Early vocal recognition of mother by lambs: contribution of low- and high-frequency vocalizations

In sheep, Ovis aries, lambs show a preference for their own mother soon after birth, which is important for their survival. The relative role of low- and high-pitched bleats provided by the mother in this early preference is not clear. While acoustic recognition is clearly established in 2-day-old lambs, it is not known whether lambs use an individual acoustic signature, or some other feature of acoustic communication (type of bleat or a combination of responses) by the mother indicating acceptance. The acoustic analysis of bleats demonstrated that both types of vocalization can support individual discrimination and that frequency spectrum and amplitude modulation exhibit an individual stereotypy. However, the characteristics of bleats and the quantity of encoding information in them differed between low- and high-pitched bleats. In two-choice procedures, we tested the ability of 48 h-old lambs to discriminate between recorded bleats of their own mother and an unfamiliar equivalent mother. Lambs showed a preference for the playback of low-pitched bleats of their own mother, but not for high-pitched bleats. Therefore, 48 h-old lambs can use an individual acoustic signature to recognize their own mother when low-pitched bleats are used. In contrast when high-pitched bleats are used, a vocal exchange between the mother and her lamb appears necessary. We conclude that 48 h-old lambs are able to learn and recognize the individual characteristics of their mother's low-pitched bleats and that this vocal recognition may be important for the establishment of the mother–young bond even at this early stage of the relationship.