Monogamous seahorses may not be that stable!

Leviathan Press:

Although most seahorses are monogamous and known for their strong relationships, recent studies have shown that their relationships are not as stable as scientists previously claimed. Zhang Dong, a researcher at the East China Sea Fisheries Science Academy and the lead author of the study, said that if a seahorse couple is forcibly separated, the female will find a new partner.

(onlinelibrary.wiley.com/doi/10.1111/jfb.14867)

The researchers placed male and female erectus seahorses (Hippocampus erectus) in a tank. After they mated, the male was moved to another tank, separated for four days, and another male was placed in a new pair with the female. After repeating the last pairing to ensure the accuracy of the experiment, they placed all the seahorses in the same tank to see which male the female would choose. In total, the team conducted experiments on 24 different mating pairs of seahorses.

They found that in almost all cases, females had no clear preference for their original partners. While the researchers acknowledge that this could be because females are unable to recognize their partners after a few days of separation, it is more likely that competition between males is balanced during forced separations. The researchers also noted that seahorse couples often perform complex dances to perform ritual greetings with each other, and if this dance is interrupted (such as during forced separation), female seahorses will consider other male partners.

Human couples can learn a lot from seahorses.

These wonders of the sea spend some quality time together. They flirt, swim together, and mate. The rest of the time, they go their separate ways, drifting through the ocean, eating and drinking at their leisure. But they do look forward to being together again.

Just after sunrise, male and female seahorses approach each other, gently rub their noses together, and then begin to twirl around each other. Many of them make seductive clicking sounds. The couples sway gracefully back and forth, as if dancing to the rhythm of underwater music. They embrace each other in a dreamy way, as if they have lost track of time.

However, love can be dangerous for seahorses. While dancing, seahorses release hormones that cause their camouflage to fade away. This causes the color of their bodies to change and begin to glow, with the contrast in their skin patterns becoming more noticeable. Researchers speculate that this is how seahorses express their willingness to mate.

Seahorse's mating dance. © BBC

The dance is also a means of seduction. Courtship can take hours before mating. Finally, the female signals that she is ready. She swims to the surface, points her nose to the sky, and stretches her body straight out like a stick - a posture that is irresistible to the male. The male seahorse presses his chin to his chest and opens and closes his tail like a switchblade. This allows him to fill his pouch with water, showing his beloved female how spacious it is.

© The Rainforest Site

Soon, the female and male seahorses were cuddling together, drifting with the water. Their noses and bellies were touching, and because of the curve of their body posture, they looked like a heart. Then, something amazing happened.

A tube appears on the abdomen of the female seahorse, which looks a bit like a penis, the so-called ovipositor. When the love scene reaches its climax, both parties raise their heads in ecstasy and arch their backs . The female seahorse injects the egg into the male seahorse's brood pouch, and the male seahorse's sperm eventually combines with the egg to form a fertilized egg.

Afterwards, the loving couple separates. Their body colors and patterns return to normal. The male seahorse will rock himself to ensure a favorable position for the fertilized eggs to slide into the pouch. The female seahorse usually swims off to find food. For the father-to-be, the difficult time of pregnancy begins now.

Scientists have long wondered why males are the ones who get pregnant in the seahorse world, but recently an explanation has emerged.

In 2001, Tony Wilson, professor of evolutionary biology at Brooklyn College, City University of New York, and his colleagues demonstrated that pregnant male seahorses can provide nutrients to the embryos in their pouches.[1]

At the same time, male seahorse pregnancies are becoming more and more like those of female mammals. For example, the male seahorse's immune system protects the embryo from infections. Scientists at the Helmholtz Centre for Ocean Research Kiel (GEOMAR) are currently investigating the microbiome (commensal bacteria) of the male's pouch in relation to seahorse pregnancy. The prospective father transfers his microbiome to the embryo via the pouch, thereby strengthening the offspring's immune system.

In addition, the pregnant male seahorse removes waste products produced by the embryo and provides it with nutrient-rich, high-energy fats. Gas exchange enables the embryo to breathe.

A male seahorse gives birth to a baby seahorse from his pouch. © George Grall

Still, there is a question of how the female's eggs are fertilized by the male. Biologists once thought that males fertilized the eggs directly in their pouches. However, researchers found something interesting in the tube seahorse (Hippocampus kuda): Since their vas deferens end outside the pouch, males cannot store sperm in the pouch. Long-nosed seahorses (Hippocampus guttulatus) and European seahorses (Hippocampus hippocampus) also cannot fertilize eggs in their pouches, which is anatomically impossible.

So, how does the sperm get to the egg?

William Holt, a professor of zoology at the University of Sheffield in the United Kingdom and an expert on seahorse reproduction, explains that in the case of tube seahorses, males expel semen during mating, but the opening of the pouch is about 4 millimeters from the end of the vas deferens.

