Western Brown Snakes

Introduction

For many years it was suspected that the widespread Western Brown Snake (Pseudonaja nuchalis) was in fact a composite species, however efforts to split nuchalis were largely defeated by the extreme level of colour and pattern variation encountered both within and between populations. Ontogenetic colour changes, suggestions of intergrades, and possible hybridisation with other Pseudonaja species added to the confusion. Despite the enormous challenge researchers were able to narrow down a number of basic colour morphs, and recent genetic studies have now built upon earlier findings to confirm the existence of at least three species within the nuchalis-complex.

Identification

Pseudonaja aspidorhyncha
A long and slender snake, with a smallish head indistinct from the neck. When viewed from above the snout has a chisel-shaped appearance due in part to the enlarged strap-like rostral scale. Scales are smooth and semi-glossy, often very glossy around the head and neck. Dorsal colour and pattern is highly variable; the base colour ranges from pale to medium brown on the body, with the head often darker brown. While some individuals may be unpatterned, there is usually a degree of dark banding present which is often more pronounced along the sides and posterior half of the body. These bands can be one to many scales wide and range from very faint to quite distinct. Broad bands are occasionally interspersed with a number of narrower bands. A dark band on the neck is often present, and there may also be a scattering of dark brown or black scales on the head and neck. The belly is dirty-cream, yellow or medium brown, becoming paler under the throat and chin. The eye is large with a dark iris and a reddish-orange ring around the pupil. The inside of the mouth is dull pinkish-grey.

Midbody scales in 17 rows, ventrals 207-226, anal and subcaudal scales divided.

Pseudonaja mengdeni
A long and slender snake, with a smallish head indistinct from the neck. When viewed from above the snout has a rounded appearance. Scales are smooth and semi-glossy, often very glossy around the head and neck. Dorsal colour and pattern is variable but almost always conforms to one of two distinct forms; the ‘Orange with black head’ or ‘Pale head, grey nape’ morph (described by Mengden, 1985).

'Orange with black head’ - base colour ranges from pale to dark yellow or orange on the body, with the head and neck being dark brown or black. The body typically has a fine to heavy black reticulated pattern, which is often absent anteriorly. Sometimes there may also be a series of broad, dark brown bands down the length of the body. The belly is cream or yellow, and is often blotched with orange, salmon, dark brown or grey. Chin and throat cream, dark grey or black. The eye is large with a dark iris and a reddish-orange ring around the pupil. The inside of the mouth is blackish.

‘Pale head, grey nape’ - base colour ranges from pale to medium brown or yellowish, with the head and neck pale to light brown. The body typically has a fine reticulated pattern, which is often absent anteriorly. An indistinct pattern of thin greyish brown bands can be present (more obvious on the sides and towards the back half). Some specimens may exhibit a pattern of broad dark brown bands interspersed with narrower bands. Typically there is a broad darker brown or greyish brown band on the neck, often bordered anteriorly by several black scales (sometimes forming a ‘V’ or ‘W’ shape). The belly is cream or yellow, and is often blotched with orange, salmon, dark brown or grey. Chin colour is cream. The eye is large with a dark iris and a reddish-orange ring around the pupil. The inside of the mouth is blackish.

Midbody scales in 17 rows, ventrals 193-224, anal and subcaudal scales divided.

Pseudonaja nuchalis
A long and slender snake, with a smallish head indistinct from the neck. When viewed from above the snout most often has a chisel-shaped appearance due in part to the strap-like rostral scale, other times the snout may appear slightly rounded. Scales are smooth and semi-glossy. Dorsal colour light to medium brown on the body, with the head and neck sometimes dark brown or black and the snout being paler than the head. Occasionally there is a series of faint to obvious broad dark brown bands along the body, and scattered dark brown or black scales on the neck. The belly is cream or pale yellow, often with salmon blotches (particularly on the anterior section); chin is cream coloured. The eye is large with a dark iris and an orange ring around the pupil. The inside of the mouth is blackish.

Midbody scales in 17 rows, ventrals 194-207, anal and subcaudal scales divided.

Habitat

All species - May be found in a range of arid and semiarid habitats, including grassland, shrubland, savannah woodland and dry sclerophyll forest. Also commonly found in pastoral areas. Western Brown Snakes will take shelter under any available groundcover, natural or man-made, e.g. fallen timber, rock slabs, corrugated iron sheeting, and will also utilize disused animal burrows and deep soil cracks.

Distribution

The extent of each species’ distribution is only roughly known at present, pending further examination of museum specimens from known localities:

Pseudonaja aspidorhyncha
Southern central Australia, south of 27°S, from Penong, western Eyre Peninsula, South Australia, eastwards to Hermidale, central New South Wales.

