Reproductive Cycles of Animals

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  • 0:01 The Circle of Life
  • 0:41 Two Cells, One Life
  • 1:33 Growing Up
  • 2:39 Meiosis
  • 3:18 Adults
  • 4:09 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

While there is a full range of animal life on this planet, the truth is that it all follows the same basic cycle: it is born, it reproduces, and it dies. In this lesson, we'll focus on the reproductive cycles, from sperm and eggs on through adulthood.

The Circle of Life

I hate to burst your bubble, but everything lives and everything dies. I know it's a really dark way to start a lesson but in doing so, I want to point out something else - every living organism seeks to pass on something to the next generation.

No, I'm not saying that earthworms create trust funds or that oak trees have rich uncles. Instead, I'm saying that one of the major goals of every generation is to pass on a better genetic code than it had to the next generation. In this lesson, we're going to look at the primary means through which that genetic code is passed on to the next generation within animals, the reproductive cycle.

Two Cells, One Life

For all the diversity of the animal kingdom, life follows a pretty basic plan. For more highly-evolved organisms, especially including practically all vertebrates, every creature begins with just two cells, neither of which actually originated from them. Instead, one parent contributes a sperm cell while another contributes an egg. These two cells bond to create a zygote in a process known as fertilization. From this point on, the zygote splits into multiple cells and becomes an embryo.

Obviously, there are some differences between different animals. For example, a human embryo does not develop the same features as that of a duck, nor does it develop within the confines of a hard shell. However, the same general cycle of development from zygote to embryo to young occurs until the developed young emerges into the world.

Growing Up

From the minute that an animal is born, it begins to mature towards the eventual goal of being able to reproduce. Obviously, newborn animals don't have this ability. Instead, they are juveniles. This period of time differs greatly between animals. Some mammals have a juvenile period of only a few months, while others require years. Ultimately, all animals transition from a juvenile state to a point at which they can reproduce. This is particularly obvious in frogs. As tadpoles, frogs are unable to reproduce. However, once they have transformed into adult frogs, they are able to make new tadpoles.

In humans, we call this change puberty. I'm going to make the assumption that you know all the details that go with puberty, but if you step back and look at them, you'll see that it's really when certain characteristics become more apparent. In short, testosterone starts to play a bigger role in the body chemistry of men, while estrogen plays a bigger role in the body chemistry of women. This is all building toward humans being able to reproduce.


Let's step back for a moment and look at the cellular difference between egg and sperm cells, and every other cell in the body. Every other cell in the body reproduces through a process called mitosis. In short, a cell splits and creates two, hopefully identical, versions of itself.

But that doesn't happen when sperm and egg cells reproduce. Instead, through a process called meiosis, each sperm and egg cell only gets half of the genetic material necessary to create another being. This material will combine with the genetic material of the mating cell to create a zygote with a mix of both parents' genetic codes.

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