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In today’s news, gender transformation has become a widely debated topic. While humans can undergo gender changes through medical interventions, it remains an unnatural process for us. However, fascinatingly, the animal kingdom is home to numerous species that possess the remarkable ability to change their gender naturally. Some of these creatures even boast both male and female reproductive organs.
Join us as we explore the diverse reproductive strategies and extraordinary adaptability found among simultaneous hermaphrodites and sequential hermaphrodites alike.
Why Can Some Animals Change Gender?
The ability to change gender is known as hermaphroditism and it’s something that is pretty common in the animal kingdom. While it might seem strange to us humans, it’s actually very beneficial and may even improve the chances of the animal’s survival.
Well, I’ll be honest, scientists have struggled to come up with a viable theory as to why evolution resulted in animals that could change gender. While there are several suggestions, a common one is having a size advantage as a specific gender which explains how hermaphroditism could enable the animal to survive.
What’s more, the ability to change gender is important when the reproductive value of one sex outweighs the other. For example, where the females are larger, it is beneficial as they’re able to lay more eggs, but on the other hand, in some species, being male is more beneficial as their superior size ensures they are better able to defend themselves.
In terms of reproduction, the ability to shift gender also means that animals have a greater chance of mating. Considering species where interactions are very limited, it may not be possible for any given individual to find a mate of the opposite gender, but hermaphroditism saves the day!
Types of Hermaphroditism
When a lot of people think about hermaphroditism, they may incorrectly assume that the phrase simply refers to animals that are both genders. While this is one type of hermaphroditism, it’s not the only one, and some creatures can switch between genders at will.
The first type of gender-changing ability is called sequential hermaphroditism. This refers to animals that are able to switch between genders either once or several times throughout their lives. In many species, this happens owing to the need for one gender or the other for mating purposes and it can be done both ways.
- Protandry – This first subtype occurs when an animal changes from male to female. The animal, usually a fish, begins life as a male and may then go on to reproduce in his male form, later switching and reproducing as a female, although there may be an overlap in this. Similarly, some animals can be born male and switch between being male and female several times for reproductive purposes.
- Protogyny – Secondly, we have protogynous hermaphrodites which are animals that are born with female reproductive parts. However, later in life, they’re able to make the switch and reproduce as a male. This is the more common of the two, and it’s thought that because males’ reproductive potential increases with age, it’s more advantageous this way.
- Bidirectional sex changers – There are some sequential hermaphrodites that are born with both male and female reproductive parts. But unlike simultaneous hermaphroditism, these organisms can only act as either a male or a female and not both at the same time. Essentially one set of organs remains dormant while the other functions.
In animals that have simultaneous hermaphroditism, both male and female reproductive organs are present. Both sets of organs are fully active and can produce male and female gametes. They can even do this within the same breeding season.
There may be some species of simultaneous hermaphrodites that are able to self-fertilize, but this isn’t true of all species.
Types of Hermaphrodite Animals
Not including insects, around 30% of all the animals in the world are hermaphrodites; I bet that’s much more than you thought. Many of these are fish, but there are also many other types, so let’s get better acquainted with some of them.
1. Clownfish (Amphiprioninae subfamily)
The clownfish was made famous in the movie Finding Nemo, but the fact that these little swimmers could change gender was never mentioned. The reality of life as a clownfish is pretty interesting, with these fish living in large groups made up of a large breeding pair and several smaller fish which are all male.
In the event that anything happens to the breeding female, the large male will transform into a female (this makes clownfish protandrous), and the next largest fish will increase in size and become the new ‘queen.’
An interesting fact about clownfish is not only can they change gender, but they’re immune to the sting of sea anemones. This is because they have a symbiotic relationship with these underwater creatures and provide them nutrients in the form of waste in return for somewhere to shelter.
2. Asian Sheepshead Wrasse (Semicossyphus reticulatus)
Found in waters around Korea, China, and Japan, the Asian sheepshead wrasse has a very distinct appearance. Both males and females have a bulbous forehead.
When these fish are born, they are always female, and it’s not until later in life that they develop their male sex organs. There was a famous example of this transformation being caught on camera in the BBC series Blue Planet ll in which a ten year old female sheepshead wrasse entered her lair and underwent her transformation.
Upon emerging, she (now he) was large enough to take on the current alpha of the group and fight for dominance. This means that the fish mated with the current largest male as a female and then took him now and continued to spread its DNA as a male.
3. Bluehead Wrasse (Thalassoma bifasciatum)
The bluehead wrasse lives in large groups of females with just one male at the helm. He’s the only one that has a blue head and this allows him to show off his dominance over his group. However, he’s not going to be around forever, and if he dies or leaves, a new male needs to fill his shoes.
Rather than a brand new male entering the harem, the largest female fish within the group begins her transformation. It takes just ten days for her ovaries to develop into testes, fully capable of producing sperm, and within just three weeks, she’s ready to take over leadership of the group.
Much of how this happens still remains a mystery but scientists believe that these social changes are influenced by the release of a stress hormone called cortisol.
4. Parrotfish (Scaridae family)
Parrotfish belong to a family of around 95 species, and while they’re all able to switch genders, some go from male to female while others go from female to male. Plus, they not only change gender, but they’ll also change color throughout their lifetime.
