In temperate climates, the sexual activity of sheep is synchronised by photo-period. Cattle do not experience periodic infertility; however their reproductive strategy is related to seasonal food availability. The herder can modulate this system only to a certain extent. Restriction of the birth period is achieved by physically controlling the animals. Extension of the birth season for sheep cannot be achieved outside of the period defined by photoperiod without the intervention of artificial light and hormonal treatments. In cattle it requires supplying the mates with food on demand the whole year round. This suggests that physiological and environmental constraints might have been strong influences on reproduction management in early sheep and cattle husbandry. It is most likely that sheep and cattle reproduction were seasonal. Within this framework, how restricted in time the birth season was remains to be defined. Assessment of birth seasonality from stable isotope measurements (δ18O) of teeth was carried out on the north-western European Neolithic assemblages of the Knap of Howar (ca. 3600 BC), the Holm of Papa Westray (beginning of the 3rd millennium BC) and Er Yoh (second half of the 4th millennium BC). The results suggest a more restricted birth season both for cattle and sheep at the Knap of Howar than seen later in time at the Holm of Papa Westray and at Er Yoh. However it is not yet possible to disentangle climatic from biological and cultural factors responsible for this difference. These observations must be re-evaluated after widening of the data set.
Birth seasonality, sheep, cattle, Neolithic, north-western Europe, oxygen stable isotope ratio, tooth enamel.