Falling in love means getting shot in the head. I can’t stand the love in the animal world…

There is no shortage of bizarre sexual behaviors in the animal world. Many animals injure females during mating (in hermaphroditic animals, both partners or the one receiving sperm), and the methods vary. These brutal mating methods are collectively called traumatic mating .

Sea Slug Love: One Head Shot!

In 2014, Rolanda Langer of the University of Tübingen in Germany and her research partners Wiminghausen and Antes published a surprising discovery.

The researchers caught some sea slugs from the Great Barrier Reef and raised them in the laboratory. They belong to the genus Siphopteron. These little guys are very cute, only 2 to 4 mm long, with white bodies with bright red and yellow edges, as bright as candy.

The appearance is soft and cute, but the heart is...丨seaslugforum.net

Despite their cute appearance, sea slugs have a very wild sex life. The sea slug family is already well-known for its sex life, and Siphopteron sea slugs are also one of the wildest of their kind. It is the only animal we know of that will pierce the other's head during mating.

Schematic diagram of the mating process of Siphopteron sea slugs丨DOI: 10.1098 / rspb.2013.2424

The sexual relationship between the two sea slugs begins with a lingering, head to tail, huddled together like two shoes in a shoebox. Because sea slugs are hermaphrodites, the two parties fertilize each other , and the two sea slugs extend their elongated male genitals at the front of their bodies. This guy is forked, and both forks have sharp "heads", one thick and blunt, and the other is sharp and thin.

The thick pointed head is inserted into the entrance of the female's genitals to deliver sperm, because sea slugs are head-to-tail, and the front male genitals are just facing the "gate" at the rear of the mate. This path is not long. The thin pointed head will go around to the front and insert into the other's head, just above the eyes. The genitals of sea slugs can stretch very long, almost as long as the body.

I didn’t expect it to be so cute丨In-Depth Images Kwajalein

During insertion, the genitals will inject some liquid. We don't know the composition and source of the liquid yet, but based on the sea slugs of the same genus, the injection is likely to be prostatic fluid. The entire insertion process lasts more than 40 minutes. When mating is about to end, the sea slug's posture will change from hugging in a ring to a "train car" with its head facing its tail. Once, Lange and others observed three sea slugs together, fertilizing each other. The function of the sea slug's "headshot" is still unclear. But it is not easy to pierce the head, and it requires extremely flexible genitals and operations, so there must be benefits to natural selection in doing so.

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In the most common household pest, bed bugs (Cimex lectularus), males use their sharp external genitalia to pierce the female's abdominal wall and inject sperm into the female's circulatory system, allowing them to swim to their destination on their own.

Bed bugs mating, the smaller male bed bug below, thrusts his genitals directly into the female's abdomen | Rickard Ignell / Wikipedia

At least nine families of snails will pierce each other's bodies with a hard "arrow" made of calcium salts during mating, which is called "Love Dart". Different snail species have different shapes of "arrows" such as needles, arrowheads, and blades, just like a ninja's hidden weapon bag.

Afterwards, the snail on the right had a love arrow stuck in it. | Eynar / Wikipedia

Some flatworms belonging to the order Polyclad, which look like brightly colored handkerchiefs, have the most uninhibited mating methods in the animal kingdom. Flatworms are hermaphrodites, and fertilization can be achieved by stabbing the other's body and injecting sperm. During mating, both parties will extend their male genitals, dodge and stab each other like a sword fight, and the stabbed party acts as the "female" and receives the sperm.

Two flatworms raise their bodies and try to pierce each other with their genitals. | Leslie Newman / Wikipedia

Why do animals hurt their partners? There are many reasons, but it is definitely not to increase the fun.

The Snail's "Cupid's Arrow"

Let's take an example, a type of animal that has been studied more thoroughly: snails. The reason why they love and hate each other is probably that the love of snails is extremely unfriendly to sperm. More than 99% of the sperm that enter the snail's body lose their lives. There is a small organ in the snail's body called the copulatory sac (Bursa Copulatrix), which secretes enzymes that dissolve sperm. The reason why snails "hate" sperm may be to give the party receiving sperm the right to choose : it can destroy the sperm it doesn't like and choose the snail with the best genes to act as the "father" (snails are hermaphrodites, so any one can be the father).

Mating snails | David Fox / gettyimages.com

As we mentioned earlier, mating snails use love arrows to sting each other. The end of the love arrow is connected to a pair of glands that secrete mucus to coat this "arrow". Ronald Chase and Katrina Blanchard of McGill University also conducted an experiment to understand the "medicinal effect" of mucus on snails.

They caught some small gray snails (Cantareus aspersus) and surgically removed the glands of the snails, then let them mate. After mating, the snails that received sperm were injected with gland extracts, and the other group was injected with salt water after mating. After the snails that received sperm laid eggs, the genes of these eggs were checked. The results showed that the "slime snail" could fertilize at most 44.1% of the eggs, while the "salt water snail" could only fertilize at most 20.4% of the eggs (some sperm from previous matings were stored in the snails that received sperm, so the "credit" for fertilization was not fully distributed to these two), that is, mucus more than doubled the chance of fertilization.

The snail said, only with "arrow" can you get it丨kiokuima / YouTube

The ingredients in the mucus allow the snail to control the contraction of the muscles of the "pipe" of the mating sac, making it difficult for the enzyme to flow out. The snail that shoots the "Cupid's arrow" may use this method to increase the survival rate of its own sperm and allow more eggs to be fertilized. The "drugs" that some snails use to control their own kind are very dangerous. After being stung, the snail with the scientific name Bradybaena pellucida lays fewer eggs and has a shorter lifespan. Although the total number of eggs is reduced, the snail that uses this "human weapon" can greatly increase its fertilization rate , so in general, it still has more offspring than the snail that does not use the arrow of love. The snail took a bigger piece of the smaller cake.

The competition behind the harmonious relationship

Mating seems to be a harmonious and harmonious relationship, but it is not. Females and males, or in hermaphroditic animals, the egg providers and sperm providers often conflict with each other. For example, the best choice for females may be to mate with multiple males and choose the best sperm, but the best choice for males is to fertilize as many eggs as possible. After mating with a female, he certainly does not want other men to grab this "cake".

So males may have evolved some methods to manipulate females. For example, some proteins in the semen of Drosophila melanogaster, a frequent visitor in the laboratory, can suppress the "desire" of females , prolonging the "refractory period" of female fruit flies after mating, and reducing the chances of "rivals" invading.

Mating fruit flies | FRANCISCO ROMERO FERRERO

However, while males evolve manipulation methods, females will also evolve new methods to counter, such as using enzymes to break down the proteins that males excrete into their bodies. Genetic research has found that the reproductive proteins of Drosophila spp., regardless of male or female, have evolved very quickly. After this evolutionary "arms race" develops to a certain extent, the female's external genitalia has become a tightly defended "pass". If the "drug" and sperm take this path together, it is easy to be killed without a trace. So we have to find another way, like the love arrow of the snail, to choose a place with weak defenses to break through. Some animals, like bedbugs, have even changed the "path" that sperm take.

Perhaps the "headshot" mating method of sea slugs is also to "administer medicine" to their mates to control them. They choose the tactic of attacking the head, directly aiming at the opponent's central nervous system.

Author: Red Queen