WHO'S YOUR MOMMA?: Lacking an appropriate mother figure, animals like ducks, pigs and even potentially humans can bond to a parent from an entirely different species. Image: ©ISTOCKPHOTO.COM/ROGER WHITEWAY
If you saw Winged Migration or Fly Away Home, which delivered the first true bird's-eye views of the world, you may have wondered how they got those wild geese to wear tiny camcorders on their heads. In fact, the cameras were in ultralight aircraft, which the birds accompanied—by choice. The crafty filmmakers took advantage of one of Mother Nature's tricks called imprinting: If you had grown up thinking your mom was inside that noisy plane—or was that noisy plane—you'd have gladly tolerated it, too.
In the mid 1930s German ethologist Konrad Lorenz popularized filial imprinting, the process by which a newborn animal learns to recognize the unique characteristics of its parent, typically its mother. This phenomenon was termed imprinting (translated from the German word prägung) by Lorenz's mentor, Oskar Heinroth, who believed that the sensory stimulus encountered by the hatchling was immediately, and irreversibly, "stamped" onto the animal's brain. Lorenz demonstrated this with his famous goslings, which had spent their first hours of life with him and subsequently followed him everywhere; as adults they preferred the company of humans over fellow avians.
Lorenz's little geese are the most well-known example of cross-species imprinting, but it can occur between other species, too. Any animal born relatively independent—not entirely relying on a parent to provide food or protection, so-called precocial species—needs to be able to discriminate between its parents and other members of its (or other) species, lest they get lost or attacked. A gosling, or other precocial animal, accomplishes this with an instinct to approach and follow a moving target after birth as well as a vague preference for objects that have particular features, such as a head and neck. In the wild, this guides a gosling to favor its mother.
In the absence of an appropriate stimulus, however, practically any object can become a source of comfort to the newborn. After one to two hours of exposure to the target, a gosling will have formed a strong preference, avoiding novel objects and showing signs of distress when the "imprinted" object is removed.
No explicit reward, such as food or warmth, is needed. In fact, some research suggests that aspects of the object itself—its shape or movements, for example—may have the capacity to stimulate endogenous opioid (endorphin) production in a newborn's brain: hence, instant comfort.
"There have been a lot of questions about whether [precocial birds] actually do have a naive preference for their own species," explains Utrecht University zoologist Johan Bolhuis. "They probably don't."
This may be true for humans as well. Cognitive neuroscientist Mark Johnson of Birkbeck, University of London, who worked with Bolhuis on chick imprinting and now studies this phenomenon in human infants, found that within minutes of birth babies show a preference for facelike over nonfacelike stimuli. And, after visual exposure to their own mothers, newborns show a strong preference for their moms' faces, likely reinforced by the flurry of activity, including protein synthesis and changes in synaptic transmission, that occurs in the brain during imprinting, as shown by University of Cambridge neuroscientist Gabriel Horn.
Because recognizing and bonding with a parent are more dependent on exposure and learning than on a genetically programmed response, it's conceivable that any animal exposed exclusively to a member of a different species might happily call it mom—witness the children purportedly raised by wolves in India and the orphaned chipmunk adopted by Buffy the Chihuahua as well as a tiger in Thailand's Sriracha Tiger Zoo suckling piglets—after being suckled as a cub herself by, naturally, a pig. Without such a promiscuous capacity for trust, an infant whose mother abandoned it or died shortly after its birth would face certain doom if it were unable to swap preferences for an adoptive parent.