Introduction

Sheep were domesticated in the Near East around 10 000 years ago and spread into Western Europe from there (J. Clutton-Brock 1981). Sheep similar to Soays had reached the Orkneys by 4000 bc and the sheep population of St Kilda may have originated around that date. In many aspects of their anatomy and physiology, they appear to be intermediate between contemporary domestic sheep and wild sheep (Boyd and Jewell 1974; Jewell 1986).

To understand the unusual dynamics of Soay sheep and their consequences for selection and adaptation, it is important to know something of their history as well as of the human inhabitants of St Kilda. The first two sections of this chapter describe the islands of St Kilda (section 2.2) and their history (section 2.3). Subsequent sections describe the appearance and anatomy of Soay sheep (section 2.4), their feeding ecology (section 2.5) and their reproductive system (section 2.6). Since variation in fecundity and neonatal survival affect the growth rate of the population, we describe the factors affecting the early development of lambs (section 2.7) as well as the factors affecting winter survival in juveniles and yearlings (section 2.8). Finally section 2.9 reviews the costs of reproduction and other factors affecting mortality in adults.

The islands of St Kilda

The four main islands of the St Kilda archipelago lie 160 km to the north-west of the Scottish mainland (Fig. 1.1).