They take their name from their strong beak-like mouths, which they use to bite off pieces of rock and coral and then poop out the leftovers as sand!
With many species of parrotfish, individuals are always born female. They live in groups of one male to several females, but if anything happens to the male, one of the females begins her transition. She will go from a dull gray color to a much more brightly colored fish and, during her transition period may even be half male and half female, fertilizing the eggs of unsuspecting harems.
5. Slipper Snail (Crepidula spp.)
The slipper snail is a species of sea snail that is able to change from male to female. While the exact signaling method is unknown, scientists did notice in studies that individuals who were granted contact with others of the same species tended to undergo transformation more rapidly. This shows that as opposed to waterborne cues, some sort of physical interaction is necessary for the process to take place.
In the wild, groups of slipper snails are often found together, with older females at the bottom of a pile and younger males on the top. As more snails join the pile, the males gradually begin to transition into females. The reason for this formation is that these snails cannot move around, so grouping together like this ensures successful reproduction without the need to look for a mate. Since they all start life as males, turning into a female is the only viable way to ensure reproduction can take place.
The females at the bottom of the stack will eventually die off, and that’s when young males can transform and take over their role.
6. Hawkfish (Cirrhitidae family)
Another interesting species is the hawkfish, a tropical species found in the Pacific and Indian Oceans. They all start life as females and will live in harems protected by one dominant male. However, as they get older, these fish are able to switch gender to become male.
As males, hawkfish will initiate a courtship ritual around dusk and may flit between different perches and groups of females. Once the courtship is over, spawning occurs. It’s thought that environmental changes are one of the triggers for gender change. However, once a female turns into a male, she may be challenged by the current dominant and if she is defeated, she will then revert back to her previous gender.
Interestingly, hawkfish spawn their eggs at the surface of the water and, once they are fertilized, they’re carried off with the current, taking around three weeks to hatch.
7. Northern Shrimp (Pandalus borealis)
Found in the northern Atlantic and Pacific oceans, the northern shrimp is a common marine species. However, their status has not been assessed by the IUCN, although it’s thought that ocean warming could be having an impact on populations.
Northern shrimp are all born as males, and it takes around two years for them to reach sexual maturity. They’re then ready to mate with an older female who will store her fertilized eggs under her abdomen over the winter, ready to be released in the spring or summer.
It’s not until a male has reached around the age of four that he will gradually start transitioning into a female. She will then continue her life for around another two to four years, mating with younger males. The process of transformation is thought to be triggered by water temperature, and when temperatures are higher, the shrimp grow at a faster rate.
8. Banana Slugs (Ariolimax spp.)
One of the only terrestrial animals on this list, the banana slug is often found on forest floors along the North America Pacific coast. As you may guess by their name, these slugs are often yellow in color, but some individuals may have black or brown markings that make them look much darker.
Banana slugs are simultaneous hermaphrodites meaning they have both male and female genitalia. Amazingly, the penis of the banana slug is so large that it’s often the same length as the animal’s body! Plus, it protrudes from an opening on their head.
If weird anatomy wasn’t enough to interest you, then you might want to consider the copulation ritual of the banana slug. Two individuals will come together to mate and will both insert their penises into one another to release sperm. Each slug will also produce around 75 eggs for fertilization.
The fertilized eggs are then laid among the leaf litter or in a log, and the adults leave them to fend for themselves.
9. Cushion Starfish (Asterina gibbosa)
The cushion starfish can be found along rocky coastlines in Europe, particularly around the United Kingdom, and is one of only a few species of sequential hermaphroditic echinoderms.
Cushion starfish all begin their lives as males and only begin to make their transformations as they age and get larger. They have gonads located in their arms which expel gametes from a central location on the body. As females, these starfish may release up to 1000 eggs which stick to the substrate with a jelly-like substance.
While we aren’t entirely sure at what age and size these sea stars change gender, studies have shown that this typically occurs once the arm length reaches somewhere between 0.4 inches (10 mm) and 0.6 inches (16 mm). Further research revealed that by the age of three years, only around 15% of cushion stars were still male.
10. Blue-Banded Goby (Lythrypnus dalli)
The blue-banded goby is a bidirectional hermaphrodite, meaning that it can freely switch between being male and female several times during its life. It’s been shown in studies that body size is one of the leading triggers for change and that larger females are much more likely to reallocate their gender, as being larger means having a higher spawn rate.
These fish are native to the eastern Pacific Ocean and are a small species that generally grow to around 2.6 inches (6.6 cm).
11. Ribbon Eel (Rhinomuraena quaesita)
The last gender-bending creature on our list is the ribbon eel which is a species of moray eel found in the Indian and Pacific Oceans.
Ribbon eels are sequential hermaphrodites, and all begin life as males. Over time, they make the transition to being female and as far as we know, they’re the only species of moray eel that display hermaphroditism traits.
What’s more, the ribbon eel also changes color as it changes gender, with young males being black with yellow fins. Once they turn into females, adults typically have a green coloration with yellow and white markings on the fins. If males do not change gender then adults normally change from black to blue but retain their yellow fins.
With all that said, being hermaphrodites is something that is only theorized owing to the color changes we see over the lifespan of the ribbon eel. This has yet to be confirmed by science.