Seasonality of birth depends on environmental (the fertility period is determined in certain species by the photoperiod) and genetic factors (the number of gestation per year differs according to the sheep breed, for example). It can also be controlled by he herder, who may group or spread the births, avoid a second gestation in the year for weak females.

Birth control on a Maasai sheep (Kenya) (Balasse et al., 2003).

The herders control the reproductive activity of their sheep by tying an apron in front of the males’ genitals, which prevents mating and facilitate the choice of the mates.

For these reasons it is sometimes difficult to predict birth seasonality. It can be investigated through isotope analyses in tooth enamel.

PRINCIPLE

In a given locality, the oxygen isotope composition (d18O) of meteoric water varies seasonally (with temperature at high and middle latitudes, with precipitation amount at low latitudes).

Recording of the seasonal variations in precipitation d18O in tooth enamel.

The seasonal cycle is recorded tooth enamel bioapatite, whose oxygen isotope composition is linked to that of ingested water (meteroric water). It can be reconstructed through a chronological sequential sampling.

Because the chronology of tooth growth is fixed within a species, individuals born at the same season record the same sequence of the seasonal cycle in their teeth. If they were born at different seasons of the year, the recording of the seasonal cycle is shifted from one individual to another.

Model for the recording of birth seasonality in a herd.

APPLICATION

Application of the method to the Late Stone Age site of Kasteelberg (South Africa, Cape province) has shown two seasons of birth for domestic sheep, separated by about six months. This result could reflect

• subdivision of the flock into two groups, with planning of two birth periods in order to spread the birth and lactations events;
• two lambings per year (or three over two years) for indigenous females, as witnessed by European colons in the XVIIIth century.

In both cases, one consequence would have been the spread of milk availability throughout the year.

RELATED REFERENCES

 Balasse M., Smith A.B., Ambrose S.H. & Leigh S.R. (2003) Determining sheep birth seasonality by analysis of tooth enamel oxygen isotope ratios: the Late Stone Age site of Kasteelberg (South Africa). Journal of Archaeological Science 30 : 205-015.