Mock cobras—their defensive posture resembles the cobra’s
Yes indeed, you may come across cobras, in Northern Thailand’s forests. Also across king cobras, kraits, pit-vipers and large pythons. These ill-reputed, deadly serpents are all there. Their names are well known to visitors, and merely mentioning them may make their flesh creep. They are certainly not rare, and occasionally hill tribe people are bitten by green pit vipers resulting into serious, albeit seldom fatal, envenomation. Yet, during a trek, encounters with these potentially dangerous animals are far from common.
On the other hand, the most numerous and widely occurring snake species in these forests are hardly known, such as the rather small-sized slug snakes and the arrowhead snake. The name slug snake refers to the eating habits of these animals, which mainly prey on slugs and snails. In the North three kinds of slug snakes occur, an orange-brownish species known as the Keeled Slug Snake (Pareas carinatus), and the greyish White-spotted Slug Snake (Pareas margaritophorus) and Spotted Slug Snake (Pareas macularius). As they are mainly active at night, one is unlikely to come across them in their natural environment, but the numerous roadkills of these species reveal that they are common.
The Arrowhead Snake (Plagiopholis nuchalis), however, is diurnal, and one may spot it when it’s crossing a path or road during the day. The diet of this harmless, beautiful little snake consists mainly of earthworms which it preys in the forest litter. Interestingly, when threatened, it seems to mimic the cobra by raising and flattening the upper part of its body.
Despite this remarkable defensive behaviour, and being a common snake in northern Thailand, little is known about this reptile. In Chris Mattison’s Encyclopedia of snakes only two lines are devoted to this and three related species: ‘Plagiopholis Four species found in China, Burma and Thailand. Small snakes, terrestrial but otherwise poorly known.’ In one handbook on Thailand’s snakes (Lai ngu thai, ‘Thai snake patterns’ by Wirot Nutphan) it is not even mentioned.
In the English literature about the snakes of this region, it is usually referred to as the ‘Assamese mountain snake’ — probably because its first description was from Assam (northeastern India). However, in two recent works on snakes in India it is not mentioned. As it occurs in much of mountainous South and Southeast Asia, I find ‘Assamese’ not appealing and suggest to call it ‘Arrowhead Snake’ — as it is called in Thai (ngu hua son, งูหัวศร), for it has a distinctive black chevron on the neck that resembles an arrowhead, the apex pointing forward. In some specimens the chevron may be reduced to a narrow dark blotch, or be completely absent, but in the majority it is quite conspicuous. Many other snake species have similar patterns on their head or neck, which may effectively break up the outline of the most vital part of their body in the surroundings. The chevron of the arrowhead snake may serve crypsis in the forest litter, or be part in the defensive display.
This rather stout snake seldom grows to more than fifty centimetres. It occurs in many of northern Thailand’s evergreen forests at an altitude of between 700-1,600 meter. I have spotted them on all of the North’s well known mountains, such as Doi Suthep, Doi Ang Khang, Doi Inthanon and Doi Chiang Dao. They seem to occur in virtually all mountain forests in the north and the west of the country. To mention just one area, in the forests of Mae Taeng district they are particularly common, so when visiting the Pong Duat hot spring, or trekking in the area, you have a chance to come across them.
The basic colour of its back is rather variable — in many individuals it is reddish brown, but it may also be dark brown or yellowish brown. Many dorsal scales have black, white, or yellow edges, which form a reticulum that may suggest a banded pattern. But the arrowhead and stout body easily give away its identity.
This snake is rather slow, but when cornered, it rapidly raises and flattens the upper part of its body. The snake’s greyish-white belly has a characteristic pattern of more or less rectangular, black blotches, shining beautifully like mother-of-pearl. Interestingly, in the upper part these rows of blotches can make place for a black band and two clear black dots at the margin of the throat, both being characteristics of the throat of the cobra’s raised hood.
