Three Soay ewes and three young rams graze on Hirta. Feral for 

 at least a thousand years, the breed is the most primitive of 

 Europe 's domestic sheep. Both sexes usually have horns. 



Tim Clutton-Brock 





Counting Sheep 



Every few years, most of the feral sheep on a Scottish island 

 perish — yet the flock survives 



by Tim Clutton-Brock 



In the Atlantic Ocean, off the northwest 

 coast of Scotland, lie the Outer Hebrides. 

 Forty miles farther out, the shattered rim 

 of an extinct volcano forms another archi- 

 pelago known as Saint Kilda. Its rugged, 

 rocky islands are home to huge colonies of 

 puffins, gannets, fulmars, and shearwaters. 

 The archipelago also contains its own sub- 

 species of mouse and a wren whose songs 

 are strikingly different from those of its 

 mainland cousins. But the most unusual 

 inhabitants of Saint Kilda are the small, 

 feral Soay sheep, named for a small island 

 in the archipelago on which they have 

 grazed since ancient times. Precisely when 

 they were introduced to Soay is unknown, 

 but it may have been as early as 3,000 

 years ago; even the most conservative es- 

 timates place them on the island for at 

 least a thousand years. Soays are the most 

 primitive breed of domestic sheep in Eu- 

 rope; their skeletons closely resemble the 

 remains of sheep from early Neolithic 

 sites. Although their fleece is generally 

 brown, it can range in color from cream to 

 black. Both sexes usually have horns, and 

 their partly woolly fleece also sports long, 

 straight hairs. 



Hirta, the largest of Saint Kilda's is- 

 lands, supported a population of crofters 

 that dwindled until 1930, when the thirty 

 remaining villagers were relocated to the 

 mainland. In 1932, 107 feral sheep from 

 Soay were introduced to Hirta by the is- 

 land's owner, the Marquis of Bute. They 

 quickly increased to colonize the whole is- 

 land, reaching 500 in less than ten years. 

 When the first organized census was taken 

 in 1952, there were 1,114 sheep on the is- 

 land. To a zoologist, however, the striking 

 feahire of Hirta's Soay sheep population is 



that it appears to rise and fall in cycles. 

 Every third or fourth winter, after numbers 

 have passed the 1,400 mark, the sheep on 

 the island begin to starve. In their weak- 

 ened condition, many seek the sanctuary 

 of the oblong dry-stone shelters, or cleits, 

 that the islanders once used to dry seabirds 

 harvested for their meat and feathers. Sev- 

 enty percent of the sheep succumb, mostly 

 in February or March. Their bodies pile 

 up, and by April, many of the cleits are 

 choked with rotting carcasses. 



Until recently, zoologists thought that 

 regular population cycles were confined to 

 small-bodied mammals in the Arctic and 

 sub-Antarctic {see "The Lemming Phe- 

 nomenon," Natural History, December 

 1989). At intervals of between two and 

 nine years, populations of voles, lem- 

 mings, and snowshoe hares commonly 

 rise and fall, with populations sometimes 

 falling to less than one-tenth of peak num- 

 bers. Cycles may have dramatic effects 

 both on these animals' food supply and on 

 the prosperity of their predators, whose 

 populations may decrease rapidly as their 

 own food supply disappears. We are not 

 accustomed to thinking of such dramatic 

 cycles in larger mammals. Imagine, for 

 example, a tenfold increase in American 

 white-tailed deer populations over three 

 years, or a sudden 90 percent reduction in 

 their numbers. 



But population cycles may not be con- 

 fined to small mammals after all. Ten 

 years ago, Rolf Peterson and his col- 

 leagues at Michigan Technological Uni- 

 versity showed that, across species, the 

 length of cycles increased with the body 

 size of animals. The most rapid cycles — 

 two to three years — are found among mice 



tiJiii; 



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