I can't bear to be in love with someone like that.

There is no shortage of bizarre sexual behavior in the animal world, and many animals injure females during mating (in hermaphrodites, both sides, or the one who receives sperm), and in different ways. These ferocious mating methods are collectively referred to as traumatic mating (Traumatic Mating).

The love of the sea slug: an explosion in the head!

In 2014, Roland Lange of the University of Tubingen in Germany and her research partners Weiminghausen and Antes published a startling discovery.

The researchers caught some sea slugs from the Great Barrier Reef and raised them in the laboratory. They belong to the genus Siphopteron and there is no official species name at present. These little guys are very cute, only 2mm to 4mm long, with a white body with a bright red and yellow rim, as bright as candy.

Soft and cute on the outside, but @ # ¥% & * on the inside. Picture: seaslugforum.net

At odds with Mengmeng's appearance, the sex life of the puffins is very wild. The sea slug family is already famous for its sex life at home and abroad. Siphopteron is also a particularly wild member of its kind. It is the only known animal that pierces each other's head during mating.

A schematic diagram of the mating process of the genus Siphopteron. Picture: DOI: 10.1098 / rspb.2013.2424

The sexual relationship between two sea slugs begins with a lingering relationship, head to tail, huddled together like two shoes in a shoebox. Because sea slugs are hermaphrodites, the two sea slugs inseminate each other. Two sea slugs protrude at the front of the body, slender male genitalia. This guy is bifurcated, and both forks have pointed "heads", one thick and obtuse. One sharp.

The thick tip is inserted into the entrance to the female genitalia to transmit sperm, because the sea slug is head-to-tail, with the front male genitalia facing the "gate" at the back of the spouse. The road is not long. The thin tip goes around to the front and inserts the other person's head, just a little above the eyes. The genitals of sea slugs can stretch very long, almost as long as their bodies.

I didn't expect it to be so cute. Picture: In-Depth Images Kwajalein

During insertion, the genitals will inject some fluid, and we do not know the composition and source of the fluid, but according to the sea slugs of the same genus, the injection is likely to be prostatic fluid. The whole insertion process lasts more than 40 minutes, and when mating is coming to an end, the posture of the sea slug will change from a hug to a ring to a head-to-tail "train carriage". On one occasion, Lange and others observed three sea slugs together, fertilizing each other. The function of the "head shot" of sea slugs is not clear. But piercing the head is not easy and requires extremely flexible genitals and operations, so there must be benefits from natural selection.

The big iron stick hospital is looking for the director of stab.

The most common domestic pest, the bedbug (Cimex lectularus), the male uses a sharp external genitalia to pierce the female's abdominal wall, injecting sperm into the female's circulatory system and allowing them to swim to their destination.

The mating of bedbugs, the smaller male bedbugs below, poke their genitals directly into the female's abdomen. Picture: Rickard Ignell / Wikipedia

Snails of at least nine families pierce a hard "arrow" made of calcium salt into each other during mating, called "Love Dart". Different snail species, this "arrow" has needles, arrows, blades and other different shapes, just like a ninja's hidden weapon bag.

Afterwards, there was a love arrow in the snail on the right. Picture: Eynar / Wikipedia

Some flat animals of the order Polyclad, whose appearance looks like brightly colored handkerchiefs, are unrestrained and unrestrained in their way of mating, crowning the animal kingdom. Flat animals are hermaphroditic and can be fertilized as long as they stab each other's body and inject sperm into them. During mating, both sides stick out their male genitals, dodge and stab each other like swordsmanship, and the stabbed party acts as a "female" to receive sperm.

Two flat animals tilted up in an attempt to pierce each other with their external genitals. Picture: Leslie Newman / Wikipedia

Why do animals hurt their mates? There are all sorts of strange reasons, but it's certainly not to add interest.

Snail's "Cupid's Arrow"

Let's take an example, a kind of animal that has been studied more thoroughly: snails. The reason why they fall in love and kill each other is probably that the snail's love is extremely unfriendly to sperm. More than 99% of the sperm that entered the snail lost its life. The snail has a small organ called the mating sac (Bursa Copulatrix), which secretes enzymes that dissolve sperm. The reason why the snail "hates" sperm may be to give the recipient a choice: it can destroy the unwanted sperm and choose the snail with the best genes to act as the "father" (the snail is hermaphroditic and any one can be a father).

Mating snails. Picture: David Fox / gettyimages.com

As we said earlier, mating snails stab each other with love arrows. The arrow is connected to a pair of glands at the end, which secretes mucus and smears it on the arrow. Ronald Chase and Katrina Blanchard of McGill University also conducted an experiment to understand the "effect" of mucus on snails.

They caught some small gray snails (Cantareus aspersus), performed surgery to remove the snail's arrow-loving glands, and then asked them to mate. After mating, one group of snails were injected with gland extract, while the other group was injected with saline after mating. After the sperm-receiving snail laid its eggs, the genes of the eggs were examined. The results showed that the "slime snail" could fertilize at most 44.1% of the eggs, while the "saltwater snail" could only fertilize at most 20.4% of the eggs (in the body of the sperm-receiving snail, some previously mated sperm were stored, so the "work" of fertilization was not fully distributed to the two). Mucus more than doubles the chances of fertilization.

The snail said that only when there is an "arrow" can it be achieved. Picture: kiokuima / YouTube

The ingredients in the mucus allow the snail to control the contraction of the "duct" muscles of the mating sac, making it difficult for the enzyme to flow out. Snails that shoot Cupid's arrow may use this method to improve the survival rate of their sperm and fertilize more eggs. It is dangerous for some snails to control similar "drugs". The snail, known scientifically as Bradybaena pellucida, lays fewer eggs and lives shorter after being stabbed. Although the total number of eggs is less, the snail that uses this "human murder weapon" can greatly improve its fertilization rate, so on the whole, it has more offspring than snails that do not love the arrow. The snail took a bigger share from the smaller cake.

Let's take a look at the facts about snail mating and what you don't know. The video is about 3 minutes long. Please watch it under wifi. Source: PBS Digital Studio

The arms race behind the harmony between harps and harps

The matter of mating seems to be harmony and harmony, but in fact it is not. Females and males, or in hermaphroditic animals, are egg providers and sperm providers, often in conflict. For example, the best choice for a female may be to mate with multiple males and choose the best sperm, but the best choice for a male is to fertilize as many eggs as possible. I'm sure he doesn't want other men to grab this cake.

So males may have evolved ways to manipulate females. For example, Drosophila melanogaster, a frequent guest in the laboratory, has some proteins in his semen that suppress the female's "desire", lengthen the "refractory period" after mating, and reduce the chance of invasion by "romantic rivals".

Mating fruit flies. Picture: FRANCISCO ROMERO FERRERO

But as the male evolves manipulation, the female evolves new ways to counter it, such as using enzymes to break down proteins that the male puts into her body. Genetic studies have found that reproductive proteins in the genus Drosophila.spp evolve very quickly, regardless of whether they are female or male. After this evolutionary "arms race" develops to a certain extent, the female's external genitals become a well-guarded "pass". If the "medicine" goes along this road with sperm, it is easy to be killed. Therefore, it is necessary to find another way, like the love arrow of a snail, choose a place where the defense is weak to break through. Some animals, such as bedbugs, even change the "path" of sperm.

Perhaps the "head shot" mating method of sea slugs is also designed to "drug" their spouses to control each other, and they choose the tactics of attacking their heads, directly targeting each other's central nervous system.

This article is a special contribution to species Calendar, from the author of species Calendar @ Red Queen.

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