Reproduction is of central importance to most animals, and they have evolved many fascinating strategies to enable it and increase their chances of success.
This might involve eye-catching ornamentation and displays to entice the other sex, such as in peacocks, weapons such as horns and tusks to fight off the competition, or reproductive strategies like promiscuity or harems.
Why do some animals have the ability to change sex?
Some animals have a more extreme way of making sure they pass on their genes efficiency - by changing their sex. There are a few reasons why changing from male to female or vice versa might be advantageous, and this depends a lot on the physical and social environment of a particular species.
For example, if a dominant male or female dies it might be useful for another to change sex to take their place, or it might benefit an animal to be male at one stage of development and female at another due to their size.
10 Animals that can change sex
Read on to learn why these fascinating animals have evolved the ability to change their sex.
Clownfish (Amphiprioninae subfamily)
I’m afraid to have to tell you that the film Finding Nemo is not quite biologically accurate. This is because clownfish are sequential hermaphrodites - they start out life as male then later change sex to become female. These fish live in hierarchical groups within an anemone, headed by a large dominant female.
If the female is removed from the group then the largest male will become female and take her place. Thus, in real life Nemo’s dad would become female (and might have even mated with his son…). You can perhaps see why this isn't in the film.
Bearded dragons
As with mammals, the biological sex of these popular terrarium pets is primarily determined by the combination of chromosomes inherited from their parents. This sets them apart from a lot of reptiles, which don't have sex-determining chromosomes and instead, their sex is decided by the temperature at which their egg is incubated.
They develop into males at certain temperatures and females at others. Bearded dragons are special in that they can do a bit of both. As well as the two sex chromosomes they also have a temperature override that kicks in when conditions are harsh (hot and dry) and creates more females.
When temperatures creep above 32 degrees Celsius a sex reversal happens in which those with the typical male chromosome will instead develop into females. The exact reason for this female-producing strategy is unknown, but it is thought that it could aid reproductive output when times are tough.
Humpy Shrimp (Pandalus goniurus)
Named after the hump along its abdomen, this shrimp also starts off as male and then becomes female, but this is their norm rather than a conditional strategy. This type of sequential hermaphrodism (male then reproducing as female) is called protandry and is more common than the opposite direction (protogyny).
The benefit of this reproductive strategy is thought to be down to the fact that when you are small it can be better to be male, and when you are larger (and older) it pays off being female. This is because eggs are much bigger than sperm, so it is better to be bigger if you want to produce lots of eggs.
This is known as the size-advantage model. This particular species is a male for the first year of its life, becomes female in the second year and in the same year lays up to 3000 eggs and dies. What a life.
Bluehead wrasse
Although protandry is thought to be more common overall, in fish protogyny is more frequently found. One example is the bluehead wrasse, a native of the tropical western Atlantic Ocean. In this species, young females can permanently turn into males. This change happens quickly, with mature ovaries becoming sperm-producing testes in just 8-10 days.
They can then mate with females and father offspring. Bluehead wrasse typically live in harems - social groups made up of one dominant male and many females. Sex change occurs in response to the loss of a dominant male - the largest female in the group will step into his spot to maintain the hierarchy and the breeding output.
Green frogs (Rana clamitans)
As with the bearded dragon, the sex of this species can be determined by a combination of both genetics and the environment they find themself in. These frogs have been found to develop the body of their genetic sex as tadpoles and then reverse under particular conditions, becoming opposite-sex adults.
It was initially thought that this happened in response to human pollution, as female-skewed populations were found in more contaminated waters. However, it has since been found to be a natural phenomenon, with sex reversal also occurring in unpolluted ponds. This shows us that fogs can adjust their sex pretty flexibly based on local circumstances - a useful ability! This frog is definitely worthy of a place on our weirdest frogs in the world list
Yesso scallop (Mizuhopecten yessoensis)
Another very flexible species when it comes to their sex is the Yesso scallop. This bivalve mollusk can change its sex in response to their local social environment. Like many sequential hermaphrodites they are male when younger and smaller, then become female to gain a size advantage.
However, the timing of this is not uniform, it also depends on the make-up of the scallop population they are in. For example, they change their sex earlier when there are more small males around (for them to reproduce with when they are female). In this way they can really get the very best advantages out of changing sex.
Starlet cushion star (Asterina gibbosa)
This cute little sea star is yet another species that starts out male then makes a switch when they have grown bigger. In this species scientists have been able to measure when this change occurs pretty precisely - when their arm length is between 9 and 16 mm. This is compelling evidence for the size advantage model, showing that being female becomes beneficial only when they have reached a certain size.
Pacific oyster (Magallana gigas)
These oysters can display a variety of sexual strategies - some remain the same sex all their lives, some change sex once or more, and a small number are simultaneous hermaphrodites, being both sexes at once. Both protandry (male to female) and protogyny (female to male) are found in this species, with the former occurring more when food is in high abundance.
A recent study has also found that sex change occurs less frequently in older oysters. Like many of the animals on this list sex is determined by a combination of environmental, age and size factors.
Swamp eel (Monopterus albus)
Swamp eels, a family of freshwater fishes from the tropics, are an example of animals that start out female then become male. In their case, all hatchlings start life as females, then as they mature some individuals will change to become male. The transformation is fairly slow, taking as much as 30 weeks to complete. During this time there is an ‘intersex’ period in which they have both male and female gonads at the same time. The change is not always permanent however - some males turn back into females again when there is a shortage of females in the population.
Star tunicate (Botryllus schlosseri)
This fascinating species is one you likely haven’t heard of. It is a colonial tunicate, and as such is not just one animal, but a whole colony of hundreds or thousands of tiny animals living as one! Each little unit is known as a zooid, and they are all clones of each other, so genetically identical.
They are very rare in that they can reproduce both asexually (just one parent is needed) and sexually. The term sex change is less applicable here than for the other species in this list, as the individual zooids dont change. It is still considered a sequential hermaphrodite however, as in this species the female phase of reproduction happens first, with the colony releasing eggs into the water, then two days later is the peak of sperm emission (the male phase).
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