Reptiles
Viviparous Lizard
Zootoca vivipara · also called common lizard
The viviparous lizard is a small, ordinary-looking brown lizard, and yet it holds two remarkable records: it lives across a wider band of the planet than any other lizard, and it is one of the few animals that, depending on where you find it, either lays eggs or gives birth to live young.
Overview
Zootoca vivipara, also called the common lizard, is about 12 to 15 cm long including the tail, brown with darker flank stripes and rows of spots, the male with a spotted orange or yellow belly. It lives across Europe and northern Asia, from Ireland to the Pacific, in heath, bog, rough grassland, woodland edges, and mountains, and it hunts spiders and insects by day.
Furthest north of any reptile
The viviparous lizard reaches inside the Arctic Circle in Scandinavia and high into the mountains, further than any other reptile, and it is the only reptile native to Ireland. It manages this because most populations give birth to live young — 3 to 11 at a time, each in a thin sac it breaks out of within minutes — so the female can carry the developing embryos to warm basking spots instead of trusting a clutch of eggs to the cold ground.
Eggs or live young
In the warmer south of the range, in parts of the Pyrenees and the southern Alps, the same species still lays eggs. Having both strategies in one species makes it a natural experiment in how live birth evolves.
Status
The viviparous lizard is Least Concern and secure across its huge range, with only local declines where rough grassland and heath are lost to intensive farming and building.
Viviparous Lizard: common questions
What does viviparous mean, and do all these lizards give birth to live young?
Viviparous means giving birth to live offspring rather than laying eggs. Most populations of this lizard are indeed viviparous, and the female keeps the fertilised eggs inside her body through development and delivers a litter of little lizards, each in a thin transparent sac it wriggles free of almost at once. But it is not universal. In the warmer southern parts of the range, some populations of the very same species lay eggs in the normal reptile way. This split makes the lizard scientifically valuable, because researchers can compare closely related egg-laying and live-bearing forms to understand why and how live birth evolves, and it is thought to have shifted from eggs to live young as the species spread into colder climates.
Why is live birth an advantage in a cold climate?
A reptile embryo needs warmth to develop, and in a cold or short summer a clutch of eggs buried in the ground may not get enough heat to hatch before winter. A pregnant female, by contrast, can carry the embryos with her and raise their temperature by basking, moving into the sun in the morning and into shelter when it is too hot or cold, effectively incubating the young with her own behaviour. This lets the species breed successfully in places where egg-laying would fail, which is why the viviparous lizard, almost alone among reptiles, can live inside the Arctic Circle and high in the mountains.
How do you tell a viviparous lizard from a slow worm or a newt?
A slow worm is a legless lizard, so it looks like a small smooth shiny snake, with no legs at all, whereas the viviparous lizard has four well-developed legs and a more textured, patterned skin, typically brown with darker stripes and rows of spots and, in males, a yellow or orange belly with dark speckling. A newt is an amphibian with soft, moist, often slimy skin, no claws, and a flattened, paddle-like tail, and it is usually found in or very near water, especially in spring; it moves slowly. The viviparous lizard has dry, scaly skin, clawed toes, a round-sectioned tail, and it is fast, darting away and basking in dry, sunny places.
Where is the best place to see a viviparous lizard?
Look on sunny mornings in spring and early summer at the edges of rough, undisturbed habitat such as heathland and moorland, the banks of bogs and ponds, coastal dunes, woodland rides and clearings, railway embankments, and south-facing road verges and dry-stone walls. The lizards bask on raised, sun-catching spots such as logs, tussocks, boardwalks, fence posts, and bare patches of peat, usually close to thick cover they can dash into. They are wary and quick, so slow, quiet approach and watching a known basking spot from a distance works better than searching on the move.