Investigating Seasonality and Season of Birth in past Herds: a Reference Set of Sheep Enamel Stable Oxygen Isotope Ratios

Intra-tooth sequential analysis of enamel δO is currently used to investigate birth seasonality in past animal populations, offering new insights into seasonal availability of animal resources, herd management and seasonality of site occupation. Reference datasets are still required to address two major difficulties: (1) inter-individual variability in the record of the seasonal cycle is affected by tooth size; (2) the season of birth cannot be directly estimated from the timing of tooth growth, because of a delay in enamel mineralization. We present a dataset acquired on the lower second molar of ten modern sheep from Rousay (Orkney) born within a few weeks in April / May and submitted to the same environmental conditions until death. All sheep have recorded a sinusoidal pattern of δO variation spanning approximately a year. From the difference between expected and measured time sequence, the delay of enamel mineralization is estimated to five to six months. The data set is then described using a model mainly based on a cosine function. The period, corresponding to the length of M2 crown formed over a year, averaged 35.8 mm. A very slight variation of tooth growth rate with time and no attenuation of the isotopic signal towards the cervical margin of the crown could be detected on this dataset. The lowest δO values, corresponding to the sheep’s first winter, were tracked at a distance from the enamel-root junction varying between 23.0 and 30.3 mm (xmin mean = 27.6 mm); the highest δO values, corresponding to the sheep second summer, between 6.3 and 11.6 mm (xmax mean = 9.9 mm). Most of the variability can be attributed to tooth size. When normalized on the period, xmin and xmax are 0.28 (± 0.05) and 0.78 ( ± 0.05) on average, meaning that the Rousay sheep have recorded the minimum and maximum δO values on average at respectively 78 % and 28 % of the end of the periodic cycle recorded in the second molar.