The first thing a certain brood of baby geese saw when they hatched was Conrad Lorenz, one of the first great behavioral theorists. As young geese, they followed him everywhere he went and became sexually attracted to him as adults. The baby geese had imprinted on Lorenz. In the process of filial imprinting, the imprinting of offspring on their parents, there is a critical period for learning that is irreversible once something has been imprinted upon. The hatchling geese imprinted on Conrad Lorenz, and nothing could de-imprint them. Ducklings have also been known to imprint on people. Both ducks and geese are precocial birds. Unlike altricial birds, which are helpless for several weeks after hatching, precocial birds quickly start walking around. They need to follow something for their own safety and thus imprinting is vital to their early survival.
The concept of a critical period for learning is not restricted to imprinting, nor to geese. Songbirds have a critical period for song learning, as we will see in Signaling and Communication. Humans also seem to have a critical learning period. In children age 4 years and younger, learning a language is almost effortless. No class is needed, no specific instruction; they pick it up on their own. After age 13, it is much more difficult to learn a language. For older people, it is virtually impossible to learn to speak as well as a native.
Neural Control of Filial Imprinting
Johnson and Bolhuis identified two independent neural systems that control filial imprinting in precocial birds. Newly hatched chicks will follow almost anything that has eyes and moves. After the chick follows something, another part of the brain, analogous to the frontal cortex, recognizes and imprints on the individual being followed. These mechanisms are independent. There is an instinct for chicks to follow, and then they learn what they are following.
It might seem odd that being able to identify and follow a mother does not have a genetic mechanism. Yet with a neural rather than genetic mechanism, the chick gains flexibility that might help in survival. If a chick's mother dies, the chick can then be adopted by another family member or conspecific. If the chick's recognition of its mother were genetic, the chick would not follow its adoptive parent, and would die. Further, detailing the recognition of a specific individual is far too complicated to achieve genetically. An individual may be able to tell what relationship he has with others, but cannot be genetically programmed to recognize a certain individual, particularly because environment plays a large part in appearance. The chick's neural imprinting system allows more adaptive flexibility and hence is an advantage.
Most animals are not monogamous. In many species, males mate and leave, while the female raises the offspring. Many animals receive no parental care at all. If a young female is raised by her mother, with no father around, how can she learn to recognize potential future mates? Females learn to recognize what an appropriate mate should look like from their siblings or neighbors. Sexual imprinting is a general imprinting; it is not specific to individuals, only species typical characteristics. If a female were to imprint specifically on his sister, or vice versa, inbreeding would result, which reduces a population's fitness. The more general system of sexual imprinting allows young to learn to recognize potential mates without inbreeding.
There are many examples of offspring raised by foster parents of a different species preferring to mate with the foster species over its own species. Lorenz's geese were more sexually attracted to humans than to other geese. Goats raised by sheep mature and prefer to mate with sheep, and sheep raised by goats prefer to have goats as mates.