Organisms have the capability of adjusting their family. Some organisms may regulate the number of offspring to maximize their fitness through miscarriages, and even cannibalism. With a miscarriage, a genetic mutation occurs in the developing fetus. This serves as a substantial penalty to the mother for carrying it, and a spontaneous death in the embryo occurs. This maintains the family size. Cannibalism occurs in a variety of species such as fish and rats. For example, in certain species of fish the male oxygenates the egg and on a certain occasion he might eat some of the eggs. This is due to the oxygen levels being too low. This condition causes the eggs to be in jeopardy, and rather than the whole clutch dying the father eats some of the eggs so others might survive. In other cases, the mother absorbs the fertilized eggs or developing embryos. If the environment gets too stressful for the mother, she has the ability to do this, and such a thing is seen in rats. This consumption of the fetus provides the mum with lots of nutrients and reduces the stress on her. She now might be more capable to reproduce again when the environment is more favorable.