Acoustic variables of high-pitched vocalizations in dairy sheep breeds

Abstract

The source–filter theory of voice production is widely used on mammal vocal communication research allowing a better understanding of individual and context variations. Generally, mammals with less complex repertoires have received little attention regarding the influence of different factors on their vocal characteristics. Acoustic cues appear to be crucial for mutual recognition from a distance in sheep. During stressful situations such as separation from the offspring and social isolation, sheep produce mainly high-pitched bleats, which can be influenced by many factors including breed, individuality, sex or body size. The aim of the present study was to analyze high-pitched bleats produced during a short isolation period from four dairy sheep breeds (38-42 days post-partum) and to investigate if breed, number of offspring, presence of sibling, sex, height, activity levels and context (short temporary or complete absence of kids), affect their vocal characteristics. Ewes (n = 49) of four different dairy breeds namely Chios, Karagouniki, Orino Epirus and Synthetic breed (50% Orino Epirus, 25% Chios breed, 25% East Friesian breed) were isolated from their suckling offspring (n = 65) and vocalizations and behavioural response of each ewe and lamb(s) were recorded. In order to examine the effect of different context on the vocal characteristics of bleats, the experiment was repeated for 11 of the above tested ewes in dry period and thus, without suckling lamb(s). Principal component analysis followed by linear mixed model analyses showed significant (p < 0.05) differences in the vocal parameters between the examined breeds in both ewes and lambs. Interestingly, ewes differentiated their bleat characteristics in regard to the number of offspring. In addition, differences were observed in acoustic characteristics of lambs that had a sibling, which produced bleats with higher mean format frequencies. Furthermore, the behaviour of ewes appeared to be related to vocal parameters. Body size affected lambs’ vocal parameters related to fundamental frequency disturbance. In addition, ewes’ acoustic parameters related to fundamental frequency and F0 disturbance were observed to differ between contexts. Finally, individuality in both ewes and lambs was also observed in regard to their vocalizations during the studied separation period. The study provides valuable novel information about sheep high-pitched bleats as well as for future development of in-situ livestock applications.