Viviparity In Early Adulthood

Why is it that reptiles like lizards and snakes are capable of laying eggs as well as performing live births but birds can only lay eggs? As with most biological phenomena the mystery of why birds do not give birth to live young can be explained once again by Darwin and his theory of evolution. As per the norm with evolution this was a matter of life or death, survival of the fittest and adapting to changing environments over millions of years and many generations.
Viviparity and oviparity. What?
Hundreds of years ago the great Greek philosopher and scientist, Aristotle, first described the terms viviparous (to give birth to live young) and oviparous (egg-laying) animals. It is a popular misconception that viviparity is restricted to only
mammals; viviparity is also present in the reptiles of the order Squamata as well as other animals. Squamates are a rather recent order of reptiles, consisting of snakes and lizards, which evolved from being oviparous to being viviparous. The fact that there is such a large variety of viviparous animal types that are so unrelated (eg. a shark and an ape) makes one wonder: What evolutionary pressures where responsible for viviparity and oviparity?
Why are some squamates viviparous but not birds?
Viviparity has evolved over one hundred times in vertebrates, most of which have occurred in squamates. This convergent evolution for viviparity was a result to both biotic and abiotic conditions that favoured the retention of the eggs within the squamates’ oviduct. The retention periods got longer and longer over time and the eggs were layed at later stages of embryonic development until the eggshells were successively thinned and/or eventually eliminated. This is supported by the fact that there are some egg-laying squamates that still lay eggs at different stages of development and that there are also some species that retain the eggs until birth. In addition to this there were other evolutionary adaptations that developed for the metabolic necessities of the embryo. These adaptations include increased blood supply to the oviduct, increased surface area of the extra-embryonic membrane and a higher affinity for oxygen-binding to the foetal blood. These adaptations are all to aid respiration of the embryo while it is in the oviduct.
The abiotic factors that drove the evolution of squamates were climate and safety.
Scientists have done research and consulted with fossil evidence and found a correlation between past climates and the convergent evolutionary move toward viviparity. Viviparity and egg retention served a thermoregulatory process; a way for the squamates to maintain optimal embryo development temperatures because laying the eggs would have left them exposed to unfavourable temperatures. If the embryo of an animal is not developing at the correct temperature their development could take longer, negatively affecting the mother or the embryo could die. Squamates are cold blooded and need to find warmth, for example by bathing in the sun they are able to maintain the temperatures at which their bodies are at. This is but one of the reasons that viviparity evolved. Another reason for the evolution of viviparity is for the protection and safety of the eggs to ensure succession of offspring. By laying eggs as opposed to retaining them they are not only exposed to the elements but also to possible predation. Viviparity allows the mother to save energy in terms of protecting a clutch of eggs from predators; the eggs are also safe from microbial attack, drying out and direct UV
exposure.

Although we have looked at the obvious advantages of viviparity we cannot assume that these advantages to apply to all vertebrate animals. This therefore takes us to the reason why birds did not evolve viviparity, the advantages of laying eggs. Birds have other ways of ensuring the safety of their eggs such as parental care; bird eggs are brooded by both or one of the parents and are therefore constantly thermoregulated as well as guarded. A brilliant example of parental care is the emperor penguin of Antarctica; the female will lay the egg and leave the male to keep it safe and warm for two months while she goes to feed. Paired with parental care the eggs are also sometimes equipped with camouflage and layed in well-engineered nests that are high up in trees which are out of reach from many predators. The embryo is protected from drying out by the eggshell and layers of shell membranes and is safe from microbial attack due to the albumen (egg white) within the egg.
By looking at the life history of an animal it is safe to say that it would only evolve certain traits if the benefits of said traits were greater than the costs. In the case of birds we can see that they never evolved egg retention and viviparity because it was not beneficial to them. If birds had evolved viviparity their ability to fly and feed would be hampered slightly as well more energetically costly to carry around the extra weight of the eggs. This would not only pose a threat to the mother’s survival but also to the survival of her offspring.

The answer is almost always evolution, especially in this case. Some species of snakes and lizards can give birth to live young and some can lay eggs; the reason for this is simply the necessity for survival over millions of years. Birds have had no need, even since the time of the dinosaurs (yes, birds are relatives of dinosaurs), to evolve viviparity because the way they are reproducing has had no negative outcomes in terms of their reproductive output.