Y)iingLizaiids CanBeBeamble 



In Australia, live-bearing skinks have evolved from egg layers. Why? 

 by Richard Shine 



One of the reasons I was attracted to the 

 study of Hzards and snakes, rather than 

 other kinds of animals, is that they gener- 

 ally like to stay in bed on cold mornings, 

 just as I do. I thought that I wouldn't have 

 to rise at dawn (like the bird watchers) or 

 muck through muddy swamps at night 

 (Uke the frog catchers) because Australian 

 skinks come out into the woods and fields 

 on sunny afternoons. 



Unfortunately, as I discovered in the 

 field, these common little hzards are so 

 elusive after they have warmed up by mid- 

 morning that they are almost impossible to 

 catch. Chilly mornings are the best time to 

 pick them up as they lie, rigid with cold, 

 under their nighttime logs. In the Brind- 

 abella Range of southeastern Australia, 

 where I study and collect skinks, I imag- 

 ined the kookaburra birds were mocking 

 me with their annoying "laughter" as I 

 turned over logs from first light until the 

 sun's rays dispersed the morning fog. Each 

 morning a few hours after dawn, I re- 

 turned to my tent, with dew-soaked socks 

 and chattering teeth, to boil the billy (ket- 

 tle), change into dry clothes, and sit by the 

 campfire gloating over the fruits of my 

 morning's labors. 



My prizes wouldn't have impressed 

 most people. Every day I caught about 

 twenty small, drab skinks, most of them 

 less than six inches long. Why endure so 

 many bone-numbing morns to collect 

 these Uttle creatures? Because to me they 

 were objects of intrigue: I hoped that they 

 might help me solve one of the great mys- 

 teries of reptiUan reproduction. 



All these unspectacular skinks may 

 look very similar, but they include several 

 species that are biologically very different. 

 About half the Brindabella hzard species 

 reproduce by laying eggs (oviparity); the 

 other half, by giving birth to fully formed 

 babies (viviparity). Those that lay eggs 

 range from the elegant little elf skink, 

 which Uves under cool, moist logs, to the 

 larger, three-lined skink, which basks in 

 the open and rapidly sprints between snow 

 grass tussocks when approached. The hve 

 bearers are all active in the daytime and 

 are generally larger than the egg layers. 



Among them are two varieties of heavyset 

 water skinks, confident lizards that are 

 undisputed owners of large logs on the for- 

 est floor. As I approached, they would fix 

 me with a balefiil glare; but reluctant to 

 move out of the warm sunlight, they were 

 easily caught with a noose of fishing line 

 at the end of a rod. 



In all skinks, egg layers as well as hve 

 bearers, the females ovulate their large, 

 yolky follicles in late springtime (Novem- 

 ber in Australia). These are immediately 

 fertilized internally by sperm that they 

 have stored either for a week or two (in 

 spring-mating species) or throughout the 

 entu-e winter (in autumn-mating species). 

 Eggs are laid in a moist, protected site 

 under a log or rock. In hve-bearing spe- 

 cies, the females retain membranous eggs 

 without shells inside their oviducts. 



The soft, leathery shells of lizard eggs 

 are much more permeable to water than 

 are the brittle eggs of birds; thus, egg lay- 

 ers need to deposit their eggs in moist en- 

 vironments. Because of this water uptake, 

 hzard eggs swell to twice their initial size 

 as they develop. Since both types of eggs 

 absorb water as the embryos grow, preg- 

 nant females of live-bearing species are 

 grossly distended by the end of the gesta- 

 tion period. 



By getting several pregnant females to 

 run along miniature "lizard raceti-acks" in 

 the laboratory, I was able to show that they 

 are much slower runners than their non- 

 pregnant counterparts, especially when 

 close to birthing time. In laboratory trials, 

 small, venomous white-lipped snakes — a 

 common predator of the Brindabella 

 skinks — were much better at catching 

 pregnant hzards than nonpregnant ones. 

 While these tests appear to confirm the 

 self-evident (ask any pregnant woman 

 how the last few weeks of pregnancy af- 

 fect her mobility), they were the first of 

 their kind. (Charles Darwin said he "loved 

 a fool's experiment," because it is remark- 

 able how often the "obvious truth" turns 

 out to be wrong.) 



I am continually amazed that lizards so 

 similar in other ways — size, shape, color, 

 diet, and general behavior — can differ so 



profoundly in the way they bring forth 

 their young. When viviparous species 

 omit an external egg stage, they also sub- 

 stantially increase the length of time dur- 

 ing which a female must carry her devel- 

 oping young. 



She pays a heavy price for this burden. 

 Not only is she slower in outrunning 

 predators and capturing prey, but her store 

 of fat may be so reduced as to impair her 

 reproductive ability the following year. 

 Why should both egg laying and live bear- 

 ing occur in otherwise similar species? 

 My curiosity was piqued about the pos- 

 sible adaptive advantages of hve bearing. 



34 Natural History 1/94 



