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.
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.
Despite its initial survival value, however, imprinting on something other than your kind can become problematic when you reach sexual maturity. Though it operates by different mechanisms, sexual imprinting—the process by which an animal learns to recognize an appropriate mate—is also strongly linked to early parental experience.
In 1976 there were about 100 whooping cranes (Grus americana), the tallest North American bird, left in the world. Conservationists tried to forestall their extinction by breeding cranes in captivity and reintroducing them into the wild, relying on one adult female to continue her rare genetic lineage. Hatched and hand-reared in the San Antonio Zoo, "Tex" wanted nothing to do with the handsome male whoopers she later met; she performed her elaborate mating dance solely for her human keepers. Only after George Archibald, one of the world's leading crane experts, literally moved in with Tex for several months, formed a pair-bond with her, and joined her repeatedly in the species-specific courting ritual, did she lay the first egg of her life at age 10.
Such sexual confusion also shows up in sheep and goats, which are, along with most ungulates, precocial species. When Keith Kendrick of the Babraham Institute in Cambridge, England, and his colleagues cross-fostered newborn kids and lambs with mothers from the other species, the infants formed strong bonds with their foster moms. The goats grew up thinking they were sheep, and vice versa.
And even though mammals are thought to exhibit more behavioral flexibility than geese, when two same-species siblings were raised together by a mother of the other species, the offspring's sexual preference in adulthood was for their foster mother's species. Further, males that had been cross-fostered preferred to mate with females of their moms' species even after living exclusively with their genetic species for three years.
Nevertheless, it remains unclear whether all parents (or foster parents) become "imprinted" on their infants' brains in a manner similar to that seen in precocial birds. In the meantime, try to avoid newly hatched chicks—unless you're ready to take on the responsibilities of motherhood.
