Role of auditory and visual cues in mutual recognition between ewes and lambs in Merino sheep

Abstract

The relative roles of visual and auditory cues in mutual recognition of Australian Merino ewes and lambs 1–4, 8–12 and 23–31 days after birth, were studied. The roles were assessed by observing behaviour when a ewe and a lamb were simultaneously released, 13 m apart, at opposite ends of a screened enclosure; in some tests the ewes had been muted or altered in appearance, while in others the lambs had been so treated. Auditory cues were muted by anaesthetising a ewe's or lamb's vocal cords and blocking the test partner's ears with putty; visual cues were altered by shearing ewes and by blackening ewes and lambs. While these treatments had little effect on the behaviour of the animals to which they were applied, there were clear effects on the behaviour of their partners. Blackening or muting lambs, either separately or together, slowed the approach of the ewe towards the lamb. Blackening alone, led to hesitation or dodging in the ewe's approach, and agnostic responses in some ewes. Intact hearing reduced the effect of blackening with own (natural) partners, but not with alien lambs, so there is some specificity in auditory cues from the lamb. Likewise, ewes' scores for interest-in-lambs were not affected by muting the lambs' voices alone, but were reduced by blackening. This reduction was less marked when the voice of the lamb, either own or alien, was intact, so a lamb's voice also has some non-specific attraction to ewes. The results for ewes were largely independent of the time since lambing. The results also show that it is not essential that a ewe hears her lamb for her to accept it. Lambs 1–4 days old were relatively insensitive to any of the treatments imposed on ewes. As judged by the speed of approach towards ewes and the incidence of hesitation or dodging, 8–12-day old lambs did not perceive the shape and colour change due to shearing, as readily as the change due to blackening, while older lambs appeared to perceive both changes equally well. Muting the ewes clearly affected the responses of 8–12-day old lambs, but not of older lambs. It appears that in Merino lambs the importance of visual cues relative to auditory cues increases between the 2nd and 4th week of life. Interest scores also indicated that older lambs rely more on visual than on auditory cues. It is concluded that both auditory and visual cues are important in mutual recognition of ewes and lambs, but auditory cues are of secondary importance if strong visial cues indicate an alien partner.