"It's hard to imagine how sperm can swim that far without getting lost in the ocean because they're so slow," Holt said. "We speculate that when the female transfers her eggs to the male's pouch using her ovipositor, she also collects sperm from the seawater, so the sperm go into the pouch along with the eggs." Holt believes that fertilization in other seahorse species is likely to occur in a similarly complex manner.

"Collecting" sperm in the water? Sounds odd. Why do seahorses expend so much energy on courtship and mating? Why can't female seahorses simply lay their eggs in the water, like almost all other female fish, and then let the male seahorse fertilize the eggs in the water himself?

Although seahorse reproductive behavior seems illogical and absurd - it's like a hunter throwing his loaded rifle at a stag instead of shooting him directly.

“Not all processes in nature are perfectly efficient,” says Axel Meyer, professor of evolutionary biology at the University of Konstanz in Germany, with a smile. “Evolution does not start from a blank slate, but makes the best use of the resources it has.”

Evolution, he explains, does not work "with precision like an engineer, but by trial and error". Organisms have to function in each generation, which means the evolutionary process is based on chance. "That's why there are random mutations in the genome, which are passed down as a prerequisite. Some change very slowly, while others don't change at all.

© National Geographic

Meyer said there are still some "design flaws" in nature. For example, albatrosses have a wingspan of up to 11 feet (3.35 meters), which makes them so heavy that they sometimes break their necks when landing. "Even our human structure is not 100% perfect. The distance between our trachea and esophagus is too close, which undoubtedly increases the risk of suffocation.

Darwin coined the term "survival of the fittest" in 1869, a term that derives from the English philosopher and sociologist Herbert Spencer. "Darwin applied this principle to individuals of the same species," explains Meyer. "The term 'fittest' should not make you think of a gym." In nature, the determining factor is how many offspring an individual has compared to the reproductive success of its contemporaries. "Fitness is measured as the relative chance of survival and reproductive success of a sample genome compared to its competition in a given population."

Meyer and other scientists believe that the poor location of the opening of the male seahorse's vas deferens is evidence that male seahorse pregnancy evolved slowly in small steps rather than being designed from the beginning.

An even bigger mystery is seahorse monogamy. Only 3% of mammals have lasting pair bonds, and even fewer of them exist among amphibians, reptiles, and fish. Yet most seahorses are model couples who never cheat on each other. Not only do they have affectionate courtship, but they also invest a lot of time and energy in maintaining their relationships, participating in a variety of activities together.

In many seahorse species, couples come together daily to greet each other and dance. Behavioral scientists believe that seahorses use these rituals to strengthen their bond, with sex playing a lesser role. These couples often stay together for life.

Seahorses form strong couple bonds and usually stay together for life. In Japan, seahorses are called "竜の落とし子" (たつのおとしご), which literally means "illegitimate child of the dragon", but seahorses do look more like dragons. © BluePlanetArchive/Steven Kovacs

In ancient legends, if one of the partners gets entangled in a net, the other seahorse will never abandon it, but will voluntarily follow into the net of captivity. Aquarium keepers report that if one seahorse dies, its partner often dies within the next few days. Is it because grief makes the surviving widow or widower lose the will to continue living?

The unique way seahorses reproduce raises another question: It is the males who get pregnant, so why don't female seahorses distribute their eggs among as many partners as possible, just as males distribute their sperm in most parts of the animal kingdom?

A dog's penis bone, with the arrow pointing to the urethral groove. © wikipedia

Walrus penis bone. © Rafael Osona Auctions

Some primates, dogs, and even whales have penis bones that allow them to maintain a permanent erection, meaning they can mate with multiple different partners in succession. In stark contrast, most female seahorses remain faithful to their partners and will not mate again until their partners have given birth and are ready to get pregnant again.

Why do most seahorses remain loyal until death?

“In general, we know very little about the origins of monogamy in the animal kingdom,” says Anna Lindholm, an evolutionary biologist at the University of Zurich. However, there is good evidence that it arose independently in several species during evolution.

A 2019 study[2] showed that 24 genes in the genomes of prairie voles, poison dart frogs, and water pipits were closely associated with monogamous behavior. It is speculated that this pattern may also exist in the seahorse genome. However, this finding still does not fully explain the purpose of monogamy.

The birth of a male seahorse is still a spectacular sight. © Tenor

"Long-lasting partnerships are beneficial because in some species, offspring can only survive to adulthood if both parents are involved in caring for them," Lindholm explains. For example, storks, wolves and beavers. This explanation might be clear, but it doesn't apply to seahorses, whose babies swim off on their own after birth without any support from their parents. Could it be that seahorses are simply more capable of love than other creatures?

