Why did the chicken cross the road?
-To get to the other side.
As we move to the Kingdom of Animals, we will see that there are two main types of life cycles: animals that hatch from eggs and animals that are born live. As we saw on our first tour of the Kingdoms, many creatures are born from eggs, including amphibians, birds, insects, and most fish and reptiles (not to mention a couple of unusual mammals). But do all of these creatures that hatch from eggs have the same life cycle?
When these creatures reproduce, the females lay eggs. All animals (except a few strange invertebrates) must have a mother and a father, so if the egg is not fertilized, it does not become a new animal. (Most chicken eggs that you eat for breakfast are unfertilized because they keep the (female) hens away from the (male) roosters.) For fish and amphibians, the eggs are usually unfertilized (the males have not yet added their part so it can grow into a new creature); birds and reptiles lay fertilized eggs.
Once the egg is fertilized, the new baby grows for a while inside the egg, often for a couple of weeks, although penguins and many reptiles stay in their eggs for more than two months. In that time some eggs are left alone (such as most fish, amphibian, and insect eggs), while others are carefully tended and guarded (most birds and many reptiles are attentive parents). Eggs might be laid among rocks or on the top of ponds, buried under the ground or settled into a meticulously constructed nest.
Regardless of how it was cared for (or not), the day will come when the baby animal, which has been growing and developing inside, must face to the world. If it’s a bird, though, it is up for a big challenge – it’s not easy to get out of those eggs! Birds grow a special spike on the end of their beak called an egg tooth to help them get through that strong, protective shell, and it can still take them a couple of days.
Once the creature has finally pushed their way out and into the sunlight, what emerges might look quite similar to or very different from its parents. How can they look different when a frog always has frog babies and a robin always have robin babies? Some animals, such as amphibians and some insects, undergo a process known as metamorphosis, a dramatic change in a creature’s body as they grow. Amphibians, as you may remember, hatch from eggs laid in or near water and look like tiny fish-like creatures. Over time, they develop legs, lose their tails (if they are frogs or toads), start living on land (usually), and eventually find a mate to have babies of their own.
Similarly, some of our favorite insects, such as butterflies and moths, begin life as wriggly, worm-like caterpillars; this is a time in their life known as the larva stage. These young creatures spend their days eating, eating, and eating some more as they continue to grow quickly and get ready for their metamorphosis, a phase known as the pupa stage. The creature might spin a cocoon (like a moth) or transform its body into a chrysalis (like a butterfly). For several dark days, the creature stays inside, growing and changing to emerge as its adult version – no longer a creeping caterpillar but a beautiful butterfly. Now that the creature is an adult, it can have offspring itself.
Of course, many creatures that are born from eggs do not go through metamorphosis. Although the baby house finches that lived in our eaves were quite ugly, when their feathers grew in after just a few weeks they looked much like their parents. Reptiles, too, look like small versions of their parents when they are first born. Even some insects look like tiny versions of their adult selves, such as the praying mantis. Like all newborn animals, these creatures spend most of their time eating and growing so that they can quickly become adults. Then the cycle of life can begin again with the egg (or perhaps the chicken?).