Piglets call for maternal attention: Vocal behaviour in Sus scrofa domesticus is modulated by mother's proximity

Plasticity in the production of “separation distress” vocalizations, and seeking of caregivers beyond mere satisfaction of immediate physiological needs, are manifestations of attachment that we investigated in the domestic pig (Sus scrofa domesticus). Female piglets from eight litters were either weaned (N = 8) in pairs or stayed with their mother and remaining littermates (unweaned, N = 8). After 48 h, they were placed individually in a test pen for 5 min in isolation (ISO) followed by 5 min in fence-line contact with their mother (Restricted Reunion, RR). The sows spent comparable time close to their weaned and unweaned piglets during the RR sessions, and gave a comparable number of vocalizations in their presence. Although unweaned piglets uttered more vocalizations overall than their weaned littermates (P = 0.008), both weaned and unweaned piglets vocalized at a higher rate during the RR than the ISO session (P < 0.02). Furthermore, during the RR session, the piglets in both groups gave more vocalizations when their mother was in close proximity than further away (P = 0.04). In the ISO session, piglets uttered only grunt vocalizations whereas, during the RR session, they uttered both grunts and screams, with the ratio of screams to grunts increasing when their mother moved further away from them than when she was nearby (P = 0.004). Thus, piglets modulated their vocalizations as a function of their mother's proximity, and psychological attachment persisted beyond weaning.