Eggs are a fascinating form of reproduction. Egg-laying can be found in EVERY group of animals in the world; fish, reptiles, insects, birds, and even mammals.
Eggs can be seen as a disadvantage; they have to be deposited and, in many cases, left whilst the parents find food to sustain themselves, leaving them vulnerable to predators or weather conditions. There are some truly exceptionally unusual eggs out there. Here's a closer look at some of them:
Walking along a beach, you may be lucky enough to find a 'mermaid purse.' These are shark egg cases; they've typically hatched by the time they wash up on the shore, but you'll notice they often have unusual shapes with long edges. This shape allows them to attach to rocks and seaweed on the sea floor. Some are even shaped like a corkscrew to ensure they get wedged somewhere safe.
The sand tiger shark retains its eggs inside, and the first baby that hatches will eat its siblings, so only one large baby is born.
The largest shark in the world - the whale shark - produces the largest eggs; some have been recorded to be 30cm long! That's twice the size of an ostrich egg, the largest terrestrial egg.
So that's the largest egg, but what about the smallest? The smallest eggs I can find are the eggs of Tachinidae (True flies). These eggs are usually around 0.02mm long!
While not all insects lay eggs, many do, and they have some of the most incredible diversity. Stick insects, quite famously, have eggs that look very much like their poo (see above), and therefore few predators locate them. Other stick insect eggs mimic seeds!
Lacewings produce a long stalk with the egg resting on the tip, keeping it out the way of other invertebrates who might want to eat them.
Butterflies lay their eggs on the food plant of the caterpillar, which gives them the best start in life. Butterfly eggs have many different shapes, patterns, and colours and are beautiful to hold.
Birds are the best-known egg-producing animal and the only class of animals that must lay eggs. In all other Classes' there are examples of both live birth and egg-laying. They are nearly always given high care unless you trick another bird into caring for your egg, as in the cuckoo case.
The poor kiwi from New Zealand lays an egg that's 20% of its body size! This allows for a larger and more developed chick. For comparison, a human baby takes up around 5% of its mother's body weight.
Most bird eggs use cryptic and mottled colours to camouflage them, but some can be pretty colourful, too; the Emu and Emu-wren have beautiful green shells that look stunning.
Who produces the most eggs? My first thought is termites - they have a giant queen, who is an egg-producing machine; she can lay up to 30,000 eggs daily for nearly 20 years! This equates to 165 million eggs in her lifetime.
That sounds a lot, but the ocean sunfish is the champion egg producer. In a single spawning, they can lay 300 million eggs! Producing millions of eggs is successful as it requires no parental care, and the sheer number of eggs can overwhelm any hungry predator.
Like the kiwi mentioned earlier, having a single egg has pros and cons. A single, well-developed offspring has a head start in life; less development must happen outside the egg but if a predator finds an opportunity, all the effort for that progeny is lost.
Temperature plays a large part in incubation. Ant eggs can take as little as a few days for the grub to develop and hatch; they will then get moved to a nursery as ants have a highly sophisticated level of care.
The longest incubation time goes to the deep-sea octopus; down in the freezing cold depths of the ocean, things can take a long time - 53 months (or around 4.5 years) for these eggs to hatch, which is exceptionally long for a cephalopod.
One last and quite important fact... who came first, the chicken or the egg?
The first hard-shelled egg was produced by proto-amphibians around 312 million years ago.
The first ever aquatic egg is up to a billion years old!
The first ever chicken, the junglefowl, diverged from their common ancestor a measly 4-6 million years ago!
So there we have it: the egg came first by a considerable margin!