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To Egg or Not to Egg
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Baby Gila Monsters, baby Banded Geckos, and baby Pythons - all hatch from eggs. Female Chameleons, female Ocellated Skinks, and female Boa Constrictors - all carry their babies in their oviducts for their entire gestation periods. Why are some reptiles egg layers and other reptiles live bearers? Richard Shine's article "Young Lizards Can Be Bearable" that appeared in Natural History Magazine has proposed an answer to this curiosity.
This egg vs. live birth question first spurred Shine's curiosity when he found that half of all Brindabella lizards lay eggs. The other half give birth to fully formed babies. Shine did further research and found:
1. Egg laying and live birth can occur in reptile species that are evolutionarily very similar. (Boas and Pythons)
2. Egg laying and live birth can occur even within the same species. (Bougainville's Skink)
3. Reptile egg layers have evolved into live bearers in at least 100 different origins. (The differences in reproductive methods have been proven to be genetic and not short term adaptations. It is believed that live birth is an advanced trait.)
4. In all cases of reptilian live birth evolution, there is a climatical pattern.
Shine has found, through his research, that the reptilian live birth evolutionary step has always occurred in relatively colder climates.
Why do reptiles evolve into live bearers in colder climates much more frequently than in warmer climates? Shine's studies have produced a common sense answer to this question: Female reptiles carry their babies throughout the gestation period in colder climates to give them a better chance of survival.
The following illustrates how live birth is advantageous to reptile babies:
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LIVE BIRTH CONS |
LIVE BIRTH PROS |
| 1. Live bearing females are slower in hunting their food. | 1. Babies receive better incubation. |
| 2. Live bearing females are slower in escaping predators. | 2. Babies are protected by their mother while they are inside her. |
| 3. Live bearing females lose more fat than egg laying females. They may not become fertile again for a longer period of time than if they laid eggs. | 3. Newborn babies are healthier than newly hatched babies. |
| 4. Live birth babies develop quicker. | |
| 5. Live birth babies are born earlier than those that hatch from eggs. | |
| 6. Earlier born babies find the best homes and feeding areas before the other babies have hatched. |
There exists an interesting side note to Shine's findings on the egg vs. non-egg question: The warm - cold climatical determination for live birth evolution has not been proven in the class of mammals and it does exist in birds, fish, or even amphibians.
Despite the inconsistencies of live birth evolution in other animal classes, it is clear that reptiles who thrive in cooler climates have evolved into live bearers in order to have healthier offspring.
