Not all snakes lay eggs. Some keep the eggs inside them until they are ready to hatch, and then give birth to live young. The marshes of northern Argentina are home to one of the largest of the live-bearing snakes, the anaconda. Here, a heavily-pregnant female is swimming underwater. But it is morning and chilly so she moves out of the water and on to the swamp to warm herself in the sun. Slowly the day begins to warm up until finally she is too hot and moves back to the water again to cool off. In this way she keeps her body at about 29 degrees centigrade, a perfect temperature for the babies developing inside her. Finally, she finds a quiet pool free of cayman and her contractions start. Her first baby is born and goes straight to the surface to take its first breath of air. Eventually she produces 15 young, which is actually quite modest as they can produce up to 40. Right from the beginning of their lives though, the youngsters are totally independent and get no care from their mother. Instead, they simply swim away.