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Why do animals change sex? -

Animals change sex through various biological mechanisms, typically in response to environmental cues, social dynamics, or changes in population structure. This phenomenon is known as sequential hermaphroditism.

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Hermaphrodism -

Hermaphrodism happens when an animal has both male and female reproductive organs, and in some cases, they can be capable of self-fertilization. Hermaphroditism provides reproductive flexibility, especially in environments where finding a mate of the opposite sex can be challenging.

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Temperature -

In some cases, temperature plays a role in the development of an animal’s sex. If an egg develops at a certain temperature, it may determine the sex of the animal that hatches. For example, at specific temperatures, an egg that would typically develop into a male could instead develop into a female.

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Clownfish -

Clownfish live in hierarchical groups within an anemone, which is headed by a large dominant female. Clownfish start out life as male, then later change sex to become female. Once the change has been made, they cannot switch back. 

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Clownfish -

The change is driven by hormones and begins in the clownfish's brain before affecting their reproductive organs. They change sex to reproduce without needing to search for a mate.

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Bearded dragons -

Like mammals, the biological sex of bearded dragons is determined by the combination of chromosomes inherited from their parents. This differs from many other reptiles, whose sex is determined by the temperature at which their eggs are incubated.

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Bearded dragons -

In addition to their two sex chromosomes, bearded dragons have a temperature override that activates in hot and dry conditions, resulting in the production of more females.

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Bearded dragons -

When temperatures exceed 89.6°F (32°C), a sex reversal occurs. Individuals with the typical male chromosomes will instead develop into females. The exact reason for this female-producing strategy is unknown.

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Bluehead wrasse -

Bluehead wrasse is a species of fish native to the coral reefs of the tropical waters of the western Atlantic Ocean. Bluehead wrasses start life as females but when a dominant male dies or is removed, the largest and most dominant female in the group undergoes a significant hormonal shift and becomes male.

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Bluehead wrasse -

The hormonal shift happens quickly, with mature ovaries becoming testes in just eight to 10 days. This process is crucial for bluehead wrasses to maintain their social structure and reproductive success.

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Green frogs -

The sex of green frogs is determined by a combination of both genetic and environmental factors. Some green frogs have been found to develop the body of their genetic sex as tadpoles and then reverse under particular conditions, becoming the opposite sex as adults.

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Green frogs -

Experts initially believed that green frogs changed sex in response to human pollution, as populations with a higher number of females were found in more contaminated waters. However, it has since been found to be a natural phenomenon, as sex reversal also occurs in unpolluted ponds.

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Humpy shrimp -

The humpy shrimp gets its name from the hump along its abdomen. The shrimp starts off as male and then becomes female. This is called called protandry and is more common than the opposite direction (protogyny).

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Humpy shrimp -

The benefit of protandry in humpy shrimps may be due to the size-advantage model, as it allows individuals to maximize their reproductive success by starting as males when they are smaller and transitioning to females as they grow larger. The increased size provides a greater reproductive output and overall advantage.

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Starlet cushion star -

The starlet cushion star is another species that begins its life as a male and then switches to a female as it grows bigger. 

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Starlet cushion star -

Scientists can pinpoint exactly when the change happens: when their arm length is between 9 and 16 mm (0.35 to 0.62 inches).

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Pacific oyster -

The Pacific oyster has a range of sexual strategies, from remaining the same sex all their lives to being simultaneous hermaphrodites (being both sexes at once.) 

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Pacific oyster -

Both protandry (male to female) and protogyny (female to male) are found in Pacific oysters, with the protandry occurring more when food is in high abundance. 

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Swamp eel -

All swamp eels start their lives as females. As they mature, some will change to become male. The transition is fairly slow, taking as much as 30 weeks to complete.

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Swamp eel -

During the transition, there is an "intersex" period in which they have both male and female gonads. The change is not always permanent; some males turn back into females when there is a shortage of females in the population.

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Star tunicate -

Star tunicates are marine invertebrates. They can transition from one sex to another in response to environmental and social factors. Star tunicates can also reproduce both asexually (just one parent is needed) and sexually. 

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Coral hawkfish -

Coral hawkfish start their lives as females and can change to males if conditions call for it. This usually happens when the harem’s male leader takes on too many females, prompting the largest female to turn into a male hawkfish and split away with half the harem.

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Coral hawkfish -

Unlike other sequential hermaphrodites that make the switch and stick with it, coral hawkish can revert back to females again. 

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Banana slugs -

Banana slugs are simultaneous hermaphrodites, meaning they possess both male and female reproductive organs and can use them simultaneously during reproduction.

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Banana slugs -

While banana slugs are capable of self-fertilization, they prefer to find a partner. 

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Black sea bass -

Black sea bass, found from Cape Cod to Cape Canaveral, are protogynous hermaphrodites. They begin their lives as females and can later transition to males. 

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Black sea bass -

As black sea bass populations are spread over a wide area, it's difficult for experts to observe them in their natural habitat. However, research on sea bass in tanks has shown that the sex change may be related to supply and demand. When female sea bass observed a decrease in the male population in an adjacent tank, they switched.

Sources: (BBC) (Tree Hugger) (A-Z Animals) (Animalogic)

See also: The weirdest ways animals kill their prey

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