Nesting behavior refers to an instinct or urge in pregnant animals associated with an increase of estradiol (E2) to prepare a home for the upcoming newborn(s). Nest building provides protection against predators and competitors that mean to exploit or kill infants. It also aids in family structure and is therefore influenced by different mating behaviours and social settings. It is found in a variety of animals such as birds, fish, squirrels, mice and pigs, and is said to be done by humans as well. However, recent research into nesting tendencies in women has questioned whether there is really any biological basis for nesting in humans, positing a social explanation instead.