There is an old riddle which we hear most frequently – “Which came first, the egg or the chicken?” There are varied thoughts and answers which can be given for this question, however it is relatively easier to answer the same in terms of evolution of birth in animals and as their adaptations for a successful reproduction. This complete article is centered around how nature has its own demarcation for various living organisms, sharing the same roof. With the advent of vertebrate life, nature procured only two ways for an individual mother to give birth. Either the mother lays an egg, where it can continue to grow before hatching, or she keeps the baby inside her until it proceeds emerging as a fully formed squirming newborn. In either of the cases, there lies a complete fundamental split.
Is there any primordial reason for this strict reproductive dichotomy between egg laying (oviparity) and live birth (viviparity)? Why did live birth evolve? These are few questions which have been explored recently while studying a remarkable lizard that can lay eggs and also give birth to live offsprings at the same time. The studies have effectively highlighted on enormous complexity and variability in mode of giving birth and in sexual reproduction.
Primitive Vs Advance strategies for birth:
Earlier the reproduction habit in female animals was found to lay eggs, where they released their ova, often thousands at a time. Subsequently, the sperms released by the male would then fertilize some of these eggs in a hit-or-miss fashion, and the resulting embryos took their chances on surviving in the hostile world until they hatched. Many organisms, especially the small and simpler ones, still reproduce this way.
But with increasing complexity in animals, preference of a less chancy strategy of internal fertilization became dominant. The phenomenon was evident specially in the vertebrate species, including many amphibians, reptiles and even some fishes, like sharks. By this phenomenon they could then ensure that a higher percentage of their eggs gets fertilized, and this additionally offers the female with a better male selective options. The embryo via internal fertilization ensures viability and develops safely inside its mother until she eventually releases it into a protective shell. It is yet not very clear as to when exactly the live birth evolved. However, it is known what forces may have been driving the transition from egg laying to live bearer and what evolutionary steps may have preceded it.
Both birth methods eventually have the same purpose i.e to maintain their population but they present contrasting advantages and disadvantages. When we look into the Egg Laying species, the female becomes physically free from their offsprings sooner. For example- Birds never evolved into a live bearer, possibly because they will have to spend a lot of energy in flight process while carrying a young individual inside them, which apparently is unsupportable. So, they never took this opportunity to make a transition from being egg layers to a live bearer. Egg bearers can also generally have more offspring in a single litter, since the size of the mother’s body isn’t a constraint. But this in turn risks the eggs of being exposed to their predators and environment dangers.
Live-bearing mothers, on the other hand, can house their embryos and protect them from predators and environmental dangers for longer period of time. Being pregnant exposes them to more predation and puts them at considerable risk from the embryo itself. As we all know that embryo acts like a foreign body inside a mother’s womb and makes the mother more prone to environmental dangers and health risks. For the length of her gestation, the mother balances on a tightrope, diverting resources to a foreign being, while keeping herself healthy. Therefore internal foetuses are more taxing for the mother than the egg bearers.
Who can give live births?
In a paper published in Nature in 2009, Chris L. Organ and his colleagues demonstrated that before a species could evolve live birth, it probably had to evolve the ability to determine the sex of its offspring genetically. Many animals still depend on environmental factors such as temperature to determine whether the embryo would develop into a male or a female. Lets consider an example for a better understanding- a sea turtle; they can barely walk on rugged land but they still cross their boundary and come to beaches to lay their eggs. You would be amazed with the beauty of how nature has finely tuned every creature with their own reproductive mechanisms. Sea turtles, if laid all their eggs in water, would be less likely to get a variety of males and females because the temperature gradient in water is much smaller than it is on land.
But once the marine species has evolved the ability to determine sex through genes, it no longer needs to venture onto land and can fully adapt to its aquatic life. As Organ and his co-authors wrote in their paper: “Freed from the need to move and nest on land, extreme physical adaptations to a pelagic lifestyle evolved in each group, such as the fluked tails, dorsal fins and wing-shaped limbs of ichthyosaurs [a group of prehistoric marine reptiles].”
Evolution in Reptiles: from Egg Laying to Live birth
For some, live birth and egg laying might seem as the only fundamental way of giving birth but for other animal species, an in between third option, which breaches the laws of nature exists. Alternatively, it creates a smooth transition between either of the two options. Some reptiles, for example, tailor their reproductive strategy depending on where they live.
Australian three-toed skink (Saiphos equalis), is a lizard with a remarkable distinction of being able to perform both procedures- lay eggs as well as give birth to live young. This species of lizard has attracted scientific attention due to its dual reproduction habit of producing both live young as well as laying eggs. The species which live in higher, colder regions bear live young, while those in warmer lowlands lay eggs because its difficult to maintain a warm ambience around eggs in colder conditions.
Evolutionary records shows that around hundreds of reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.
Oviparous to Viviparous: Posing a Nutritional constraint:
In case of mammals, embryo stays connected to the mother via a highly specialized vascular connection known as placenta. This placenta fulfils oxygen as well as a wide range of nutritional demand for the growing embryo and acts as a channel to exchange all the nitrogenous waste back to the mother. For vertebrate animals which lay eggs, the embryo gets most of its nutrition from the yolk and the calcium surrounding it in form of a protection via porous shell. This calcium is secreted by the special gland present in the mother’s oviduct.
The thickness of this shell plays a vital role in the survival of an embryo because it acts as a filter which selectively allows gaseous exchange. If the shell becomes too thick it becomes impervious for gaseous exchange and if it is too thin then it cannot withstand the extreme environmental conditions. Therefore a fine tuning in thickness of calcium deposition is the basic need. The major difference between oviparity and viviparity therefore centers on a strategic evolutionary decision about when the mother should deposit her embryos. If she deposits them early, she’s an egg layer, and if she deposits them late, she’s a live bearer. Many reptiles find this mechanism of reproduction as the simplest way to overcome the geographical challeges thrown by nature.
References:
- The Conversation
- Oh Baby! Which Animal Families Lay Eggs and Live Birth?
- Evolution of vertebrate viviparity and specializations for fetal nutrition
- PLOS MEDICINE
This is very informative.