Stephanie Moriceau and Regina Sullivan of the University of Oklahoma tested how baby rats responded to the pairing of an unfamiliar odor--peppermint--and a weak electric shock to their tails. The charge-laced scent attracted the youngest pups without exception while repelling their older siblings of 21 days--the age when rats become fully independent. But young rats between 12 and 15 days old either learned to love the peppermint despite the shock if their mother was present or learned to fear it if she was not. When presented with the odor later in a Y-maze, the mothered pups would invariably move toward it while their motherless counterparts would move away.
This learning appears to be mediated by stress hormones. Giving corticosterone to seven-day-old pups allowed them to begin to fear the peppermintaroma ahead of schedule while removing corticosterone rendered slightly older pups immune to fear conditioning. Other tests showed that a mother's presence reduced the release of corticosterone and thus dispelled the fear that would otherwise condition the pups against the smell. "The mother's ability to modify fear learning circuitry may provide clues to abusive attachment and predisposition for mental illness and altered emotional expression later in life," the researchers write, proving that in some cases mother may not know best.