Pseudonaja mengdeni
Western and central Australia, from Carnarvon, Western Australia, to Mootwingee National Park, western New South Wales.

Pseudonaja nuchalis
Tropical Northern Territory, north of 17°S.

Feeding and diet

Western Brown Snakes are known to feed on a wide variety of vertebrate prey, including lizards (geckos, skinks and goannas), other snakes, fledgling birds, and small mammals such as rodents (in rural areas they predate heavily on the introduced house mouse). In captivity juvenile snakes appear to prefer small lizards, moving on to mice and rats as they mature. Western Brown Snakes are generally diurnal hunters; however during periods of hot weather they may forage late in the evening and at night. They have keen eyesight, and are quickly alerted to any movement that could signal a potential meal. When these snake strike they hang on to their prey and will often then use constriction as a further means of securing the prey until the venom takes effect.

Other behaviours and adaptations

There is a record of a captive Western Brown Snake (originally from Renmark, SA) exhibiting seasonal colour change, becoming a dark chocolate colour over the cooler months and then changing to a paler brown as the warm weather returned. The change appeared dramatically when the animal sloughed its skin in autumn and then again in spring, and this pattern was repeated for several years following the initial observation. It is suggested that in the wild these snakes may adopt a dark phase during winter to help absorb heat, and then become lighter to reduce the risk of overheating in summer.

Western Brown Snakes appear to show a degree of sexual dimorphism in that females tend to be more slender than males of similar length. One reptile keeper noted that female Westerns are particularly nervous and "fiery", and do not settle down in captivity as readily as males.

This group of snakes is primarily terrestrial, however they are known to occasionally climb, with one individual (?species) being found in the branches of a tree 3m above the ground.

Breeding behaviours

The following is a generalized account of the reproductive biology and behaviour of the nuchalis-complex as a whole:

In the south of the range, courtship and mating begins in spring and may extend into summer. In central and northern Australia the breeding season is less restricted and may take place whenever conditions are suitable. In captivity males have been observed engaging in combat, which involves “plaiting” the bodies together and pushing down the opponent’s head, however prolonged bouts of biting have also been recorded (generally unusual in male combat). Between 6 and 9 weeks after copulation the female produces up to 38 eggs (average of 12). In a good season a female may produce a second clutch 40-60 days after the first.

The incubation period may be quite variable, e.g. at 30ºC individual clutches took between 61 and 83 days to hatch. The emerging young measure just over 30cm, and siblings often show a high degree of polymorphism (varying colour and pattern forms). It is interesting to note that in any clutch there are nearly always more males than females, regardless of incubation temperature.

Conversation status

All species of Western Brown Snake are fairly common and their status is secure. Population numbers may have even increased in some rural areas due to the abundance of rats and mice.

Predators

The recorded predators of the species include the Mulga Snake Pseudechis australis and various birds of prey.

Danger to humans

Western Brown Snakes in general are fast moving, nervous snakes. They will quickly take cover if given the opportunity, however if caught out these snakes will rear up in a typical “S” stance and face the threat, often with a gaping mouth. Although reportedly not as aggressive as the Eastern Brown Snake Pseudonaja textilis, the Western will readily defend itself if provoked, striking quickly before turning to attempt an escape. The snake’s fangs are quite short (only 2-3 mm), however the venom is very potent and has high neurotoxic and haemolytic activity. A bite from any species of brown snake should be treated as life-threatening and medical attention sought without delay.

References

Cogger, H. (2000) “Reptiles and Amphibians of Australia”, Reed New Holland

Greer, A.E. (2006) “Encyclopedia of Australian Reptiles – Elapidae”, Australian Museum

Ehmann, H. (1992) “Encyclopedia of Australian Animals – Reptiles”, Australian Museum, Angus & Robertson

Mengden, G.A. (1985) “A Chromosomal and Electrophoretic Analysis of the Genus Pseudonaja”, in “Biology of Australasian Frogs and Reptiles”, ed. by Grigg, G. et. al., Royal Zoological Society of New South Wales

Mirtshin, P. and Davis, R. (1991) “Dangerous Snakes of Australia”, revised edition, Ure Smith Press

Skinner, A. (2009) “A Multivariate Morphometric Analysis and Systematic Review of Pseudonaja (Serpentes, Elapidae, Hydrophiinae)”, Zoological Journal of the Linnean Society, vol. 155, pp.171-197

Wilson, S. and Swan, G. (2010) “A Complete Guide to Reptiles of Australia”, third edition, Reed New Holland