Vocalisations between mother and young in sheep: effects of breed and maternal experience

Abstract

This experiment was designed to investigate the function of vocalisations of the parturient ewe to her lambs in two breeds of sheep, a highly selected lowland breed (Suffolk), and a less selected hill breed (Scottish Blackface). Sheep have a specific lambing vocalisation, the low-pitched bleat or "rumble", that is made almost exclusively to the lamb. The hypothesis was that this "care-giver" bleat will be important in the formation of the ewe-lamb bond. Ewes also make high-pitched bleats, considered to be "protest" or distress bleats, after the birth of the lamb. Vocalisation data were collected from inexperienced and experienced ewes of both breeds, and their lambs. Additionally, an embryo transfer study between the two breeds was carried out to investigate interactions between ewe and lamb bleating. Low-pitched bleating was higher in primiparous ewes than multiparous ewes, and highest in primiparous Blackface ewes (mean bleat rate: primiparous Blackface=7.04, multiparous Blackface=2.73, primiparous Suffolk=3.99, multiparous Suffolk=2.18; P<0.005). An increase in litter size, or raising lambs of a different breed, had no effect on the rate of low-pitched bleating. The number of Suffolk ewes making high-pitched bleats was greater than Blackface ewes (61.4% vs. 35% for Suffolk and Blackface ewes respectively, P<0.01) and the rate of bleating increased with twin litters. Suffolk lambs had a higher bleat rate than Blackface lambs (mean bleat rate: Blackface=0.252, Suffolk=0.622, P<0.01), and lambs born to a primiparous ewe had a higher bleat rate than lambs with a multiparous dam (P<0.05). Birth of the second twin lamb caused a doubling in individual lamb bleat rate in both breeds. Ewe low-pitched bleat rate is, therefore, affected by intrinsic factors, ewe breed and experience, but not by characteristics of the lamb in the immediate postnatal period. The function of the bleat may be to strengthen bonding with the lamb, and the higher rate in primiparous ewes may reflect both the immaturity of the system in inexperienced animals and a slower rate of bond formation. High-pitched bleating is affected by ewe breed and may reflect the likelihood of separation of the ewe from the lamb. Lamb vocalisations are modified by their rearing experience, and appear to represent a signal of need. Increased domestication of the Suffolk breed may have led to an increase in lamb vocalisations.