One is inclined to consider this as a case of mimicry, the harmless arrowhead snake imitating the pattern and behaviour which seem to advertise the cobra’s deadly bite. However, it may have evolved independently, as the ‘big eyes’ of the raised hood are threatening enough in itself. In Central America, where no cobras occur, a small snake (Ninia sebae) also spreads its neck in defense. As Chris Mattison writes ‘mimicry may be involved, but there is a strong possibility that spreading and raising the neck serves the simple function of making the snake appear larger and more fierce than it really is.’ There are also caterpillars that behave this way and, if further disturbed, may even ‘strike’.
In South and Southeast Asia another mountain snake raises its body in a defensive posture similar to the arrowhead snake. In India, where it is common in Darjeeling, it’s therefore called ‘false cobra’, but in works about Thai snakes it is usually referred to as Big-eyed Mountain Keelback (Pseudoxenodon macrops) as it has large eyes and keeled dorsal scales. Thai call it ngu lai sap ta to ( งูลายสาบตาโต ), which means ‘big-eyed striped smelly snake’, for other keelbacks (snakes with keeled dorsal scales) may emit a foul odour from musk glands at the base of their tail — for example, the very common Striped Keelback (Amphiesma stolatum) does so when handling it badly.
The Big-eyed Mountain Keelback may grow much larger (to 120 cm) than the arrowhead snake. It was considered a harmless, non-venomous species, but the presence of much enlarged teeth in the rear of the upper jaw is suspicious. Yet, in Thailand it is rather uncommon, and only occurs at high elevations (1200 m and higher) in rather undisturbed evergreen mountain forest in the north and west, so bites by this snake are probably unknown. Thusfar, I only came across specimens in Doi Inthanon and Doi Suthep – Pui National Parks and Chiang Dao Wildlife Sanctuary (Chiang Mai), Phu Hin Rong Kla National Park (Phetchabun) and in the mountains of Umphang district (Tak).
The colouration and pattern of the back shows even more variation than in the arrowhead snake, but usually there is a row of red and black dots along each side of the body. It also has a black chevron, which looks like an ‘arrowhead’ but is longer and less clear than the one in the arrowhead snake. Even the throat often shows a crossband and two lateral spots. So, also in this species behavioural adaptations and skin patterns have evolved that are similar to the cobra’s raised hood.
As a matter of fact, only one species of cobra is sympatric with these ‘mock cobras’ in the mountains of the North, which is the medium-sized Indo-Chinese Spitting Cobra (Naja siamensis). The colouration and pattern — even the hood mark — of this cobra are also highly variable.
When cornered, many other snakes will raise the anterior part of their body. For example, the very common, non-venomous Copperhead Racer (Coelognathus radiatus) throws it into loops while expanding its throat longitudinally, and eventually fiercefully strikes, making it a favourite in the snake shows around Chiang Mai. The widespread and mildly venomous Oriental Whip Snake (Ahaetulla prasina) will defend itself in a similar way. But, as far as I know, only cobras and King Cobras and the two mentioned, harmless mountain snakes raise and flatten their anterior body into a kind of hood to their opponents.
The cobra’s defense in combination with its frightening hissing — in Thai cobras are called ngu hao ( งูเห่า ) or ‘hissing snake’ — and its fatal bite has inspired awe in man since time immemorial. Nagas, mythological serpents venerated all over South and Southeast Asia, have been modeled after the cobras. In Thailand, the cobra has somehow become the embodiment of all that concerns snakes. When showing Thai from the countryside a fresh roadkill and asking them what kind of snake it is, they surprisingly often answer ‘ngu hao’, even when the snake has hardly any resemblance to the cobra. Apparently, local wisdom has sharply declined, whereas the respect or fear for the cobras has not. However, in case of an arrowhead snake, raised in defense, one can hardly blame a person for mixing it up with a cobra. As the picture shows, its cobra appeal is all too clear.
Sjon Hauser@text and images
 In the recently published Field guide to the reptiles of Thailand and South-East Asia (Das, 2010) the snake is called the Common Blotch-necked Snake, a renaming that does not acknowledge the characteristic arrowhead-shape of the blotch on the haed and neck.
 I once observed that an aroused specimen switched from the characteristic defensive display to another. It lowered its head, the snout touching the ground, while the neck, still expanded, showed the arrowhead-sign upside-down, somewhat resembling the spectacles of a spectacled cobra from India, Naja naja.