c 

 o 



Survival 



of the 

 Smallest 



When Pleistocene seas rose, 

 diminutive'island deer gained a 

 competitive edge. ..temporarily 



by Adrian M. Lister 



►^ Islands have long been favorites among 

 [T^l biologists for the study of evolution. Be- 

 cause the number of species is low and the 

 habitats relatively simple, islands are ideal 

 for thorough surveys of both ecology and 

 genetics. They also provide a perfect 

 model for one of the most popular theories 

 of how species come into being: a small 

 population of plants or animals, isolated 

 from the main range of the parent species, 

 can rapidly evolve into a new form. The 

 widespread phenomenon of endemic 

 species, plants or animals found only on 

 particular islands or island chains, testifies 

 to the power of this process. 



A common island phenomenon is the 

 evolution of unusual body size in mam- 

 mals. Small mammals, such as dormice, 

 shrews, and hamsters, often grow larger 

 than usual, while larger, herbivorous 

 mammals tend to become smaller than the 

 norm. A living example of the latter can be 

 seen in the Florida Keys, a chain of narrow 

 islands off the southern coast of Florida. 

 Key deer are miniature versions of the 

 common white-tailed deer of mainland 

 North America. 



Further examples of dwarfing come 

 from the fossil record, particularly from 

 the Pleistocene, the period that lasted 

 much of the last two million years and in- 

 cluded the ice ages. During this time, 

 many islands in the Mediterranean were 

 home to dwarf forms of deer, elephants, 

 and hippos. Dwarf stegodons, which were 

 mastodonlike proboscideans, lived in In- 

 donesia, and dwarf mammoths inhabited 

 islands off California and northern 

 Siberia. 



For the past six years, I have studied 

 fossil deer from caves on Jersey, an island 

 politically affiliated with Britain but geo- 

 graphically close to the coast of northern 

 France. The recent work there is the con- 

 tinuation of excavations first begun in a 

 cave at Belle Hougue on the northern 

 coast of the island in 1914; a second cave 

 with deer remains was discovered nearby 

 in 1965. Anatomical studies show these 

 bones to be closely related to Cervus 

 elaphus, the red deer common today in 

 Europe and Asia, and differing only 

 slightly from the North American elk. The 

 bones from Belle Hougue belonged to an- 

 imals that, fully grown, stood less than 

 two and a half feet at the shoulder and 

 weighed eighty pounds, only one-sixth the 

 body weight of red deer from other Pleis- 

 tocene fossil deposits in mainland Europe. 



Dating of the Belle Hougue deposits, by 

 analysis of uranium isotopes, indicates 

 that the dwarf deer lived about 120,000 

 years ago. However, at another site on the 

 island. La Cotte de Saint Brelade, older 

 red deer fossils have been discovered in 

 deposits spanning the period from about 

 230,000 years ago to shortly before those 

 of Belle Hougue. These older remains are 

 much larger than the bones from Belle 

 Hougue; indeed, they are the same size as 

 bones of red deer on the mainland. Jersey, 



then, was once home to ordinary-sized red 

 deer. How and why did the transition to 

 dwarf deer on Jersey occur? The clue 

 comes in the climate of the Pleistocene. 



For much of the time represented by the 

 older La Cotte deposits, global climates 

 were colder than those of today, and ice 

 covered the higher latitudes. So much of 

 the world's water was locked up in the ex- 

 panded polar icecaps that global sea levels 

 fell to as much as 300 feet below present 

 levels. Even today, the seaway between 

 Jersey and France is relatively shallow — 

 only about thirty feet deep at low tide. But 

 during the ice ages, this sea floor was ex- 

 posed, and Jersey became part of a broad 

 plain connecting Britain and France across 

 the dry Channel. The large red deer recov- 

 ered at La Cotte were therefore part of a 

 widespread mainland population free to 

 roam over a broad range and walk be- 

 tween present-day France and Britain. 



About 125,000 years ago, the climate 

 warmed up, the icecaps melted, and as sea 

 levels rose, Jersey became an island. The 

 dwarf deer bones from Belle Hougue, 

 which date from this period, are embedded 

 within a deposit of pebbles and seashells 

 above the reach of modem tides. Now fos- 

 sihzed, these deposits were a beach when 

 sea level was a few yards higher than it is 



Left map: Some 150,000 years ago, sea 

 levels were about 300 feet lower than 

 today's, and the coast of Europe (dark 

 green) was farther west Britain, France, 

 and the island of Jersey were part of one 

 land mass. Map at right: As sea levels 

 rose about 125,000 years ago, Jersey was 

 joined to the mainland only by an 

 isthmus. By 120,000 years ago, the 

 island's isolation was complete. 



60 Natural History 6/94 



