Fig. 2. Translating mother–infant reunion phase of the Strange Situation Procedure across species.

The SSP uncovers deficits in attachment behavior in children through a series of exposures to a stranger and separation–reunion with the mother, culminating in a final epoch of the child’s reunion with the mother^2,10. Child behaviors during this reunion can signify the quality of attachment to the caregiver. While minor variations in attachment quality suggest individual differences in mother–infant interactions, attachment categories such as disorganized attachment highlight disruptive mother-infant attachment quality and predict later-life pathology^2,68. Adaptation of the SSP to rodents (rSSP) uses a similar series of epochs with the mother. In rodents, where infants rely on the maternal odor for mother–infant interactions, the mother possessed the natural diet-dependent, learned maternal odor. A “stranger” was produced by feeding another lactating dam a different diet (Tekland) that suppressed the natural maternal odor and provided this mother with a novel maternal odor, which was unfamiliar to test pups. Importantly, pups reared with the “stranger” mother show strong attachment behaviors to this mother, while our test pups unfamiliar with this “stranger’s” maternal odor do not. Only the final reunion data from the SSP and rSSP is used to determine attachment quality (outlined in red). The SSP child subjects were 11–28 months old, while rat pups were PN13–14, an age range across species associated with some mobility and complete dependence on the caregiver for survival¹⁵.