains are almost exclusively giving birth to live young (viviparous).

Taxonomy and nomenclature

''Saiphos equalis'' was originally described by the British people, British

zoologist Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological clas ...

John Edward Gray John Edward Gray, FRS (12 February 1800 – 7 March 1875) was a British zoologist. He was the elder brother of zoologist George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray (1766–1828). The same is used for ...

in 1825 as ''Seps equalis''. In 1831, Gray reclassified it under a genus he separately established, ''Saiphos''. ''Saiphos equalis'' is now considered to be the only member of the

''Saiphos''. It belongs to the

subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end subfamily botanical names with "-oideae", and zoologi ...

Lygosominae of the skink

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Scincidae.

Phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

analysis in 2003 showed that the closest sister taxa of ''Saiphos equalis'' are '' Coeranoscincus reticulatus'' and members of the genus '' Ophioscincus''. They are part of the Australian ''Sphenomorphus'' group, a large

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

within Lygosominae.

Distribution and habitat

''Saiphos equalis'' are common in

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

and

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...

of eastern Australia.

Reproduction

''Saiphos equalis'' includes populations of three main reproductive modes: oviparous with long (15 days) incubation periods, viviparous with no incubation period (0 days), and intermediate populations that are oviparous with short (~5 days) incubation periods. No populations of this skink exhibit normal scincid oviparity behavior with greater-than-30-day incubation periods, which could further indicate this skink is truly making the transition to exclusively viviparity. In a study, published in 2001, of coastal ''Saiphos equalis'' populations, mitochondrial nucleotide sequences ( ND2 and cytochrome b) were used to organize the relationships among the various populations. According to Smith ''et al.'' the analysis suggests that the long incubation period oviparous lineage is the sister group to the other short-period oviparous and viviparous populations. These clades are consistent and correspond to variation in reproductive modes as well as geographic location according to latitude and altitude. Lizards from high elevation sites (greater than ) in north-eastern

are viviparous, while low-elevation populations from northern and southern in New South Wales exhibit short-period oviparity, an intermediate between viviparity and typical oviparous behaviors. The viviparous populations give birth to fully developed offspring in transparent membranes, while the short-day oviparous populations give birth to partly shelled eggs that contain mostly developed

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s. The embryo continues to develop in the egg prior to hatching. In the northernmost coastal region of New South Wales, the lizards have relatively long incubation periods (approximately 15 days), and the eggshells are thicker. In April 2019 ''Saiphos equalis'' made news when researchers from the

University of Sydney The University of Sydney (USYD), also known as Sydney University, or informally Sydney Uni, is a public research university located in Sydney, Australia. Founded in 1850, it is the oldest university in Australia and is one of the country's si ...

reported observing a female laying eggs and giving birth to live young from the same pregnancy, the first reported observation of a vertebrate doing this.

Description

Taxonomy and nomenclature

Distribution and habitat

Reproduction

References

Further reading