Two Taipans - Coastal and Western

There are two taipan species in Australia:

The Coastal Taipan, Oxyuranus scutellatus from the near coastal, open forests of northern and eastern Australia.		The Western Taipan, Oxyuranus microlepidotus from the ashy downs of the Channel Country, south-western Queensland and north-eastern South Australia.

What follows is a technical account of the features distinquishing them.

Relationships between the two species

In 1981, examination of external features, facial muscles, skull measurements, venom, hemipenis anatomy, numbers and shapes of chromosomes, and behaviour of the two species then known as Oxyuranus scutellatus and Parademansia microlepidota, showed they should be placed together in one genus. They were more closely related to each other than to any other species. Oxyuranus (1923), was the appropriate generic name for both species because it was pre-dated Parademansia (1955). The work of Covacevich, McDowell, Tanner & Mengden (1981) led to the change (see further information).

Size
Both taipan species are large. Females of each reach similar maximum sizes (mean snout-vent length approximately 145 cm), but male Coastal Taipans grow much larger than male Inland Taipans (mean snout-vent length 156 cm and 132 cm). Males of both species become sexually mature at smaller body sizes than females. (In O. scutellatus, 78 cm for males and 101 cm for females; in O. microlepidotus 92 cm for males and 133 cm for females).

Scale Counts
Midbody (O. scutellatus 21 or 23, O. microlepidotus 23, rarely 25); anal (single, single); ventrals (220-248, 211-224); subcaudals (48-76 paired, 54-66 paired).

Colour
O. scutellatus: unmarked light olive to dark russet brown dorsally (specimens from the Tully area, NEQ, almost black); head usually lighter coloured, especially in the rostral and labial regions, ventrally cream, usually with pink or orange flecking; mouth pink; eye reddish.

O. microlepidotus: pale to very dark brown dorsally, often with dark flecks that may form distinct bands posteriorly; head glossy black in freshly collected specimens, lighter in captive specimens; ventrally, behind the black neck region mustard yellow with or without flecks; mouth of dark specimens blue-grey to pink, of lighter specimens greyish pink to off-white; eye black.

Skulls

	Coastal and Western Taipan skulls

Muscles are very similar. In both species the venom gland muscle is very broad and arises from the entire length of the parietal.

Skull measurements, ratios and dental characters of both species similar. They differ, however, in size and proportion and there are differences in numbers of teeth. The fangs in adult O. scutellatus are very large (7.9-12.1 mm) while those of O. microlepidotus are of moderate length (3.5-6.2 mm). All posterior maxillary teeth in O. scutellatus are solid, while those of O. microlepidotus appear to be fully functional fangs.

Venom
Very similar. They are closer to those of each other than to those of any other large Australian elapid, despite differences in the relative proportions of components. Amino acid compositions of the neurotoxins are also very similar. The venoms of O. scutellatus and O. microlepidotus differ, however, in toxicity, yield, and hyaluronidase activity. (This reflects the rate at which venom disperses through tissue. Hyaluronidase is an enzyme). The effects of the venom of both species in humans are primarily neurotoxic. Coastal Taipan venom also has myotoxic and coagulant effects, whereas only the latter has been reported for the Western Taipan (Sutherland & Tibbals, 2001).

Hemipenis

	Coastal and Western Taipan hemipenes

In both species the hemipenis is moderately long, narrow and simple. There are minor differences in the patterns and numbers of spicules and spines.

Chromosomes
Gross comparisons show the two species share the same paired number (2n = 35) and that their chromosomes are similar in form.

Behaviour
Activity times: Period of greatest activity for O. microlepidotus is 8.00 - 10.00 am, occasionally late afternoon in early Spring. For O. scutellatus, 9.00 - 11.00 am, rarely late afternoon and early evening throughout the year, but most commonly encountered in late Winter and early Spring.

