How Animal Imprinting Works

Konrad Lorenz was born in 1903 in Vienna. From an early age, Lorenz was fascinated with animals, collecting a menagerie of various animals he'd found and nursing sick animals at the local zoo. While studying medicine, he continued his work observing animals and became particularly interested in their behavior. After finishing his medical degree, Lorenz went on to attain a PhD in zoology in 1933 and quickly got to work publishing some influential papers on animal behavior [source: Hess].

Oskar Heinroth, a fellow scientist and friend of Lorenz, was actually the first to identify and record the phenomenon of (what would later be known as) erroneous imprinting. Heinroth noticed that, unlike certain other species, greylag geese can attach to humans instead of their own mother straight out of the egg. Lorenz would be the one to experiment with the idea and name the phenomenon "imprinting" (he used the German word prägung). Lorenz studied this process in greater depth to discover the exact conditions necessary for birds to attach to humans and the various effects a human parent can have.

In one experiment, Lorenz separated a nest of goose eggs into an experimental group and a control group. He took the experimental group to raise apart from the mother but left the control group with her. The experimental geese only met Lorenz — not their goose mom — when they hatched and attached to him as their mother. To test his hypothesis about the hatchling-mother bond, he marked the two groups and put them together under a box. Sure enough, when he brought in the mother goose and lifted the box, the control group waddled back to their mother, but the experimental group came to him. He named this phenomenon of certain birds learning who their parents are filial imprinting.

Through further experiments, this time with jackdaws, Lorenz attempted to learn about what he called sexual imprinting, an animal's process of figuring out its proper mate. Indeed, he found that when birds sexually imprint on another species, they will try to mate with members of that group. Interestingly, though, a bird sexually imprints on a species and not on an individual. For example, Lorenz found that when a bird sexually imprints on a human, the bird will try to mate with a human — but not the one who raised it. In addition, different types of imprinting occur at different times as birds mature: One of Lorenz's jackdaws learned to eat with humans and fly with crows but mate with its own species [source: Harre].

Lorenz emphasized that imprinting was unlike other forms of learning for two reasons. First, it happened during what he called a critical period — a definite phase during which the learning had to occur (although this varied depending on the species). Second, Lorenz argued that imprinting was permanent and irreversible. Next, we'll explore how further research called into question some of Lorenz's conclusions.

Further experiments in the 1950s revived interest in the imprinting phenomenon. German-born scientist Eckhard Hess built a laboratory in Maryland with his partner, A. O. Ramsey, a high school biology teacher. The lab setting allowed the researchers complete control of the environment to study imprinting in mallard ducklings. They found that the ducklings' most sensitive period of imprinting was actually 13 to 16 hours after hatching. Furthermore, the ducklings that had to climb hurdles or travel farther to follow the model duck formed a stronger attachment to it. Not only would ducklings imprint onto papier-mâché ducks, but also onto colored spheres. Unsurprisingly, Hess also found in later experiments that ducklings became more attached to models that moved and made noise than models that were more stationary or quiet [source: Price].

Although Hess accepted Lorenz's theories that imprinting happened during a critical period and was irreversible, other researchers questioned these conclusions. In the 1960s, other experiments revealed that social isolation changes a duckling's window of "imprintability." When kept socially isolated, for instance, the duckling can still imprint 20 hours after hatching. This means that experience alters imprintability, suggesting that the window isn't entirely genetically determined [source: Hardy]. As a result, researcher Wladyslaw Sluckin proposed using the term sensitive period rather than critical period. What's more, other experiments suggested that imprinting could be reversed by gradually introducing a bird back to its own species.

Research has revealed that sexual imprinting is also possible in altricial birds (those that are more helpless at birth). Experiments during the 1960s and 1970s revealed that a bird can show sexual preference for its own species without having any experience with another of its own species. Researcher Patrick Bateson wanted to reconcile the ideas that sexual preference is partly genetically determined but also capable of being influenced by experience via imprinting. His experiments suggested that birds prefer sexual mates that are within their own species but don't prefer those of the opposite sex to which they were exposed early in life [source: Bateson]. So, he theorized that the purpose was to balance inbreeding and outbreeding. Birds are genetically predisposed to mate with their own species but also must rely on experience to ensure they don't mate within their immediate family. Sexual imprinting has also been grouped with other methods of how animals learn socialization under the more general term of species imprinting.

Strictly speaking, imprinting is a phenomenon exclusive to certain bird species, just as Lorenz meant it when he coined the term. But as we've seen, subsequent research has revealed imprinting to be more flexible than Lorenz originally thought. This calls into question the phenomenon's rigid definition. Furthermore, researchers have borrowed the term in studying how early experience can affect behavior in other types of animals.

One of the most fascinating studies involved cross fostering sheep and goats. During the 1990s, researcher Keith Kendrick and his colleagues switched sheep and goats at birth. The animals were allowed social contact with their own species while being raised by their adoptive species. It turned out that the adopted animals preferred to mate with the species of their adoptive mother [source: Price].

Interestingly, however, the male animals were more affected by this social imprinting than the female animals. Males were more likely to prefer to socialize and mate with their adoptive species, and researchers found it was harder to reverse their imprinting. After the first year of being raised by a different species, the adopted animals were reunited with their own species and removed from contact with the other species. Once a year, the researchers would allow the adopted animals contact with their adoptive species to assess their preference. Females could reverse their sexual preference back to their own species in one or two years, whereas males' sexual preferences still hadn't changed after three years [source: Goodenough et al.].

Other landmark studies have explored the effects of mother-infant bonding among mammals. Famously, researcher Harry Harlow discovered rhesus monkeys preferred surrogate model mothers wearing terrycloth as opposed to surrogate model mothers made of wire but providing food. It's believed that giant pandas won't prefer to mate with each other if handled by humans from a young age. As a result, some zookeepers dress in panda suits.