Tuesday, 4 June 2013

Serpentes ...

West Sussex, 1 June 2013

Natrix natrix (Linnaeus, 1758)

The Grass snake, Natrix natrix, is Britain's largest native terrestrial reptile, and probably its most common species of snake. It is non-venomous and completely harmless to man; and rarely bites, even when handled.

With regards to the British subspecies, helveticaN. natrix is most easily identifiable by the interlinked black and yellow collar, which usually forms a band or ring immediately behind the head. The upper body is typically olive-green, olive-brown or greyish in colour, with a variable row of black bars along the sides; occasionally with smaller round markings along the back in double rows. The ventral scales are off-white or yellowish, with dark triangular or rectangular markings. The face has marked black bars on each side below the eye and these provide a key method of individual identification. Melanistic and albino forms are rare but occasionally arise. The sexes are similar in appearance, although females are usually larger. Males can be identified by the presence of a swelling at the base of the tail and by the fact that they have longer tails relative to females. In addition, the head of the female is much more pronounced in being triangular in appearance with the eyes deep set into the cheeks; whereas the male has an altogether more slender head with eyes that often protrude beyond the line of the face and jaw. The majority of females also tend to possess two postocular scales (behind the eyes), as opposed to the males three. The female may grow up to 1.2 metres in length (snout to vent); however, more mature individuals have been reported up to 2 metres in length. The male is generally much smaller in comparison, and individuals of around 600mm are usually among the largest specimens recorded.

Ponds, lakes and rivers are their preferred habitat. They are strong swimmers. Not only are features such as bank-side vegetation important to provide cover against their predators, there must also be readily identifiable passages of ground cover in the form of ditches, hedges or banks of brambles which lead up to the banks. If such passages are removed from the surrounding environment, then the frequency at which N. natrix will visit a particular water body will reduce.

From March, when the snakes emerge from hibernation, until mid may, they eat large quantities of small fish; mainly because these are spawning during this period and are easier for the snakes to catch amongst the weedy fringes of water bodies. From May to early July, the diet emphasis falls onto newts, and again, this is because the newts own life cycle doesn’t include a land-borne phase until after July. From July, N. natrix gradually disperses away from ponds and into forest clearings, wooded copses and longer grasses / ground cover and a higher number of frogs and toads will then be taken as a result. The frogs and toads themselves have generally moved away from the ponds by April, although some are obviously predated upon between the cross over of snakes arriving at ponds in spring and while the frogs or toads are migrating away from them. Nests of very young chicks such as moorhens are sometimes consumed (mainly to supplement an otherwise restricted diet), as are mice and voles, especially by the more mature female snakes, but these incidents are rare.