Responsiveness to simple releasers is sufficient for certain behaviors such as begging behavior in gull chicks, but more complex information that cannot be genetically programmed is required for other behaviors. An example is parent–offspring recognition. When animals live in close proximity to other individuals, as in a herd or a nesting colony, it is important for parent and offspring to learn each other’s identity soon after birth so they will be able to find each other in a crowded situation. In many such cases, a parent–offspring bond is formed by imprinting. What characterizes imprinting is that an animal learns a specific set of stimuli during a limited time called a critical period, also known as a sensitive period.

Konrad Lorenz demonstrated that young greylag geese (Anser anser) imprint on their parents between 12 and 16 hours after hatching. By positioning himself to be present during this critical period, Lorenz succeeded in imprinting goslings on himself. The imprinted goslings followed him around as if he were their parent (Figure 52.6A). In a subsequent experiment his assistants wore boots with different patterns on them. The goslings imprinted on the boots, and even in a situation that mixed different groups of goslings, they always sorted themselves out by following their “parental” boots.

Imprinting requires only a brief exposure, but its effects are strong and long-lasting. Emperor penguins (Aptenodytes forsteri) reproduce during the coldest, darkest time of year in Antarctica. The parents walk up to 150 kilometers inland to form a dense colony, where the female lays her egg. She then walks back to the ocean to feed while her mate incubates the egg. By the time she returns, the chick has hatched. She then takes over its care and feeding, and the father walks back to the ocean to feed. Generally he is away so long that the mother must leave to find food to avoid starvation. Thus after being away for weeks, the father must find his chick in a crowded, milling colony of chicks, all calling for their parents (Figure 52.6B). Yet he can unerringly locate his own offspring by recognizing its call, which he imprinted on before he left to feed.

The critical or sensitive period for imprinting may be determined by a brief hormonal state. For example, if a mother goat does not nuzzle and lick her newborn within 10 minutes after its birth, she will not recognize it as her own offspring later. For goats, the sensitive period is associated with peaking levels of the hormone oxytocin in the mother’s circulatory system at the time she gives birth and at the same time she is sensing the olfactory cues emanating from her newborn kid. A female goat rendered incapable of smelling before giving birth is unable to differentiate between her own kid and other kids after giving birth.