Lorenz and the Goslings
In 1935, Konrad Lorenz published his landmark paper on imprinting, describing how newly hatched greylag geese would follow the first large moving object they encountered — whether their mother, Lorenz himself, or even a moving box. This rapid, seemingly irreversible form of learning, occurring only during a brief critical period after hatching, challenged the behaviorist view that all learning follows the same associative rules. Imprinting demonstrated that evolution has prepared certain neural circuits to acquire specific information at specific times.
The Critical Period
The critical period for filial imprinting in precocial birds (those that can walk shortly after hatching) typically opens within hours of hatching and closes within 24-48 hours. During this window, the young bird's brain is primed to form a representation of 'parent' — whatever stimulus happens to be present. Eckhard Hess showed that the strength of imprinting peaks at about 13-16 hours post-hatching in domestic chicks and declines rapidly thereafter. The closing of the critical period may involve active fear responses to novel objects that prevent new attachments from forming.
Stimulus Features and Salience
Not all stimuli are equally effective as imprinting objects. Movement is crucial — stationary objects produce much weaker imprinting. Sound, especially species-typical calls, enhances imprinting when paired with visual stimuli. Object size, color, and speed of movement all influence salience. Interestingly, the effort required to follow an object (what Hess called 'the law of effort') strengthens imprinting — goslings that had to climb over obstacles to reach a moving model imprinted more strongly than those with an unobstructed path.
Beyond Filial Imprinting
Imprinting is not limited to parent-offspring attachment. Sexual imprinting determines mate preferences later in life — cross-fostering experiments show that birds raised by another species often prefer to mate with that species in adulthood. Habitat imprinting influences where animals choose to settle. Song learning in oscine passerines involves a sensitive period during which young birds memorize the song of a tutor. These diverse forms of imprinting share the common features of rapid learning, critical timing, and relative irreversibility, suggesting a conserved developmental mechanism shaped by natural selection.