Twin/Non-Twin Discrimination By Lambs: an Investigation of Salient Stimulus Characteristics

Abstract

Abstract A series of experiments was conducted to elucidate the stimuli that mediate twin/non-twin discrimination among 2-3 week old lambs. After being separated from their mother, lambs emitted more distress bleats when tested in isolation than when penned with their familiar twin, but separated from that lamb by a barrier of evenly spaced wood bars. A similar, albeit weaker effect was found for paired vs isolated non-twins. Twins separated by the barrier bleated less than did the non-twins in this condition. In contrast, bleat rates did not differ when subject lambs were tested in the presence of their twin vs a non-twin on the opposite side of a solid opaque barrier. Intact lambs in a final experiment bleated less when tested with their tranquillized twin vs a tranquillized non-twin agemate. The tranquillized stimulus lambs displayed atypical behavior patterns, including suppression of bleats and reduced activity levels. These data indicate that full bodily contact, vocalizations or normal behavioral patterns are not necessary for lambs to be recognized by their twin. Rather, twin/non-twin discrimination (and the effect of a twin on the rate of distress bleating) appears to depend upon visual, and possibly olfactory cues.