Photographs by Richard Shine 



The oviparous skink Saproscincus mustelinum with her eggs, right, and 

 the viviparous species Pseudemoia entrecasteauxii, below, 

 are abundant in many parts of the Australian high country. 



Mell, Claire Weekes, and A. M. Sergeev) 

 who had studied reptiles in three different 

 countries (China, Australia, Russia) and 

 published their results independently in 

 three different languages (German, Eng- 

 lish, French). All had noticed that vivipar- 

 ity was more common in cold-climate rep- 

 tiles, and each suggested that embryos 

 developed inside the female would have 

 better odds of surviving the cold. 



Their explanation was that the pregnant 

 female's warmth insures that the young 

 develop not only more safely but also 

 much more quickly. Whereas cold soil ex- 

 poses eggs to dangerously low tempera- 

 tures and the embryos develop slowly, 

 eggs kept inside the female's body are 

 warmed whenever she basks in the sun- 

 light. Even when air and soil temperatures 

 are close to freezing, many reptiles can 

 keep their own body temperatures at about 

 85° F by judicious basking. 



This idea is supported by the timing of 

 Uzard embryo development. In the Brind- 

 abellas, embryos of egg layers may spend 

 as much as 50 percent of their develop- 

 ment time in the mother's oviducts — 

 about a third longer than in warmer habi- 

 tats. After fertilization, oviparous females 

 deposit a thick, calcareous shell around 

 their eggs, and lay them from forty to sixty 

 days later. Eggs laid during the AustraUan 

 midsummer (December-January) in the 

 mountains, where soil temperatures are 

 low, develop slowly, and may hatch late in 

 autumn (March-April). By contrast, vivip- 

 arous females can keep their babies much 

 warmer, so they develop more quickly. 

 Live-bearing females usually give birth to 

 living young in late summer (February) or 

 early autumn (March), at least a month be- 

 fore the eggs of their oviparous cousins 

 will hatch. 



This head start for the young may be the 

 most important advantage of viviparity. 

 Babies bom early have more time to grow 

 before the onset of winter and more time 

 to locate safe hibernation sites where they 

 are less likely to freeze. Young lizards that 

 emerge earUer can set up and defend terri- 

 tories against later arrivals. Also, being 

 kept warm during incubation may result in 



the young being somehow "better" — per- 

 haps larger or smaller or fatter or thinner 

 or quicker or smarter. Fitness may involve 

 all of these attributes at various times or at 

 different stages of the life cycle — and it is 

 always relative to the environment. Many 

 aspects of an individual reptile's hfe are 

 determined by the temperatures it experi- 

 ences while still in the egg. Incubation 

 temperatures can affect the animal's size, 

 shape, color, basking behavior, agiUty, and 

 strength. In all crocodiUans, many turtles, 

 and some lizards, incubation temperatures 

 even determine the sex of the individual. 



The lizards that I found during those 

 frosty Brindabella mornings helped to 

 confirm some of the earlier investigators' 

 ideas. Pregnant females were slowed 

 down by their babies, making them easier 

 for predatory snakes to capture. Embryos 

 that remained inside viviparous females 

 were kept warmer, and did develop much 

 faster than did eggs laid in natural nests 

 under rocks and logs. Overall, this warm- 

 ing reduces the total incubation period by 

 about one month. Without this accelerated 

 development, eggs of most of the ovipa- 

 rous species would not have enough time 

 to hatch before the onset of winters (at 

 least in cooler years), and thus would be 

 killed by freezing. Short summers may be 

 the reason that so few species of oviparous 

 reptiles reproduce successfully in very 

 cold areas, where soU temperatures are fa- 



vorable for only a brief period each year. 



The data from the Brindabella skinks 

 also supported my hunch that retaining 

 eggs inside the female's body might di- 

 rectly influence the quality of the hatch- 

 lings. I checked this possibility by testing 

 the development of eggs laid in captivity 

 by oviparous skinks. I incubated some at 

 normal (soil) temperatures, and others 

 from the same clutch at hotter tempera- 

 tures — simulating the warmth of a basking 

 mother's oviducts. Compared with their 

 siblings from cool-temperature incuba- 

 tion, the artificial viviparous babies were 

 shorter, fatter, and generally less active, 

 but were much faster runners when tested 

 on my lizard racetracks. They also devel- 

 oped more quickly and hatched earlier. I 

 carried out a similar experiment with one 

 of the Uve-bearing species by giving some 

 pregnant females access to more basking 

 time, and again found that the higher tem- 

 peratures affected the shape and behavior 

 of the newborn Uzard. 



I still don't know if these characteristics 

 of artificially warmed babies would help 

 them survive any better or grow any faster 

 in the wild. By marking and releasing lab- 

 incubated young of both types and then re- 

 capturing them later, I hope to learn more. 

 Meanwhile, my hopes of sleeping until 

 midmoming have faded away, and I am 

 resigned to enduring more dawn laughter 

 from the Brindabella kookaburras. D 



38 Natural History 1/94 



