A count of the whole of Hirta’s Soay sheep population has been conducted most years since 1952 by the same method. It was these counts that revealed that Soay sheep on St Kilda have rather unusual population dynamics. Unlike many other populations of ungulates (e.g. the red deer on Rum), the population rises to maxima and then crashes, at irregular intervals. It is this population dynamic behaviour that makes Soay sheep so interesting for ecologists.
The first phase of ecological research on Soay sheep on St Kilda was led by Peter Jewell, John Morton Boyd and Peter Grubb from 1959 to 1967. In a study that was much ahead of its time, they tagged cohorts of lambs with different tag colours and followed the survivorship of different cohorts through their lives. This work was extensively published, including in the 1974 monograph, Island Survivors, but by the time it ended in 1967 the causes of crashes and an explanation for their frequency was still unclear. Tagging of some Village Bay lambs continued, partly as a warden’s duty.
The original Soay Sheep Project team: from left to right, Morton Boyd, Peter Jewell, Tex Geddes, Bill Smith and Graham Gunn.
In 1985 Tim Clutton-Brock and Steve Albon from the University of Cambridge restarted intensive research on the Soay sheep by achieving complete tagging of all the sheep living in Village Bay, so that individual life histories could be followed from start to finish. Inspired by the sister project on red deer on the island of Rum, the Soay sheep project sought to explain how the population dynamics came about, as a function of individual life history variation.
In brief, it became clear that the population dynamics of Soay sheep happen because virtually all mature females conceive each year, regardless of density, and as a result, the population can increase in one breeding season to a size which greatly exceeds the winter carrying capacity, when it may crash. Crashes are more likely to occur when there is bad winter weather, and when the population contains a large proportion of vulnerable sheep such as lambs and males. The population then increases again, over several years, before another crash. See Population Ecology in Current Research for more details.
As ecological research proceeded, it became clear that the Soay sheep population also offers remarkable opportunities for understanding the progress of natural selection and evolution in real time. Population crashes are a period of intense selection, could they have anything to do with the maintenance of genetic variation, for example in coat colour and horn type? Does the low life expectancy of most individuals select for early reproductive effort? Likewise, the population dynamics research inspires numerous questions about the relationship between the sheep and their biotic environment, including the plants on which they feed and the parasites, both of which are the subject of current research. For more information on research themes, see Current Research. To learn more about how we study the sheep go to Data Collection. For more information on the current Soay sheep research team, please go to the People section.