Survival and Behaviour of Castrated Soay Sheep (Ovis Aries) in a Feral Island Population on Hirta, St

Sulphur0815 J. Zool., Lond. (1997) 243, 623-636 Survival and behaviour of castrated Soay sheep (Ovis aries) in a feral island population on Hirta, St. Kilda, Scotland P. A. JEWELL Department of Zoology, University of Cumbridge, Downing Street, Cambridge CB2 3EJ (Accepted 19 March 1997) (With 5 figures in the text) The free-living population of Soay sheep on the island of Hirta, St. Kilda, in the Outer Hebrides, has been intensively studied since 1959. The present study was initiated to throw light on the causes of the high 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 or two of birth. The survival of these castrates has been much longer than 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 of 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 and involved in agonistic and sexual encounters. This study substantiates the earlier assertion that the costs in energy of reproduction are a major cause of mortality in temperate zone ungulates. The social organization of some castrates was similar to that of females in that they remained with the home-range group of ewes into which they were born, but other individuals resembled males in that these castrates clubbed together in their own groups. These patterns of behaviour persisted throughout life. Introduction Many species of ungulates exhibit marked sexual dimorphism, and there is a higher mortality rate among males than among females: there is intense competition between males to become successful sires (Jewell, 1976). In these ungulates, the ratio of males to females at birth is usually close to 1:l (Parkes, 1926) but, in the adult breeding population, females may greatly outnumber males. The feral Soay sheep that occupy the island of Hirta, St. Kilda, the most westerly of the Outer Hebrides in Scotland, show this condition to a marked degree. This population, showing cyclic fluctuations around 1200 head, has been studied continuously since 1959 (Jewell, Milner & Boyd, 1974; T. H. Clutton-Brock et al., 1991, 1992; Jewell, 1995), and it is recorded that mortality among rams is very high with a heavy incidence in ram lambs and yearling rams: the ratio of adult rams to ewes varies from I :3 to 1:12 through the phases of the cycle (see Fig. 2). Most sheep die in late winter when all suffer from inanition but it is presumed that the events of the rut, occurring in November, further reduce the chances of males surviving over winter. In the rut, not only are males in physical competition with each other for access to females, but they spend much less time feeding (Grubb & Jewell, 1974). In Soay rams, as the mating season approaches, testosterone levels rise and there is a marked increase in aggressive behaviour (Grubb & Jewell, 1973; Lincoln & Davidson, 1977). It is well known that castration reduces libido and aggressiveness (Mattner, 1980; Wodzicka-Tomaszewska, Kilgour & Ryan, 1981), and that castrated rams (also commonly called wethers) are placid animals, although sexual drive can be restored in them by the injection of testosterone (Fulkerson, Adams & Gherardi, 1981). In an attempt to reveal some of the sex-specific factors involved in the differential mortality of males, a number of male lambs on Hirta was castrated in the years 1978, 1979 and 1980. It 623 0 1997 The Zoological Society of London Sulphur0815 Sulphur0816 624 P. A. JEWELL was hypothesized that the castrates would not attempt to participate in the rut and that their survival rate would be much better than that of ‘entire’ (uncastrated) rams. One of these castrates of 1980 is still to be seen alive today, nearly 17 years later (spring 1997)*, but all the entire rams of the same cohort are long since dead. It seems an appropriate time, therefore, to record the results: a preliminary note on the greatly enhanced survival of castrates has been published (Jewell, 1986). Intriguingly, there was an early observation on the survival of castrates on Hirta: when Soay sheep were introduced to Hirta from the adjacent island of Soay, in 1932, the consignment of 107 sheep included 22 wethers, these being male lambs of the year that had been castrated. Sixteen years later, James Fisher visited Hirta (Fisher, 1948) and afterwards remarked to Morton Boyd (Boyd, 1953) that he had seen some castrates still alive, recognizable by their ‘knob horns’. There is a very extensive literature describing the behaviour of sheep (recently reviewed by Lynch, Hinch & Adams, 1992), mostly relating to husbanded animals. While much