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A member of the viper family, the adder has a relatively stout body for a snake and a short tail. An average male is 21 inches long while the average female is 2 feet 8 inches long. The adder has a flat head that broadens behind the eyes to form an arrow shape. Color and body markings on this snake can vary considerably with both sexes being colored differently. In general they are brown, olive, reddish brown, gold, grey or cream, but there is a black variety that is fairly common. With the exception of the black adder, which has no patterns or markings showing, the most characteristic marking is the dark zigzag line down the back with a series of spots on either side. Adders have a pair of dark bands on their heads that often form a V or X. Males tend to be cream, dirty yellow, silvery, pale grey, or light olive with black markings while females are red, reddish brown or gold with darker red or brown markings. In most cases the throat of the male is black or whitish with the scales spotted or edged with black. Females have a yellowish white chin that is sometimes tinged with red.

Adders range throughout Europe, Asia and Scotland. They can usually be found in dry places such as sandy heaths, moors and the sunny slopes of hills where they often bask in the sun on hedge banks, logs and piles of stones. The adder has a high tolerance for cold, which allows it to live as far north as Finland, beyond the arctic circle. It escapes cold weather by hibernation, which starts when the shade temperature falls below 49 degrees fahrenheit. It will emerge again when the temperature rises above 46 degrees fahrenheit, even coming out into snow. Because of this the duration of their hibernation depends on the climate. At times it can last up to 275 days, while in other areas it will last to as little as 105 days. Adders do not burrow like many other snakes, but instead seek out crevices and holes where they can stay for the winter. The depth at which they hibernate depends on the temperature. Some have been found at depths of up to 4 feet in extreme cold. It is not uncommon to find several adders in one den during hibernation. As many as 40 have been found coiled up together as a method of preventing heat loss.

The eyes of the adder are typical of nocturnal animals although it is uncertain whether they are nocturnal or diurnal. It is certain, though, that they can see well during the night and need some protection during the day. The main prey of the adder is lizards, mice, voles and shrews. Young adders live at first on insects and worms. The larger victims of the adder are killed by a poisonous bite but their bite is rarely fatal to humans. The mating period for the adder starts at the end of March and ends in early May. At the beginning of the breeding season there is a good amount of territorial rivalry between the males. Two males will face each other with heads erect and the front part of their bodies held off the ground. Swaying from side to side they will entwine their bodies and attempt to force each other to the ground by pushing and thrusting. Then the female, who has been waiting, will accept the victorious male. Adders are ovoviviparous, which means that the eggs remain inside the female until they are fully developed. Young adders are born coiled up in a membrane in August or September and are immediately capable of independent existence.