Did sexual selection influence giraffe neck lengthening?

When paleontologist Jin Meng discovered a strange skull in the vast, dry expanse of the Dzungarian Basin in northern China in 1996, he had a hunch about the pet activity of the ancient animal to which it belonged. The skull, robust and heavy, had a bone plate about 2.5 centimeters thick in the area that would have corresponded to the animal’s forehead. In addition, the neck vertebrae that Meng had found in the vicinity were also noticeably thickened, implying that they were designed to withstand great force. The expert deduced that the new species could even have surpassed the dinosaurs in the violent sport of head butting.

For years, Meng, now curator of the fossil mammal collection at the American Museum of Natural History in New York, and his collaborators at the Chinese Academy of Sciences simply called the specimen wow shou, or “strange beast.” However, that strange beast now has an official name: Discokeryx xiezhi. As described by Meng and his collaborators in an article published in Science, d.xiezhi It lived about 16.9 million years ago and is an ancient ancestor of modern giraffes.

Unlike today’s giraffes, whose necks (according to most experts) evolved to allow the animals to forage in the treetops, the thickness of the skull and vertebrae of d.xiezhi It was surely the result of sexual competition. According to the researchers, males of d.xiezhi The females would have disputed headbutting each other with a force perhaps unprecedented in the animal kingdom and that has not been observed since then.

“When talking about giraffes, we immediately think of their elongated necks,” explains Meng. “But this new species provides another example of extreme adaptation and shows that animals, even those that are phylogenetically related, can evolve in completely different directions.”

In the mid-Miocene, the present desert habitat of North China was a hot and humid place where a diverse set of species lived. Meng and his collaborators followed a series of clues to reconstruct the history of d.xiezhi. They analyzed the enamel of one tooth and performed CT scans of two skulls to learn about their internal structure. The researchers also compared the remains of the animal with the fossils of more than 50 species they found in the same area, most of them ungulates, such as d.xiezhi. Taken together, the evidence suggested that d.xiezhi it shared certain morphological characteristics with modern giraffes and that it was a herbivore, which may have fed on a mixture of leafy and herbaceous plants.

d.xiezhi It wasn’t a very bulky animal, perhaps the size of a large sheep, but Meng and his collaborators found that the species’ head and neck were among the strongest ever in a mammal, and perhaps in any earlier creature. The researchers characterized d.xiezhi for possessing “the most complex head-neck joints observed in mammals to date”.

To get an idea of ​​the extreme morphology of the head of d.xiezhiit is interesting to compare it with that of Pachycephalosaurus, a dinosaur known for powerful headbutts (its name means “thick-headed lizard”). Dinosaur experts with whom Meng and his collaborators consulted confirmed that the unique structure of the head and neck of d.xiezhi it would have allowed him to withstand an even greater force.

Current male giraffes (genus Giraffa) also fight fierce battles for females. But although d.xiezhi share family tree with Giraffa, modern giraffes are not direct descendants of the ancient species. In their fights, male giraffes use their necks instead of their heads. Its elongated neck, the group explains in their article, could have evolved for fighting and not just reaching for foliage. “In this case, as in other classic examples, the behavior could have had a notable influence on the morphological evolution […] The extreme behavior of the giraffes would have led to an extreme morphological evolution”, they point out.

“The bottom line is that the head-neck structure of the different giraffe families is highly diverse, as evidenced by the new fossils,” says Meng. “These specialized morphologies reflect the diverse lifestyles of these animals.”

Advait Jukar, a paleobiologist at Yale University who was not involved in the study, notes that the evolutionary cause of modern giraffes’ long necks remains an open question, since female giraffes also have elongated necks, and both males and females have elongated necks. females have equally long limbs. “Actually, the evolution of the neck and limbs of modern giraffes is most likely due to a combination of natural selection […] regarding a food preference and a sexual selection in that lineage”, he summarizes.

However, as for d.xiezhi, “Their head shell almost certainly evolved as a result of sexual selection and male-male combat,” says Jukar. “If modern giraffes seem peculiar to you, their relatives from ancient times were even stranger.”

Rachel Nuwer

Reference: “Sexual selection promotes giraffoid head-neck evolution and ecological adaptation». Shi-Qi Wang et al. in Science, vol. 376, art. 6597, June 3, 2022.

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