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Photo: Unsplash since Darwin's Origin of species was published, the giraffe's long neck has been used as a prominent example of adaptive evolution. Because individuals who can eat high leaves, individuals with long necks, are chosen by nature to survive and pass on their dominant traits to future generations, so evolution moves in the direction of longer necks, and that's what we learned at school.

But not everyone supports this view. After all, giraffes eat high leaves at an obvious cost of pumping blood two or three meters above the heart, and such high blood pressure is a challenge to the animal's survival itself. In addition, previous studies have shown that giraffes are more likely to look for food lower in an environment where food competition is high, and taller individuals are less likely to survive drought.

So, when the humid rainforest slowly turns into a dry savanna, is it not enough to get high food as a driving force for evolution?

Now, a team from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences has found another explanation from an ancient relative of giraffes: long necks may give males an advantage in mating battles. In other words, sexual choice may have given birth to a longer neck.

How does this monster look like a giraffe? In 1996, scientists from the Institute of Palaeovertebrae of the Chinese Academy of Sciences discovered a wonderful fossil in an early Miocene stratum on the northern margin of the Junggar Basin and called it "monster".

The fossil comes from a large ruminant about 1690 million years ago. its skull wall is very thick and the top of its head is covered with a huge disk. Next to the skull, scientists also found four cervical vertebrae, which are also very stout. In addition, the joints between the head and the cervical vertebrae and between the cervical vertebrae and the cervical vertebrae are significantly more complex and larger than those of other mammals.

The researchers feel that such a strong head and neck structure may help cope with fierce collisions, so the monster should be an animal that needs to fight by hitting its head violently. it was suspected that it was an ancient relative of some cattle that courted by collisions. However, the shape of the fossil is so special that scientists are unable to determine its identity for the time being.

Amurh is a bone labyrinth in the inner ear of animals. An is a fossil monster studied by scientists, B is a living giraffe, and C is a giraffe (photo source: original paper). Over the next 20 years, paleontologists have found similar animal fossils in similar sites many times. Until one day while scanning the skull structure with CT, scientists noticed the behemoth's bone labyrinth, the hard bony shell of the animal's inner ear. Scientists have reconstructed the shape of the bone labyrinth in fossils and found that its shape is significantly different from that of musk oxen, pronghorn and some deer, but more similar to that of living giraffes and marmosets.

The giraffe, belonging to the giraffe family, is the only non-extinct relative of the giraffe. The small part in the fossil ear sent a strong signal to scientists that the fossil monster they found may have come from the giraffe superfamily.

Photo source: Daniel Jolivet via flickr,cc-by-2.0) at this point, some things that were once incomprehensible began to make sense. For example, a large disk-shaped horn like a helmet grows above the parietal bone. Such structures are rarely seen in the known bovine, pronghorn, and deer families, whose horns grow on the frontal bone (the upper anterior part of the skull), while the horn of the living giraffe happens to grow on the parietal bone (the top of the skull), as does the extinct genus Sivatherium in the giraffe family.

Giraffe's horn is very special, called ossicone, which is a kind of bone structure wrapped in skin. Scientists have found discontinuous bone tissue on the discoid horns of fossil monsters, which scientists think means that the horns of fossil monsters may also be covered with skin like the skin-bone horns of modern-day growing giraffes.

The similarities found in the schematic map of fossil species (photo source: original paper) have led the research team to increasingly believe in the genetic relationship between the fossil species and giraffes. However, the monsters that lived about 1700 million years ago may not have evolved extra-long necks.

The gifted head-bangers scientists named the monster "Discokeryx xiezhi". Yuanyu is an ancient Chinese legendary immortal beast with a pointed horn on its head. when two people fight with each other, it will hit the other side with its horn, so it has also become a symbol of fair law enforcement.

The cymbals on the robes of the imperial officials of the Qing Dynasty (photo: Wikimedia Commons) but is it enough to judge that an animal is good at head-hitting movement just by looking at disc-shaped horns, thick skulls and stout cervical vertebrae? To confirm this, the team also conducted physical simulations to assess how adaptable the body structure of the deer is.

