What gains could justify tiie costs? 

 Both egg laying and live bearing can 

 occur even within a single species, as in 

 Bougaineville's skink, an almost limbless 

 burrowing lizard from southeastern Aus- 

 tralia. While mainland populations are egg 

 layers, those on isolated southern islands 

 produce their young alive. In some cases 

 these islands are only a few miles off the 

 coast, and the egg layers and live bearers 

 live in very similar habitats. Studies have 

 shown that the difference in reproductive 

 modes is genetically determined and not 

 subject to short-term change. Since egg 

 laying in reptiles is believed to be the 

 primitive, or ancestral, condition, the is- 

 land populations must have evolved vi- 

 viparity in fairiy recent times — certainly 

 since the last Ice Age. 



When I combed the literature to find out 

 how many times viviparity had been 

 known to evolve within lizards and 

 snakes, I found many more examples than 

 I had expected: about a hundred separate 

 origins of this characteristic. Furthermore, 

 they occurred in a definite pattern. 



In ahnost every part of the world where 

 there are lizards and snakes, live bearers 

 are the dominant type only in colder parts 

 of their ranges. In the tropical rain forests 

 of northern Australia, less than one-third 

 of the lizard and snake species are live 

 bearers, while the vast majority lay eggs. 

 But in the cold and windy mountains of 

 southern Australia, the proportion of live 

 bearers rises to almost 100 percent of the 

 indigenous reptiles. Among the few spe- 

 cies that brave even colder habitats, in- 

 cluding European adders inside the Arctic 

 Circle, Canadian garter snakes in the 

 frigid fields of Manitoba, or the small 

 lizards that scun^y across snowdrifts at 

 12,000-foot elevations in the Andes, all 

 are viviparous. Almost a// of the live bear- 

 ers that are closely related to egg layers — 

 presumably those that most recently 

 evolved from them — are found in colder 

 habitats. 



Although viviparous species have 

 evolved in many other animal groups be- 

 sides reptiles, any correlation between 

 their reproductive pattern and cold cli- 

 mates isn't apparent. In some cases, the re- 

 verse trend appears. Viviparous sharks and 

 rays, for instance, tend to inhabit tropical 

 or subtropical oceans, while egg-laying 

 species live in cooler waters. Among am- 

 phibians, there are no clear correlations. 

 Some European salamanders that Uve at 

 low elevations are egg layers, while their 

 high-elevation relatives produce well-de- 

 veloped offspring. 



Viviparity has also evolved at least 

 twice in tropical amphibians, where pro- 

 tection of the eggs against drying out may 

 be the most important advantage for these 

 animals. Oddly enough, cold climates 

 have not led to viviparity in any species of 

 birds, although such cold-adapted flight- 

 less birds as penguins would seem likely 

 candidates. Mammals evolved viviparity 



A pregnant female agama lizard from Africa 

 is an egg layer She carefully regulates her 

 temperature by pushing her body away 

 from the hot rock while she basks. 



Don W. Fawcett 



only once — early among their egg-laying 

 ancestry — and it eventually spread 

 throughout almost the entire group. (Platy- 

 puses and echidnas are the only surviving 

 egg-laying mammals.) 



Reptiles aside, the number of times vi- 

 viparity has evolved in living vertebrates 

 is small — about ten instances in sharks 

 and rays, a dozen in bony fishes, four in 

 amphibians, one in mammals, and none in 

 birds. Because the trait spread throughout 

 mammaUan Uneages so long ago, we have 

 lost any basis for a comparative study 

 within that group. 



With a hundred origins of viviparity in 

 reptiles, however, we have at least some 

 hope of finding a plausible explanation as 

 to why this characteristic has evolved so 

 often. Correlation with climate may pro- 

 vide a starting point. Why should reptiles 

 show so many striking examples of evolv- 

 ing viviparity in colder habitats? Under 

 cooler conditions, what factors could en- 

 able live-bearing reptiles to become more 

 successful than their egg-laying cousins? 



One answer was proposed more than 

 fifty years ago by three scientists (Rudolf 



37 



