RESUMEN
We previously observed 2- to 3-day-old (P2-3) rats perform maternally directed orienting behaviors (MDOBs)-locomotion, torso flexion, rotation to the supine orientation, and snout probing, accompanied by audible vocalizations (barking)-under surfaces having properties of the dam's ventrum, which result in close contact between the pup's ventrum and the surface. We now report on the development of MDOBs from birth through P2. Wistar rats (24 litters) were tested at one of four times: 1 hr after birth before the first nursing bout (P0pre), on the day of birth after the first nursing bout (P0post), on postnatal Day 1 (P1), or on postnatal Day 2 (P2). Pups were placed prone on a warmed platform inside a corral where, for 4 min, they encountered one of two motherlike surfaces at head height: (a) a soft cotton roof or (b) a patch of the pup's own anesthetized dam's ventrum. Control pups received no overhead stimulus. The behaviors we had identified as MDOBs and others, including limb behaviors, are easily recognized at P0pre, and, in aggregate, are more frequent then than at any later age. The differential responsiveness of most of the behaviors to the dam's ventrum compared to the cotton roof or open condition is relatively low at P0pre, but rises rapidly through P2. Behavioral structure develops quickly from a rudimentary principal component with two behaviors at P0pre to two principal components incorporating seven behavior classes by P2. MDOBs are discussed in relation to late-term fetal behaviors, early postnatal righting responses, and the oral nipple grasp, and as a model of the development of filial attachment in an altricial mammal.