Similarly, given the choice between the nest built by their mother and the nest of another mother, normal baby mice always chose the familiar nest while only a third of the receptor-deficient mice chose mom’s nest. The receptor-deficient mice don’t lack in the smelling department. Nor do they cry any less when exposed to dangers such as the presence of a threatening adult male mouse or cold temperatures. It seems that the opioid system is indeed connected to social bonding.