quantitative ethology has been performed on ewes and lambs, there is little specifically about wethers. Much research has been carried out on the relationship between ewes and lambs, and on the sexual behaviour of rams (Lindsay & Pearce, 1984), but in commercial flocks the age structure and sex ratio are strikingly dissimilar to those in wild populations. The feral Soay sheep have afforded an opportunity to study behaviour in free-living conditions and with a full complement of rams (Grubb & Jewell, 1966, 1973; Jewell, Milner & Boyd, 1974; Jewell, 1995), and it is in this context that the behaviour of the castrates is particularly interesting. I know of no other study of a large mammal in which a group of castrates has been liberated into a free-living, unmanaged population in this way. Methods Soay sheep Soay sheep are a relic population of primitive domestic sheep similar to those that were widespread in Britain in the Bronze Age (Ryder, 1983). In their feral form they survived only on the small island of Soay in the St. Kilda group, but Soays were being brought to parks in mainland Britain at the beginning of the century (Elwes, 1913). After the evacuation of people from the nearby larger island of Hirta in 1930, a founding population of Soay sheep was established there. Over the years their numbers fluctuated between 600 and 1800 on the whole island, but the study was carried out on the sub-population that occupied the Village Glen and the nearby promontory of Ruaival (see Fig. l), numbering 300 to 500 animals. Soay sheep are polymorphic for fleece colour and horns (Boyd et al., 1964). The 2 basic fleece colours are light fawn (L) and dark brown (D) and, as with species of wild sheep, the fleece is usually cast completely every summer. The rams usually carry heavy horns in a close spiral, although occasionally crumpled horns are to be seen, and, very rarely, polled. Almost half the ewes have horns that are finer and more upright than the rams horns and almost half are completely polled (without horns); the remainder have small knobs of horn, or very poorly developed horns, called scurs. A description of home-range organization among Soay sheep is given in Jewell (1966) and Grubb & Jewell (1966), and the social behaviour of Soay sheep is described in detail in Jewell, Milner & Boyd (1974). Castration In 1978, in a pilot study, 14 male lambs were castrated within 1 or 2 days of birth using the usual method for husbanded sheep of applying a rubber ring round the scrotum (‘Elastrator’, Elastrator Co., Blenheim, *This castrate died on, or a day or two before, 23 March 1997. He was born on 22 April 1980 and so was one month short of 17 years old. Sulphur0816 Sulphur0817 SURVIVAL OF CASTRATED FERAL SHEEP 625 Mulloch Mor Old village at the centre of the 'Onachai\ Village Glen study area \ / FIG.1. Block diagram of Hirta and Dun giving an oblique aerial view from the south. A scale is provided by the island of Dun which is 1425 m long (after Macgregor, 1960). The dashed line encloses the Village Glen study area. New Zealand). Fourteen other male lambs, caught concurrently, were tagged but not castrated to serve as controls (entire rams) see Table I. These lambs were caught over what proved to be the peak of the lambing period between 14 April and 22 April (median date April 19/20, see Jewell, 1989). An additional 16 male lambs (not shown in Table I) and 54 ewe lambs were tagged. No problems having arisen in the pilot study, a second visit was made in 1979 . This proved to be a crash year for the sheep population, however (see Fig. 2), and there had been heavy mortality in February and March. The surviving ewes were in poor condition, the lambs were light in weight, and many died before the end of the lambing period. No attempt was made, therefore, to castrate a large sample and only 8 castrates and 8 controls (3 of which died neonatally) were tagged, together with 33 other ram lambs and 44 ewe lambs. The following year, 1980, was one of recovery for the sheep (see Fig. 2). No dead sheep were found in the village area. A hundred male lambs were caught of which 50 were castrated and 50 became controls; 83 ewe lambs were caught . The male lambs were assigned to be castrated at random and the colour and horn types into which they grew had to be recorded later when they became mature. Forty-two castrates were seen in July 1983 and categorized in this way (see Results). Tagging Lambs were given small Dalton plastic ear-tags each with a unique number and of a different colour for each year.

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