Reproduction in animals

It’s spring, and Molly and Paul are in the forest with their newborn baby. It seems as if all the animals around them have recently had offspring too! There are chirping baby birds in the nest up that tree, and frogspawn in the nearby lake. The biological process of animals having offspring is reproduction. All living things reproduce, but they do so differently.

Let’s find out how animals reproduce. There are two main ways in which animals reproduce. The most common way for animals to have offspring, is for the sex cells — gametes — of two different individuals to combine. This type of reproduction, where two sex cells combine to create a fertilised cell that will develop into a new organism, is called sexual reproduction. Sex cells carried by female animals are called egg cells, and those carried by male animals sperm cells.

Gametes contain unique combinations of genetic material. So when the two sex cells fuse to create a fertilised cell – a zygote – its genetic material will be unique. But how this process of fertilisation happens, varies for different animals. For most frog species, for example, fertilisation happens outside the body – externally. A female frog lays eggs into the water, and a male frog releases semen containing sperm over the eggs, fertilising them.

Most fish species reproduce with external fertilisation too. But not all of them! For sharks to reproduce, fertilisation needs to happen inside the body. For birds, mammals, and insects, fertilisation also happens internally. Not just fertilisation, but the way offspring is delivered varies between types of animals too!

In birds, for example, fertilisation happens internally. But then, birds lay eggs, so development of chicks happens primarily outside of the bird parent’s body. Most mammals, on the other hand, grow and develop inside the body — in the womb. So sexual reproduction results in offspring with unique genetic material. But some animals can reproduce in another way too!

These animals can reproduce by creating a new organism that is genetically identical to them, and for that only one individual is needed. How does that work? This is a hydra, an invertebrate animal that lives in the sea. Here, a section of it, a bud, is breaking off — this section will grow into a whole new hydra! The new hydra will have identical genetic material to the parent, because really, it’s just a fragment of its parent that developed into a new organism.

Reproduction that produces new organisms that are identical to the parent animal is asexual reproduction. Some sea stars can reproduce asexually too — for example, if an arm or part of the middle of a sea star becomes detached from the organism, it can develop into a whole new sea star! But asexual reproduction can also occur when sex cells are involved, specifically egg cells — so what makes it different from sexual reproduction? The answer — there’s no sperm involved! The egg doesn’t need to be fertilised in order to develop into a new organism.

And the new organism will be genetically identical to the one that produced the egg. This type of asexual reproduction most often occurs in species of insects, such as stick insects or some ants. In summary, without sexual reproduction, unique genetic material would form at a much slower rate, only through occasional mutations. And without asexual reproduction, some species wouldn’t be as numerous as they are. Reproduction is a complex, yet extremely important, process.

It ensures diversity of species on Earth, and all organisms do it.