Evolutionary biologists are skeptical. However, the monogamy of seahorses does give them an important advantage. Pregnant males can be sure that their pouches carry only their own biological offspring, not any competitors' so-called "cuckoo eggs" (the biggest feature of cuckoos is that they "do not build nests and incubate eggs themselves", but lay their eggs in the nests of birds such as great reed warblers, gray magpies, and brown fantails. Generally, the chicks can hatch quickly in about 10 days, and then instinctively push the other eggs out of the nest with their backs and smash them until only their own are left. The selected bird parents raise the cuckoo chicks, which are not their own children at all and are the murderers of their children, as "the only surviving children". Editor's note).

In salmon, for example, after a territorial male or "official father" has successfully mated, other males will often flash and release sperm in the female's "nest." However, if the sperm of male seahorses (at least for tube seahorses) is not deposited directly into the pouch, how certain is it that they will get pregnant?

Could other males also join in on their sperm? Holt denies this. The reproductive strategy of the tube seahorses does give them a certain degree of certainty. "When seahorses mate, they stay very close to each other." No competitors can intervene. "In addition, the male's pouch closes again after a few seconds, so no more eggs can be injected," Holt emphasizes. "It is not possible for more than one male to fertilize the female's eggs, nor for more than one female to inject her eggs into the male's pouch." It's a fair deal.

Experts say the main reason many seahorses practice monogamy is for other reasons. Most seahorses live in sparsely populated habitats, but they are slow swimmers and, depending on the species, some are very rare.

"Since seahorses cannot travel long distances to find mates, any available sexual partners are valuable resources to both males and females and are not easily relinquished," Meyer said. "Even at lower densities within a population, they can still pass on their genes efficiently thanks to persistent pair bonds."

Male anglerfish are significantly smaller than female anglerfish. During mating, the male will bite the female's body with his teeth. Eventually, the male anglerfish and the female complete the fusion of skin and blood. Existing records show that eight male anglerfish are attached to one female anglerfish... © DSM Observer

Meyer talks about the extreme case of deep-sea anglerfish. In the vast darkness, an anglerfish rarely encounters its own kind. If a male anglerfish stumbles upon a female, he is not picky. He immediately attaches himself to her ... At first, the couple's skin grows together, and later even their circulatory systems merge. The male's jaw gradually degenerates and eventually merges with the female organism. Meyer explains that in some deep-sea anglerfish, the male's body completely disintegrates, leaving only the testicles.

It doesn't sound like a dream wedding, but it's hard to imagine a more intimate relationship...

For seahorses in love, the "married couple" will practice the dance technique every day, as well as the longer wedding dance, to ensure that the female's eggs can be technically accurately transferred to the male's pouch. Experts speculate that this also allows them to adjust their reproductive cycle driven by sex hormones. So that the female's next batch of eggs will mature after her "husband" has finished giving birth to the previous batch of pups.

Dennis's pygmy seahorse. It is usually orange, with a darker tail ring. It is only 2.2 cm long, which is very small. © Wikipedia

As the saying goes, all roads lead to Rome—or to reproductive success. In fact, not all seahorses are faithful. A restless female Denise's pygmy seahorse (Hippocampus denise) proves to be always on the go.

Scientists observed her reproductive behavior. This mini seahorse "dates" two males at the same time. She performs a wedding dance and mates with both. This biological advantage means she reduces the risk of losing her mate and all her young. This savvy seahorse took a strategy favored by stock market speculators and didn't put all her eggs in one basket!

A puffy seahorse. © Wikipedia

Some male seahorses are even particularly philandering.

The puff-bellied seahorse (Hippocampus abdominalis) of Australia and New Zealand does not seek out lasting partnerships. Instead, it will mate with almost any female nearby.[3] This fits in well with the biologists’ theory of fidelity due to scarcity, since puff-bellied seahorses live in smaller areas than most other species.

Many other species of seahorses—with or without their romantic nuptial dances—have their place amid the aquarium bustle. If there are enough partners, some male seahorses will mate with as many as 25 different females a day.

"Morality means lack of opportunity", this famous human saying may also apply to seahorses.

© Imgur

References:

[1]onlinelibrary.wiley.com/doi/abs/10.1002/bies.20626[2]www.pnas.org/doi/full/10.1073/pnas.1813775116[3]www.ncbi.nlm.nih.gov/pmc/articles/PMC2714888/

By Till Hein

Translation/Yuba and Thin Bamboo

Proofreading/Rabbit's Light Footsteps

Original article/nautil.us/seahorse-love-works-in-mysterious-ways-389285/

This article is based on the Creative Commons License (BY-NC) and is published by Yuzhu and Shouzhu on Leviathan

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