The ultimate goal of palace fighting: to undergo sex change and become emperor!

There are many problems in the world that come from the relationship between sex and marriage. So, those hermaphroditic creatures that do not distinguish between sexes seem to be able to avoid these troubles and are very lucky...

But is it really so?

Transgender, can't just lie down

To discuss the hermaphrodite lifestyle, we must first distinguish between two types of hermaphroditism. One is sequential hermaphroditism, which is changing from male to female, or from female to male, such as many fish. The other is simultaneous hermaphroditism, which has both male and female reproductive organs, such as sea slugs, marine planarians, earthworms, and most plants.

The double-banded fish (Thalassoma bifasciatum) living on coral reefs is born female. When it grows up, it can choose to change its sex to male, occupy a territory, drive out other male fish, and collect many female fish into its "harem". If it is not strong enough, it can also choose to continue to be a female fish and pass on its genes by laying eggs.

The territorial male has a blue head and bright stripes | Kevin Bryant mentalblock / Flickr

There are ways to advance and retreat, and the Double-banded Brontosaurus seems to be able to lie down and rest assured, but in fact it is not the case.

When it comes to breeding offspring, the efforts and rewards of males and females are completely different. Eggs are much larger than sperm, and the cost of producing eggs for female fish is much more expensive; female fish can mate twice in three days, while male fish can mate hundreds of times a day. If you can become the "master" male fish that dominates the harem, the spread of genes can achieve "class leap". Every fish can change from female to male, and has the potential to lead the harem. It can be imagined that everyone is coveting this position of dominating the harem.

Females and non-territorial males are yellow | Nhobgood Nick Hobgood / Wikimedia

On the surface, there is no battle royale in the fish school all day long - in social animals, after the status is determined, everyone stays in their position to avoid damaging their vitality by fighting. However, if we intervene, we will find that this calm is just an appearance.

Once someone captured a male fish from a school of fish, and after just a few minutes, the largest female fish would behave aggressively like the male fish; in as little as eight days, "she" would produce sperm and become a real "he" to take over the harem. Sex change not only does not allow the double-banded brontosaurus to rest easy, but it also pins them on the road to competing for male fish, making it impossible to escape.

The higher the virtue, the higher the evil.

For hermaphroditic animals, the problem of choosing which sex to be remains, since eggs are also more expensive than sperm.

Hypoplectrus nigricans solves this problem through mutualism: last time you were the male, next time I'll be the male. Some marine planarians use violence to solve the problem: two planarians use their sharp external genitalia to stab each other and inject sperm into each other, a process called penis fencing.

There are also many hermaphroditic species that mate in a (seemingly) egalitarian way—both partners receive sperm and both partners produce eggs. However, another form of conflict is also rearing its head at this time.

Animals that produce sperm need to increase the success rate of insemination as much as possible, or "defeat" other "rivals" who compete with them for the right to be inseminated; while animals that receive sperm need to choose sperm that they like to produce the next generation, or simply reject too much semen. Therefore, both sperm producers and sperm receivers are caught in competition.

Scientists compared sea slugs from the Aglajidae and Gastropteridae families and found that the size of the penis and prostate gland of the same species was positively correlated with the size of the bursa copulatrix, which is a very thought-provoking phenomenon.

Reproductive system of sea slugs | Anthes N et al. / Evolution: International Journal of Organic Evolution (2008)

The special shape of the penis can improve the efficiency of insemination; the secretions produced by the prostate may have hormonal effects, increasing the probability of safe insemination or reducing the other party's desire to mate again, which can be said to be a "male" weapon. The copulatory sac can digest and reabsorb semen, thereby eliminating unwanted sperm, which can be said to be a "female" weapon.

The sizes of the two are positively correlated, suggesting that they may be in a competitive relationship with each other; moreover, for hermaphroditic sea slugs, the costs of both "armaments" are borne by the same body.

Tubulophilinopsis reticulata is a brightly colored, innocent, and cute sea slug | Sébastien Vasquez / Wikimedia

Hermaphrodite, strange tricks

Even stranger tricks can be seen in the mating of hermaphrodites.

For example, some snails use their sharp spikes to inject a liquid that can increase their fertilization rate, called a love dart. For example, after mating, the Macrostomum worm bends its body over and uses its throat to aim at the genital opening to suck out the semen - the purpose of this is still unknown, perhaps to eat unwanted sperm or to expel the substance used by the other party to control itself.

The mysterious sex life of planarians. The black frame at the bottom of the picture shows the planarian sucking semen, and the red circle shows the sperm exposed from the reproductive opening after sucking | Schärer L et al. / Marine biology (2004)

However, the most courageous one is a sea slug called Chromodoris reticulata, which cuts off its penis after mating. The penis has many barbs, which can dig out the sperm of the previous sea slug, thereby increasing the fertilization rate. Because the structure of the penis is highly specialized, it is difficult to retract it after it is extended, so it can only be cut off.

Figures a and b show mating sea slugs. Figures c and d show a broken penis. Figure e shows the barbs on the penis. Figure f shows sperm hanging from the penis. | Sekizawa, Ayami, et al. / Biology letters (2013)

Fortunately, the broken penis of the sea slug is only a small section (about 1 cm long), and there is a longer section hidden in the body (about 3-4 cm long) that can be used next time. Because it is too long, this part of the penis is curled and coiled in the body.

If you ask a sea slug whether being a hermaphrodite can help it avoid many sexual problems, it will surely cry and tell you: No, it's too inward-looking!

References

[1] Anthes N, Schulenburg H, Michiels N K. Evolutionary links between reproductive morphology, ecology and mating behavior in opisthobranch gastropods[J]. Evolution: International Journal of Organic Evolution, 2008, 62(4): 900-916.

[2] Schärer L, Joss G, Sandner P. Mating behavior of the marine turbellarian Macrostomum sp.: these worms suck[J]. Marine biology, 2004, 145(2): 373-380.

[3] Schärer, Lukas, Tim Janicke, and Steven A. Ramm. "Sexual conflict in hermaphrodites." Cold Spring Harbor Perspectives in Biology 7.1 (2015): a017673.

[4] Sekizawa, Ayami, et al. "Disposable penis and its replenishment in a simultaneous hermaphrodite." Biology letters 9.2 (2013): 20121150.

[5] Warner RR, Swearer S E. Social control of sex change in the bluehead wrasse, Thalassoma bifasciatum (Pisces: Labridae)[J]. The Biological Bulletin, 1991, 181(2): 199-204.

Author: Little Wombat

Editor: pee pee shrimp

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