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The Sluggish Hunter - White-spotted Slug Snake

Jun 2021
Author : Green Power

“Snake” connotes agility. There is, however, always an exception. White-spotted Slug Snake (Pareas margaritophorus) is rather unrivalled among the order of snakes in terms of speed—the species moves incredibly slowly, and is slow to capture its prey—the similarly sluggish gastropods.

White-spotted Slug Snake is a nocturnal species widely distributed in woodlands and shrublands in Hong Kong—mostly found near slowly flowing streams or still water bodies. When it is threatened, such as when approached by humans, the docile snake seldom launches an attack as it lacks venom or any strong odour. The slug snake will instead curl up to protect its most vulnerable head.

White-spotted Slug Snake is small, growing to up to about 45 cm in length. Grey in colour, it is accurately described by both its Chinese and English common names—speckled with irregular fine black bars as its Chinese name indicates, and having white spots as its English name suggests. The English name also tells of its diet of slugs.

Specialised teeth for snail eating

Talking about the diet, there were some interesting findings published by Taiwanese scholars earlier this year. There are three species of Slug Snakes in Taiwan, all with similar behaviour, and feeding on slugs and snails. The distributions of the three species are slightly different. The study discovered that at a site where only one species of slug snake is found, the snake species has symmetrical teeth. And in area where two species of slug snakes overlap, one of the species will develop highly specialised, asymmetrical teeth!

Slug Snakes feed on slugs and snails—the former is easy prey with a soft body and lacking a shell, while the latter is a more difficult target with the tough shell. The Slug Snake has to put its lower jaw into the shell to extract the snail from its shell. As the snail shell has a spiral shape, a slightly bent lower jaw with more teeth—through evolution of asymmetrical teeth—is more efficient in feeding on the snail. Therefore, when two species of slug snakes overlap in a territory, one of them develops specialised asymmetrical teeth to feed on snails, while the other species preys on slugs. The two species do not have to compete for food, and enter a win-win situation where survival rates of both species are raised.

There is no similar study in Hong Kong. However, by observing the scales in the lower jaw, we know that slug snakes in Hong Kong also have a lower jaw bending to the right. Most snails in the world have clockwise coiling shells, which means the aperture opens to the right, and fits the right-bending lower jaw of the Slug Snakes. The evolutionary story does not end here. In South East Asian areas, we are able to find a higher proportion of snails with counterclockwise coiling shells. This probably shows that the numerous species of Slug Snakes in these areas have pushed the evolution of snails too. Every species strives hard for its survival!

© Ray So
White-spotted Slug Snake extracts a snail by putting its lower jaw into the snail shell.
© Ray So
Asymmetrical teeth help prey on snails
Most snails have clockwise coiling shells
© Ray So
The lower jaw scales of the slug snake bend slightly rightwards.

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