makes an ideal model subject for scientists researching evolution, population dynamics and demography because the population is unmanaged, is closed (no emigration or immigration) and has no significant competitors or predators. The sheep exhibit a phenomenon known as overcompensatory density dependence, in which their population never reaches equilibrium. The population growth is so great as to exceed the carrying capacity of the island, which eventually causes a dramatic population crash, and then the cycle repeats. For example, in 1989, the population fell by two-thirds within 12 weeks. The age and sex structure of the population are important in determining when a crash happens; for instance, adult males enter winter in a poor condition after the autumn rut, whereas females have been grazing all summer and so enter winter in a good condition. Survival rates of males (and lambs) are influenced by weather throughout winter (dependent on the strength of the North Atlantic Oscillation), whereas survival rates of females (and adolescents) are influenced most strongly by rainfall at the end of winter, when they will normally be heavily pregnant (the rain soaks the wool, increasing energy expenditure). Another factor in mortality rates is the loading of intestinal nematode parasites, particularly damaging in malnourished hosts. The breed is becoming smaller because of the change in climate. Breed characteristics The sheep have short tails and naturally shed their wool, which can be hand plucked (called rooing) in the spring and early summer. About one kilogram of wool can be obtained from each animal per year. Ewes are polled, scurred or horned and rams are either horned or scurred. They are most commonly brown or tan with a white belly, white rump patch and/or white patch under the chin (referred to as the Mouflon or wild pattern). Occasionally white markings on the face and/or body and legs occur. Rarely self-coloured (solid colour with no markings) black or tan individuals are seen. This breed has extremely fine fleece and, in contrast to mouflon, the inner fleece is highly developed and it is difficult to distinguish an outer coat. This is a clear indication that the Soay are indeed the product of a domesticated breed in prehistoric times. The breed also lacks the flocking instinct of many breeds. Attempts to work them using sheep dogs result in a scattering of the group. The lambs are late maturing and produce smaller carcasses relative to commercial breeds. The