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

Abstract

Intra-tooth sequential analysis of enamel d 18 O 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 data sets are still required to address two major difficulties: (1) that inter-individual variability in the record of the seasonal cycle is affected by tooth size; and (2) that the season of birth cannot be directly estimated from the timing of tooth growth, because of a delay in enamel mineralization. We present a data set acquired on the lower second molar of 10 modern sheep from Rousay (Orkney) born within a few weeks of each other in April/May and submitted to the same environmental conditions until death. All sheep have recorded a sinusoidal pattern of d 18 O variation spanning approximately a year. From the difference between the expected and the measured time sequence, the delay of enamel mineralization is estimated to be 5-6 months. The data set is then described using a model mainly based on a cosine function. The period, corresponding to the length of the 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 in this data set. The lowest d 18 O values, corresponding to the sheep's first winter, were tracked at a distance from the enamel/ root junction that varied between 23.0 and 30.3 mm (x min mean = 27.6 mm); the highest d 18 O values, corresponding to the sheep's second summer, were between 6.3 and 11.6 mm (x max mean = 9.9 mm). Most of the variability can be attributed to tooth size. When normalized on the period, x min and x max are 0.28 (1 0.05) and 0.78 (1 0.05) on average, meaning that the Rousay sheep have recorded the minimum and maximum d 18 O values on average at 78% and 28%, respectively, of the end of the periodic cycle recorded in the second molar.

Animal Board Invited Review: Sheep birth distribution in past herds: a review for prehistoric Europe (6th to 3rd millennia BC)

“…This variability may be described through the observation of the position of the optimum value of the δ 18 O cycle in the tooth crown (Balasse et al, 2003). A quantitative estimation of the variability involves a normalization procedure, where the δ 18 O sequences are modelled using an equation derived from a cosine function (Balasse et al, 2012b). This allows to objectively define the position of the Balasse, Tresset, Bȃ lȃ şescu, Blaise, Tornero, Gandois, Fiorillo, Nyerges, Frémondeau, Banffy and Ivanova optimum value and to determine the period of the cycle (the distance over which the isotopic record covers one annual cycle), which may be used to link distance to time (Supplementary Figure S1a).…”

“…In sheep, these include the Carmejane (CAR) reference set for late winter and late summer births (Blaise and Balasse, 2011) and the Rousay (ROU) reference set for mid-spring births (Balasse et al, 2012b). Analyses are conducted on the second (M2) and the third molars (M3).…”

“…They provide a reference for April/May births (Balasse et al, 2012b). The CAR is composed of five M2s from Préalpes du Sud ewes raised on an experimental farm in southern France (Digne, Alpes de Haute Provence, Figure 2).…”

“…The analytical precision within each run, estimated from repeated analyses of our laboratory carbonate standard (Marbre LM, normalized to NBS19) was lower than 0.06‰ (n = 8). The modelling procedure was applied to the data sets from KMZ, CTD, CR, HPWN, KH, SKB, PBORD, HVA and ALS (Table 1) following Balasse et al (2012b). The data set from Cheia (CHE) (Tornero et al, 2013) was revised in order to correct the estimation of the location of the enamel root junction in teeth where mineralization was incomplete.…”

Calving seasonality at Pool, Orkney during the first millennium AD: an investigation using intra-tooth isotope ratio analysis of cattle molar enamel

“…Increasingly, birth seasonality of domestic herbivores has been investigated using intra-tooth isotope ratio analysis of molar enamel (e.g. Balasse et al 2003;Balasse and Tresset 2007;Blaise and Balasse 2011;Towers et al 2011;Balasse et al 2012a;Balasse et al 2012b;Tornero et al 2013;Towers et al 2014;Buchan et al 2015;Gron et al 2015).Each of the aforementioned techniques to identify prehistoric dairying has its limitations: faunal remains may be misinterpreted due to equifinality (Halstead 1998), potentially relevant to both mortality profiles and birth seasonality estimations, whilst neither lipid nor protein analyses provide information on the intensity of milk production and, consequently, its significance to ancient economies. However, a multi-2 proxy approach that combines evidence from a range of different techniques has the potential to produce a more accurate and nuanced understanding of the economic basis of a prehistoric site or, at the very least, will act to generate a fresh series of archaeological questions, thus contributing to the debate on dairying in the past.…”