The Evolution of Eggs: PT. 2

Egg Designs – Part Two.

Last month, we discussed the arrival of the first amphibians and the story of how they conquered the evolutionary hurdle of beginning to breed on land. This signalled a major step towards life as we know it today.

This cataclysmic event resulted in huge levels of diversification, creating a large part of the animal kingdom.

In part two, we are going to look at how egg shape protects the young, why bird eggs are different colours and what reasons lie behind the fact that some reptile eggs are hard-shelled and others are soft and leathery.

Part Two

As discussed in part one, the whole evolution of eggs was centred on improving the survival rate of offspring. The intent was to remove them from predators and protect them from drying out in the hot sun. Yet today the land isn’t as desirable as it was once.

That’s mainly because now it’s filled with predators that weren’t present when the first amphibians made their leap onto land. Yet, as a result, the arms race of “predators versus parents” continues on land.

Egg shape has fallen victim to this eternal battle.

Significant differences can be seen in the egg shapes of various species. Rounded eggs are generally laid by birds that nest in holes, where rolling isn’t an issue. Additionally, their shape can prevent long-billed birds from getting a firm hold on the eggs within the hole. Conversely, birds that nest on steep cliffs, such as guillemots, have sharply tapered eggs, which roll in tight circles. This prevents them from falling off the steep cliffs on which they are laid.

In this same vein, the locations where eggs are laid can be represented in their colour. For example, white eggs are most likely to be found among birds that guard their eggs. Ducks and geese are good examples. They tend to hide their eggs within their own feathers or bury them under vegetation while standing guard. This means that any colouration would be redundant, if not downright contradictory to their survival strategy.

On the other hand, birds whose strategy is to lead predators away from the nest tend to have patterned eggs. The patterns camouflage the eggs by breaking up the line of sight. Cuckoos generally lay eggs which are the same colours as the species that raised them. Consequently, to lay their eggs, they must find a nest of the same species that raised them.

Otherwise, they risk detection by having different egg coloration.

 

 

From these examples, we can see a pattern.

Egg strategies have emerged which suit the preferences and needs of their own particular ecology. Colour and patterning can camouflage or fool; shape can save them and halt the progress of predators.

Egg structure can also reveal the environment that the egg inhabits. Commonly soft-shelled eggs are better equipped to handle humid environments, as they have the ability to absorb moisture from the area in which they are laid. Yet, shelled eggs tend to not be particularly absorbent.

Generally, they are laid in dry areas, which often requires a sealed environment that contains its own moisture. Note that this is not a steadfast rule. Some species, such as the Central American turtle (Dermatemys mawii), lay hard-shelled eggs to prevent them from soaking up too much water.

However, this is the exception that proves the rule. Almost all eggs have evolved to manage the conditions.

Some eggs preserve vital moisture in a sun-baked desert. Others prevent overlogging in boggy and sodden marshes. The eggs of nesting species are often hard-shelled to provide protection from the inquisitive pecking of birds, insects and other predators. Thus, we can conclude that since the arrival of eggs on land, an arms race has continuously escalated. Colouration, shape, thickness and texture have all changed and diversified, with the goal of allowing the young a greater chance of survival in an increasingly hostile world.