The Unbelievable Secrets of Sex-Changing Animals

There are many different animals that sex change on a regular basis, so how and why does that happen?
Trevor English

Sex change is a pretty self-explanatory concept. It is when an adult animal changes from one sex to another. But there are some interesting secrets behind the sex-changing capabilities in the animal kingdom.

Note: Commonly the term is used to refer to the procedure of humans changing sex, but for the purpose of this article, we'll focus on non-human sex change. 

Sex Change in Animals

Take, for example, the common clownfish. These lovable orange and white fish are well-known in pop-culture in large part because of the movie Finding Nemo, but they also can change sex. 


These fish are protandrous, which means that they contain both male and female sex organs, but the male reproductive organs come to maturity before the female. They live in schools made up of a large number of males, and one breeding pair consisting of a dominant male and a female. The rest of the school are not sexual and tend to be smaller. If the female of the breeding pair dies, the dominant male will generally become female and chose a new breeding partner from among the remaining males, usually the largest and most aggressive one.

In addition to the clownfish, there are actually many other fish that can naturally change sex. These include wrasses, moray eels, and gobies. 

Natural sex change, going in either direction, has even been reported in certain types of corals. Usually sex change in animals is due to necessity, from changes to environmental or social conditions.

One animal you might be surprised can sex change are chickens, which occasionally switch genders from female to male. Normally, female chickens have just one functional ovary, on their left side. Two sex organs are present during the chickens' embryonic stage, but the one on the right is a gonad that has yet to be defined as an ovary, a testis, or both. This left-side organ is usually dormant inside of the chicken. 

However, certain medical conditions—such as an ovarian cyst, a tumor, or a diseased adrenal gland—can cause a chicken's left ovary to regress. In the absence of a functional left ovary, the dormant right sex organ may then begin to grow, according to Mike Hulet, an associate professor at Penn State University's department of poultry science.

The Unbelievable Secrets of Sex-Changing Animals
Source: Klimkin/Pixabay

If that right gonad turns out to be a testis, it will start secreting male hormones called androgens. These are homones that are largely responsible for male characteristics. The production of androgen causes the hen to act more like a rooster

Even with all of that, the chicken does not change into a rooster, rather it just makes the bird phenotypically male, meaning that although she looks physically male, she remains genetically female

Secrets of How it Works

While scientists and researchers have been aware of sex-changing animals for some time, the biological process of how it works in different animals has only been worked out more recently.

For example, New Zealand scientists have recently discovered the exact biological processes that certain fish use to change sex, published in the journal, Science Advances.

La Trobe University geneticist and winner of the 2017 Prime Minister's Prize for Science, Professor Jenny Graves, helped lead the study.

"I've followed the bluehead wrasse for years because sex change is so quick and is triggered by a visual cue," Professor Graves said.

"How sex can reverse so spectacularly has been a mystery for decades. The genes haven't changed, so it must be the signals that turn them off and on."

The wrasses that the team studied live in reefs in the Caribbean. They usually live in groups with a single dominant male and a number of females. If for some reason the dominant male is removed from the group, the largest female wrasse becomes male in only 10 days. In fact, the researchers observed behavior change in just minutes, and a change in color in hours. 

By day 10, the female's ovary has turned into a testis and has started producing sperm.

The researchers were able to sequence the fish's RNA and discover which genes are turned off and on in the brain and gonad when the sex change is triggered.

Not only does this research help us better understand sex change in animals, it also helps us understand how genes are turned off and on during the development of animals and humans. 

"Genes needed to maintain the ovary are first turned off, and then a new genetic pathway is steadily turned on to promote testis formation," said Graves.

You can read more on the study here.

6 Animals You Might be Surprised can Sex Change

Moving on from scientific discovery and understanding of sex-changing animals, let's take a look at a few animals that you might be surprised (or not) to find can change sex.

6. Fish

Fish, as we've mentioned already in this article, make up the largest number of the sex-changing animals. Most prominent are clownfish and wrasses, but there are more than 500 species of fish that can sex change.

5. Corals

Many people do not realize that corals are actually animals, not plants. In 2008, it was discovered for the first time that mushroom corals can change sex from male to female, and female to male. As of yet, little research has been done to study the sex lives of corals and how or even why they make this change.

4. Slugs

Slugs are naturally hermaphrodites, always possessing both male and female reproductive organs, which can be in use at the same time.

One type of slug, the banana slug, even practices apophallation. For anyone with some knowledge of Latin, you might know where this is going. This species bites off their partners' penis following sex, possibly to prevent their partner from ever mating as a male again. Many people just think this is weird.

3. Frogs

The common reed frog has been observed to spontaneously change sex. Traditionally, this is a natural process in the frog, possibly in response to changes in the environment, but it has also recently been found that it can be caused by pesticide exposure.

2. Snakes

Some female snakes have been observed to 'miraculously' become pregnant when alone and kept in captivity. This is known as parthenogenesis, a form of asexual reproduction that may be a strategy for species continuation. In parthenogenesis, an embryo develops from an unfertilized egg cell, producing a clone of the parent.

This process has been observed in sharks and amphibians, but never naturally in mammals. 

1. Butterflies

Occasionally some types of animal can be born that are both male and female, a condition known as gynandromorphy, which means outwardly having both male and female characteristics. When this happens in butterflies, each wing is a different color, signaling male and female wings. Gyandromorphy is generally the result of an early mistake in cell division, when the sex chromosomes fail to separate, a process known as nondisjunction.

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