Peter Lewis, age 12, of Portland, Maine, for his question:

Do all the reptiles lay eggs?

All the crocodiles and their kinfolk lay soft shelled reptilian eggs. So do all the turtles and tortoises. Some of the snakes and lizards lay eggs and some bear live young. In some cases, the eggs stay in the female reptile's body until they are ready to hatch. In a few cases, the developing embryos are partially nourished inside the female's body.

Frogs lay their eggs in the water because these soft, jellified blobs could not survive in the dry air. The amphibian life cycle depends upon both water and land. The reptiles graduated from the water some 300 million years ago    when egg shells came into being. Since that time, the majority of snakes and lizards, all crocodiles and turtles have left their soft shelled eggs to hatch in the warm ground.

However, reptile eggs drown in the water and many reptiles spend their lives in the sea or in fresh streams. Different species solved this serious family problem in different ways. Sea going turtles come ashore to conceal their eggs in warm sandy beaches. So do some sea snakes. But about three quarters of the sea snake species retain the fertilized eggs in the female's body. The young are born alive in the water, all ready to gasp for air.

Reptiles are cold blooded creatures who depend upon the seasonal warmth of the earth. In temperate zones, they hibernate through the winter. Strange to say, an adult snake can survive a period of being very cold. Their eggs cannot. They need to spend weeks and sometimes months in very warm earth before hatching. A common lizard of northern Europe solves this problem by keeping the eggs in her body until they are fully developed and ready to hatch. So do several European vipers.

Sea going snakes, cold country lizards and vipers solve their family problems by bearing live young. However, many snakes and lizards that live in tropical and warm temperate zones also bear live young. In our south western deserts, some kinds of horned toads give birth to about a dozen live young,' others lay 30 or so eggs.

Obviously, reptile eggs left buried in the ground are at the mercy of hungry predators. Eggs retained inside the female's body are more protected and litters can be smaller. This may explain why boas, rattlers and many other warm climate snakes bear live young.

Most North American lizards lay little round, soft shelled reptilian eggs. One live bearing exception is the alligator lizard of the western mountains, another is the night lizard of Arizona. Among our native snakes, the boas and venomous rattlers are live bearers. So are most of our water snakes and many of our harmless snakes.