A hornless ewe suckles her lamb, below. Between 10 and 20 

 percent of mothers produce twins, which weigh less at birth and 

 are somewhat less likely to sun>ive than are singletons. Bearing 

 the remains of winter fleece, a two-year-old ewe, right, licks her 

 newborn lamb. 



Tim Clutton-Brock 



i^l^Jffi 



telopes of the Asian steppes, for instance, 

 conceive in their first year of life and usu- 

 ally produce twins; their numbers, like the 

 sheep's, can increase very rapidly. Their 

 populations are unstable, but we don't yet 

 know whether they oscillate regularly. 

 White-tailed deer, too, commonly con- 

 ceive in their first autumn of life, and ma- 

 ture females often produce twins. But 

 here, natural predators and human hunters 

 constrain population growth, usually pre- 

 venting local populations from exceeding 

 their food resources. 



One other ungulate population that ap- 

 pears to cycle is the Corsican mouflon 

 sheep, which was introduced to the sub- 

 antarctic Kerguelen Islands in the 1950s. 

 As on Hirta, there are no effective mam- 

 malian predators, and mouflon numbers 

 have increased rapidly. Unlike the Soays, 

 however, Kerguelen mouflon do not con- 

 ceive until their second year. But twins are 

 common and neonatal mortality is low. 

 Patrick Bousses, of the French National 

 Museum of Natural History, has recently 

 shown that population crashes comparable 

 to those we have observed on Hirta occur 

 every fourth year among the mouflons. I 

 am not surprised that the periodicity of 

 these cycles is rather longer than in Soay 

 sheep, for the mouflon are larger animals 



and their delayed age of first breeding 

 slows the population's growth rate. (Simi- 

 larly, as Peterson has suggested, the rela- 

 tionship between small body size, high fe- 

 cundity, and rapid population growth 

 probably explains why smaller rodents re- 

 cover from crashes more quickly than 

 larger ones, generating shorter cycles.) 



So might population cycles be a much 

 commoner phenomenon than we imagine? 

 Can we expect to find thirty-year moose 

 cycles and seventy-year elephant cycles, 

 as Peterson and his colleagues suggest? 

 That is not inconceivable, but I'm skepti- 

 cal. As body size increases and fecundity 

 falls, we see a decline in a population's ca- 

 pacity to exceed winter food suppUes by 

 multiplying during the boom months of 

 early summer. Weaning occurs later, limit- 

 ing mothers' ability to regain condition be- 

 fore the autumn rut. Populations increase 

 more slowly, providing more opportunities 

 for density-dependent changes in preda- 

 tion or starvation to depress further in- 

 creases in numbers. Although moose and 

 elephant populations may oscillate, and 

 crashes may occur when winter or dry- 

 season food supplies are suddenly re- 

 stricted, I doubt that future generations of 

 wildlife biologists will discover that they 

 show regular cycles. D 



34 Natural History 3/94 



