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In rodents and lagomorphs, the nesting instinct is typically characterized by the urge to seek the lowest sheltered spot available; this is where these mammals give birth. Female dogs may show signs of nesting behavior shortly before their due date that include pacing and building a nest with items from around the house such as blankets, clothing, and stuffed animals. (They also sometimes do this in cases of false pregnancy, or pseudocyesis). Domestic cats often make nests by bringing straw, cloth scraps, and other soft materials to a selected nook or box; they particularly are attracted to haylofts as nest sites. In birds it is known as "going broody", and is characterized by the insistence to stay on the nest as much as possible, and by cessation of laying new eggs. Marsupials do not exhibit a nesting instinct per se, because the mother's pouch fulfills the function of housing the newborns.
Hormones and nesting behavior
Maternal nest-building is regulated by the hormonal actions of estradiol, progesterone, and prolactin. Given the importance of shelter to offspring survival and reproductive success, it is no wonder that a set of common hormonal signals has evolved. However, the exact timing and features of nest building vary among species, depending on endocrine and external factors.
In rabbits, nest building occurs towards the last third of pregnancy. The mother digs and builds a nest of straw and grass, which she lines with hair plucked from her body. This sequential motor pattern is produced by changes in estradiol, progesterone, and prolactin levels. Six to eight days pre-partum, high levels of estradiol and progesterone lead to a peak in digging behavior. Both estradiol and progesterone are produced and released by the ovaries. One to three days pre-partum, straw-carrying behavior is expressed as a function of decreasing progesterone levels, maintenance of high estradiol levels, and increasing prolactin levels. This release of prolactin (from the anterior pituitary) is likely caused by the increase in estrogen-to-progesterone ratio. One day pre-partum to four days post-partum, hair loosening and plucking occur as a result of low progesterone and high prolactin levels, together with a decrease in testosterone. In house mice and golden hamsters, nest-building takes place earlier, at the start or middle of pregnancy. For these species, nest-building coincides with high levels of estrogen and progestin.
External factors also interact with hormones to influence maternal nest-building behavior. Pregnant rabbits that have been shaved will line their straw nest with available alternatives, such as male rabbit hair or synthetic hair. If given both straw and hair, mothers prefer straw during the straw-carrying period, and prefer hair during the nest-lining period. If given hair as the only material, shaved mothers collect the hair even when it is the straw-carrying period.
Interestingly, research on avian paternal behavior shows that nest-building is triggered by different stimuli in the two sexes. Unlike the case for females, male nest-building among ring doves depends on the behavior of the prospective mate rather than on hormonal mechanisms. Males that are castrated and injected daily with testosterone either court females or build nests, depending purely on the behavior of the female. Hence, the male avian transition from courtship to nest-building is prompted by social cues and not by changes in hormone levels.
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In human women, the nesting instinct often occurs around the fifth month of pregnancy, but some intuitive mothers can start as early as a couple of weeks[non-tertiary source needed] and can occur as late as the eighth month, or not at all. It may be strongest just before the onset of labor.
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