Rats

The colony's size depends on two factors: the density of the population and the food supply. When the colony's population is low, such as at winter's end, the females will bear more young and thus the population increases steadily throughout the summer. As the population and density increase, the pregnancy rate declines accordingly.

Similarly, the greater the food supply, the larger the rat population. Female rats living near an abundant supply of food bear more young than females living further away from or without such a supply. If there is little food available, both sexes will become infertile, postponing reproduction in favor of individual survival.

The female's estrus lasts about six hours, during which she mates with several males, copulating frequently during the heat. After a gestation period of 22-24 days, the female gives birth to 6-12 blind, naked, pinkish, helpless young. By the time they are two weeks old, the young are fully furred and their eyes are open. After 22 days, they leave the nest. Males are sexually mature at three months, females slightly later.