The adder is Britain’s only venomous snake. Durrell is, however, the only place that you can see adders in Jersey as they are entirely absent from the Channel Islands. They are also entirely absent from Ireland.
They will be surprisingly small in size to many people with males obtaining an average adult length of just 50cm and females 55cm. At birth they are only 14 - 18cm.
Unusually for a snake, the adder is sexually dimorphic. This means that you can tell the sexes of adults apart by visual differences. They generally differ in both size, as the females are bigger than the males and also in colouration (dichromatic); females are usually brownish in colour whilst males are normally greyish. Both sexes have the iconic zigzag patterning.
Throughout their range, melanistic (black) adders are not uncommon in the wild and they may even be more successful than the normal colour form as their darkened skin cells means that they can have a thermoregulatory advantage, and can warm up quicker. Black individuals can therefore feed on days which would be too cold for other individuals. These advantages can be offset by an increased risk of being discovered and eaten by predators compared to the normal-coloured specimens which are better camouflaged.
The adder is ovoviviparous which means that although they give birth to live young, the babies actually develop within eggs inside the females. They give birth to between 3 and 18 young, of which the sex ratio is around 1:1. The babies then have a 3-6 week period before they go into hibernation. In this time they do not feed but survive, and even grow, off yolk stored from the ovoviviparous birth.
Their diet mainly consists of small rodents but they will occasionally feed on nestling birds, lizards and frogs. They hunt both actively and also by ‘sit and wait’ strategies. They strike at prey, injecting their venom, and immediately release it to avoid being bitten back. The envenomed prey then wanders off to die and the adder uses its amazing sense of smell to follow the scent trail and eat the dead prey.
In the wild adders hibernate for 5-7 months, ending when the weather turns favourable for basking. Even snow does not prevent the adder from coming out of their burrows as long as the sun presents the opportunity for the snakes to bask and thermoregulate. At Durrell they can often be seen basking in their outside enclosure during the spring and summer months. We are able to keep our adders outside year round as we have built them a specially designed hibernaculum at the rear of the enclosure. It has been designed to maintain suitable humidity and temperatures regardless of the outside conditions so they that can safely hibernate when they need to.
The adder occurs in a wide range of habitats including heathlands, moors and coastal dunes. They mainly require undisturbed areas with uncovered slopes or glades to bask on. There must also be thick cover for the snakes to disappear into when they feel threatened. Its’ preference for undisturbed habitats means that it is rarely encountered in urban areas.
Being venomous gives the adder a bad reputation with people but they are very shy and timid animals and there have been no reported fatalities from an adder bite for over 20 years. Adders have an important role in controlling rodent numbers, their main prey item. Numbers of adders are currently plummeting at an alarming rate and the fragmentation (breaking up) of their habitat means that their populations will become more susceptible to disease and genetic defects which is likely to further harm their numbers.
IUCN Red List (May, 2011) http://www.iucnredlist.org/
Beebee, T. and Griffiths, R. (2000) Amphibians and Reptiles: a Natural History of the British Herpetofauna. Harper Collins Publishers Ltd., London.
Boulenger, G. A. (1913) The Snakes of Europe. Methusen & Co. Ltd., London.
Wirth, M. (2009) Vipera berus. Common Vipers of the Northern Black Forest.
Reptilia (Münster) 14 (2): 34-40
Wildlife Extra (2011), viewed May 12 2011, http://www.wildlifeextra.com/go/ news/adder-dna.html
Photo by Dan Lay www.dan-lay.com