If you recall that mentioned above, the head and neck joints and intercervical joints of this fossil species are more complex than those of other mammals. Between the head and neck, it has additional joints, and between the cervical vertebrae and cervical vertebrae, it has additional joints. What the scientists need to do is to add or subtract some of the joints peculiar to the deer in the model and see if the existence of these joints makes the animal more suitable for a head hit.

The researchers found that after removing the additional joints between the head and neck, the impact on the model during a high-speed head impact would lead to a very serious flexion of the Atlanto-occipital joint between the occipital bone of the head and the first cervical vertebra-atlanto. the angle of rotation can be as high as 54.7 °. Generally speaking, this means "breaking your neck".

Flexion of the head and neck joint (photo source: original paper) and if the rotation angle of the atlantooccipital joint is limited to less than 5 °, then the peak strain energy of each bone increases significantly in the model without additional joints, especially for the atlas. The risk of bone injury is greatly increased. In other words, the extra joints between the heads and necks of the deer may reduce their injuries during the impact and avoid neck breaks.

In addition, the scientists also found some contestants who are good at hitting their heads in the living musk oxen, sheep and rock sheep genera, and asked them to compare them with the deer in the simulation experiment. It turned out that the horned deer were more adapted to high-speed head strikes, or better protected from impacts, than all their opponents.

Scientists believe this can be used as evidence of male competition for courtship among giraffe's fossil relatives. More importantly, fierce fighting for mates can be a driving force for animal evolution, and discoid horns and complex head-neck and intercervical joints may be the result of sexual selection.

How did you get a long neck? The growing giraffe is also very good at fighting. On the grasslands of Africa, they can vigorously wave their necks about two meters long, or they can stab their opponents with a pair of skin and bone horns. So, could their necks be the result of sexual selection?

The giraffe's long neck may have appeared in the early Pliocene about 5 million years ago, the researchers said. About 7 million years ago, the forests of East Africa began to be replaced by savannas, the ancestors of giraffes slowly lost the favorable environment they once enjoyed, and the pressure of competition increased. Some of the taller individuals may have acquired the extreme strategy of swinging their necks to attack their competitors in such a special period.

Under sexual selection, tall giraffes are more likely to pass on their genes to the next generation. Scientists speculate that it may be in this way that they evolved into living giraffes in about 2 million years.

Their speculation is not groundless, and the horned deer may have experienced severe environmental changes. The researchers extracted enamel from fossils and determined their isotopes. The results showed that the stable carbon isotope δ 13C in the enamel of antler was the second highest among all the herbivores investigated, which indicated that they lived in an open and sunny land. In addition, the stable oxygen isotope δ 18O in the enamel of this animal has a wide range, indicating that they have many different sources of water.

Scientists analyze that the deer may live in open grasslands where the climate is relatively dry. Although the earth as a whole was warmer and forests were widely distributed in that period, the living area of the deer became drier in Xinjiang because the uplift of the Qinghai-Tibet Plateau blocked the spread of water vapor. And the fighting behavior of hitting the head at high speed may also be caused by environmental pressure.

The moose has acquired discoid horns and special head and neck joints to save itself in the courtship struggle, while the growing giraffe gets an elongated neck, which is more likely to cause severe damage to its opponent. The stories behind them may be the same.

So is it true that the length of a giraffe's neck has nothing to do with eating high leaves? In the interspecific competition, if you can get high food, you can naturally occupy a different niche from other species. But in the view of the research team, it is driven by sexual selection that they gain the advantage of eating high leaves.

This advantage also intensifies intraspecific competition, allowing the neck to evolve towards a longer trend.

Original paper:

Https://www.science.org/content/article/ancient-giraffe-relative-head-butted-rivals-amazing-sexual-weapon

Reference:

Https://www.nature.com/articles/d41586-022-01565-7

Https://www.cas.cn/syky/202206/t20220603_4836846.shtml

Https://mp.weixin.qq.com/s/6v2FKpiT8NJwWQ3HWb2y8A

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by: chestnut, revision: Erqi

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