The viviparous lizard or common lizard (Zootoca vivipara) is a Eurasian lizard. It lives farther north than any other non-marine reptile species, and is named for the fact that it is viviparous, meaning it gives birth to live young (although they will sometimes lay eggs normally).
Zootoca vivipara lives in very cold climates, yet participates in normal thermoregulation instead of thermoconformity. They have the largest range of all terrestrial lizards which even include subarctic regions. It is able to survive these harsh climates as individuals will freeze in especially cold seasons and thaw two months later. They also live closer to geological phenomena that provide a warmer environment for them. They exhibit no particular colour, but can be brown, red, grey, green, or black.
The underside of the male is typically more colourful and bright, with yellow, orange, green, and blue, and the male typically has spots along its back. Males with larger heads are more likely to be successful in mating and male-male interactions than smaller-headed Z. vivipara. The lizard thermoregulates by basking in the sun for much of the time. In colder weather, they have been known to hibernate to maintain proper body temperatures. They hibernate between October and March. Their typical habitats include heathland, moorland, woodland and grassland.
The viviparous lizard is native to much of northern Eurasia. In Europe, it is mainly found north of the Alps and the Carpathians, including the British Isles but not Iceland, as well as in parts of northern Iberia and the Balkans; In Asia it is mostly found in Russia, excluding northern Siberia, and in northern Kazakhstan, Mongolia, China, and Japan. Z. vivipara has the largest distribution of any species of lizard in the world.
Home range
The size of the home range of the lizard ranges from 539 m<sup>2</sup> to 1692 m<sup>2</sup>, with males generally having larger home ranges. Their diet consists of flies, spiders, and various other insects, including hemipterans (such as cicadas), moth larvae, and mealworms. The species is a predator, so it actively hunts down all of its prey.
Predation
Birds are common predators of Z. vivipara. Male-biased predation of Z. vivipara by the great grey shrike (L. excubitor) has been studied, finding that adult males, over adult females and juveniles, were preferentially preyed upon. This bias may be due to increased activity of adult males during the reproductive season.
Predators of this species include birds of prey, crows, snakes, shrikes, hedgehogs, shrews, foxes, and domestic cats. This same study shows that there was not a significant difference between the parasitization of male and female Z. vivipara. The origin of this characteristic is under debate. Some scientists argue that viviparity evolved from oviparity, or the laying of eggs, only once. Some research in the Italian alps has suggested that distinct populations of oviparous and viviparous Z. vivipara should be considered separate species. Cornetti et al. (2015) identified that viviparous and oviparous subpopulations in contact with each other in the Italian alps are reproductively isolated. Hybridization between viviparous and oviparous individuals of Z. vivipara leads to embryonic malformations in the laboratory. However, these crosses do produce a "hybridized" generation of offspring, with females retaining embryos for much longer in utero than oviparous females, with embryos surrounded by thin, translucent shells.
Fertilization
Z. vivipara juveniles reach sexual maturity during their second year of their life. A study that explored the presence of male sex cells in reproducing males found that for the two weeks following the end of hibernation, males are infertile, and therefore incapable of reproducing. The first is that Z. vivipara has remarkable behaviors to combat the cold, and there are geological phenomena in their distribution that maintains their habitats at a temperature that the species can survive in.
thumb|Female
Colour polymorphism
thumb|On logs in [[Estonia]]
The colour polymorphism of female Z. vivipara has not been thoroughly studied in past years, regardless of the extensive research done on the species itself. Females exhibit three types of body colouration within a population: yellow, orange, and mixture of the two. These discrete traits are inherited maternally and exist throughout the individual's lifetime. Increased competition among individuals results in lower survival rates of lizards. Additionally, female lizards disperse through habitats based on the frequency of colour types that are already present in the population.