Infant rats, or pups, will form attachments to familiar smells even when those smells are paired with electric shocks, according to a new study published in Behavioral Neuroscience (Vol. 118, No. 2). This suggests that pups' brains, and perhaps the brains of other young animals, are primed to ensure rapid attachment to their mothers regardless of how the mother treats the infant, say researchers.
University of Oklahoma psychologist Regina M. Sullivan, PhD, and graduate student Stephanie Moriceau injected 99 eight-day-old rats with either the stress hormone cortico-sterone or saline. They then placed the animals in a 600-milliliter glass beaker permeated with a citrus smell and exposed them to an electric shock for one minute. Researchers repeated this procedure 11 times, taking the pup out of the beaker between trials.
One day later, the researchers placed the pups into a two-armed maze, where the animals could choose to move toward the citrus smell or toward a pine smell. The saline-injected pups chose the citrus smell about 80 percent of the time, whereas the pups primed with the stress hormone failed to learn the odor preference necessary for attachment, choosing to move toward the citrus odor only 30 percent of the time.
Rat pups, says Sullivan, produce naturally low levels of corticosterone up until they are about 11 days old, so this experiment suggests young rats can be made to learn aversions more like older ones by introducing the stress hormone.
In a second experiment, Sullivan and Moriceau devised a way to extend what they term the "sensitive learning period" of young rats--the period of time when odor attachments are easily formed regardless of pairings with aversive stimuli, such as tail shocks.
Specifically, they removed the adrenal glands of 50 12-day-old rats and left another 50 animals intact. The rats then underwent the same procedure as those in the first experiment. Rats with no adrenal gland, and therefore no corticosterone, acted much like younger animals, choosing to move toward the citrus odor about 80 percent of the time; the normal group chose the citrus odor at a rate of only 20 percent.
These experiments, explains Sullivan, point to the importance of corticosterone in determining rats' sensitive learning period--which is key to their survival. For example, says Sullivan, deaf and blind rat pups must quickly learn to identify their mothers' smell in order to attach to nipples and suckle. Similarly expedited attachment behaviors have been observed in chicks, puppies and young primates, she notes.
"This suggests that the brains of infants are probably organized in such a way that ensures attachment to the caregiver," says Sullivan. "Of course, it would make sense that evolution would create a rapid reliable system for attachment."