	Western Taipan food - the Plague Rat

Feeding: Both species feed only on mammals, mainly rats and mice, a feeding strategy unique amongst Australian elapids. Apparently, this has led to an increase of Coastal Taipan numbers where the Cane Toad, Bufo marinus, is common. The Cane Toad is highly toxic to all but one species of frog-eating snake and populations of species which feed on toads, appear to have declined. Taipans may have benefitted from decreased competition for food and gained living space left vacant.

Sugar cane growing may also have increased availability of sheltering sites and food supply. Populations of O. microlepidotus appear to rise and fall with those of the Plague Rat, Rattus villosissimus. This species is renowned for its adaptation to life in the arid zone, through rapid response to a series of good seasons in an area where such seasons are a rarity. Adaptations of both species to feeding on mammals include large size, highly toxic venom, and a snap - release bite.

Breeding: Both species are oviparous: O. scutellatus has a clutch size of 7-17; O. microlepidotus lays from 12 to 20 eggs. Egg size is similar (O. scutellatus, 5.0 - 6.2 x 2.4 - 2.6 cm; O. microlepidotus average, 6.0 x 3.5 cm), as is the incubation period of approximately 66 days (for both species) and length of hatchlings - about 44.0 cm in both species.

Summary

Coastal Taipan (Oxyuranus scutellatus)

Size (maximum total length): 290 cm

Scale count: mid-body 21 or 23, ventrals 220-250, anal single, subcaudals 57-75 (paired).

Colour: unmarked light olive to dark russet brown dorsally (specimens from the Tully area, NEQ, almost black); head usually lighter coloured, especially in the rostral and labial regions; ventrally cream, usually with pink or orange flecking; buccal cavity pink; eye reddish.

Distribution: Mitchell Plateau (WA); near Darwin, Birdum and Oenpelli (NT); Normanton across Cape York Peninsula and coastally through eastern Queensland to Grafton (NSW); well known in sugar-growing areas; in southern Queensland, common near Beaudesert, Esk, Gympie; recent records close to Ipswich at Brisbane - Toowoomba bypass and Cedar Creek, near Samford; found on Fraser, but not on other sand islands of south-eastern Queensland.

Habitat: open forests, dry closed forests, coastal heaths, foredunes and cultivated areas.

Food: mammals - rats (Melomys spp., Rattus spp.), House Mouse (Mus musculus), bandicoots (Isoodon macrourus, Peramelas nasuta), Quoll (Dasyurus hallucatus).

Venom: strongly neurotoxic; the third most toxic terrestrial snake venom known; yield 120-400 mg.

Breeding: oviparous; 7-20 eggs; 66 ± 2 day incubation period, average snout-vent length of hatchlings 31 cm.

Size (maximum total length): 170 cm

Scale count: 23 infrequently 25, ventrals 212-237, anal single, subcaudals 54-65 (usually paired, but sometimes with 3-5 anterior scales single).

Colour: pale to very dark brown dorsally, often with dark flecks that may form distinct bands posteriorly; head glossy black in most freshly collected specimens (this sometimes fades with captivity); ventrally (behind the black neck region) mustard yellow without flecks; mouth lining in dark specimens blue grey shading to pink, in lighter coloured specimens greyish pink shading to off-white; eye black.

Distribution: Cooper Creek, Diamantina and Georgina River drainage systems; between Boulia and Hamilton (W.Q.) and Goyder's Lagoon (S.A.); and near Coober Pedy (SA), as a population isolate; old records, Bourke (NSW) and, possibly, junction of Murray and Darling Rivers.

Habitat: ashy downs with saltbush (Atriplex spp.), Channel Millet (Echinochloa turnerana), Rat-tail Couch (Sporopolus mitchelli).

Food: Mammals - Plague Rat (Rattus villosissimus), House Mouse (Mus musculus), Kultarr (Antechinomys laniger).

Venom: strongly neurotoxic; the most toxic land snake venom known; yield 30-110mg.

Breeding: oviparous; 9-12 eggs; 66 day incubation period; average snout-vent length of hatchlings 40 cm.