SURVIVAL AND BEHAVIOR OF CASTRATED SOAY SHEEP (OVIS-ARIES) IN A FERALISLAND POPULATION ON HIRTA, ST. KILDA, SCOTLAND

The free-living population of Soay sheep on the island of Hirta, St. K ilda, in the Outer Hebrides, has been intensively studied since 1959. The present study was initiated to throw light on the causes of the hi gh mortality rate of adult rams in comparison to that of ewes. In 1978 , 1979 and 1980, a total of 72 male lambs was castrated within a day o r two of birth. The survival of these castrates has been much longer t han that of the entire rams, marked as controls, and longer than that of ewes of the cohorts of the same years. The daily activity pattern o f the castrates was similar to that of ewes rather than that of rams. In particular, during the rut the castrates spent most of the daylight hours grazing, in contrast to the rams who were continuously moving a nd involved in agonistic and sexual encounters. This study substantiat es the earlier assertion that the costs in energy of reproduction are a major cause of mortality in temperate zone ungulates. The social org anization of some castrates was similar to that of females in that the y remained with the home-range group of ewes into which they were born , but other individuals resembled males in that these castrates clubbe d together in their own groups. These patterns of behaviour persisted throughout life.