LIFE SCIENCE

The “inner volume” of the animal kingdom is the real “volume”


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Ball Golden Turtle Zhang Weiwei photography

Humans are “rolled inside”, animals are also “rolled inside” – the master can even roll himself into a “ball”. Armadillos, pangolins, hedgehogs, ball horses, ball rat women, etc., can quickly roll into tight spheres when encountering danger, protecting vulnerable parts such as abdomen and limbs.

“This behavior even has a technical term — Conglobation.” Bai Ming, a researcher at the Institute of Zoology of the Chinese Academy of Sciences, told China Science Daily.

Recently, a paper led by the Institute of Zoology of the Chinese Academy of Sciences and co-authored by scientists from China, Italy and Germany was published in the journal Communications Biology. The paper targets a tiny insect, the bulbous golden turtle, revealing the secret of their “ball-forming.”

What is the difference between “ball” and “non-ball”?

In various animal groups, the body structure of insects is not suitable for globules.

“Because their torsos are only three segments, head, thorax, and abdomen, it is difficult to form a tight spherical shape. In most cases, the ventral surface, feet, and other parts of the body will inevitably be exposed. Bai Ming explained that he was one of the co-corresponding authors of the paper.

But there are some little guys who do.

Bulbophyllum is a collective name for insects of the family Golden Turtle, the family Camelback, and the subfamily Of The Golden Turtle, and is known to have 448 species in 44 genera and distributed in the pantropics. These insects are rare and live mainly in hidden habitats such as termite nests, dead branches and deciduous leaves, and decaying wood.

Although they are all called ball golden turtles, their difference in skills is still relatively large. Some bulbous golden turtles can only form loose spherical shapes, or cannot form balls at all, while others can form tight spherical shapes.

What is the difference between “ball” and “non-ball”?

The researchers quantitatively analyzed the morphological characteristics of 74 species of bulbous golden turtles around the world through geometric morphology, and found that among the three different types of spheres (cannot be formed, loose spherical, compact spherical), the structure of the head, forebreast dorsal plate, elytra wings and feet of the bulbous golden turtles had stable differences.

Further combining micro-CT and three-dimensional reconstruction techniques, the researchers found for the first time that compact bulbous golden turtles have abnormal growth in body wall thickness, that is, the outer surface of the ball (anterior thoracic dorsal plate and elytra) is significantly thicker than the inner body wall (thorax and abdominal ventral surface), and their body wall thickness is also significantly different from that of golden turtles that cannot become balls.

“We speculate that this may be a trade-offs strategy in the case of limited energy.” Lu Yuanyuan, the first author of the paper and a postdoctoral fellow at the Institute of Zoology of the Chinese Academy of Sciences, said, “When the materials that make up the body wall are limited, more material is distributed to the outside of the ball, and the body wall composition materials that are wrapped in the ball are relatively small.”

Where does ball-forming behavior come from?

Prior to this, the evolutionary history of the ball-forming behavior of the bulbous golden turtle was not clear. The only fossil record suggests that loose spherical species may have first appeared in the Cenozoic Era (about 15 million to 25 million years ago).

This time, the researchers investigated in detail the fossils from different origins and geological times. Fortunately, they found the presence of bulbous golden turtles in both Kachin amber and Dominican amber, and for the first time recorded new species of loose spherical shapes in the Mesozoic Era (about 100 million years ago), as well as new species of intact spherical shapes in the Cenozoic Era.

These findings not only advanced the origin of the ball-forming behavior of the bulbule turtle by about 80 million years, but also one of the earliest records of the globulizing behavior among insects.

The fossils initially connected the ball path of the ball turtle, revealing the corner of the glacier for the evolution of the ball behavior.

“Our findings suggest that the sphere-forming behavior is of a single origin in the bulbous golden turtle, i.e., the evolutionary sequence from non-spherical to loose spherical and then to tight spherical, which is also accompanied by inversion on a small number of nodes.” Lu Yuanyuan said.

Body structures closely related to globules include the head , the dorsal plate of the forebrea , and the elytra , which do not evolve in the same way. Zou Zhengting, an important author of this article and a researcher at the Institute of Zoology of the Chinese Academy of Sciences, said: “The Mesozoic ball golden turtle first experienced the rapid adaptation and evolution of the elytra, laying the foundation for the formation of the ball behavior, followed by the dorsal plate of the forethia, and finally the rapid evolution of the glans of the Cenozoic ball golden turtle made up a key link for the formation of a compact spherical shape.”

Why become a ball?

The seemingly simple roll of ball-forming behavior has such a complex structural evolution behind it. Why did animals evolve such behavior?

“Scientists speculate that glomerating behavior may have defensive and physiological functions, such as water retention and temperature regulation.” Lu Yuanyuan said, “For example, the rat woman that many people are familiar with lacks the valve system that terrestrial arthropods should have, so it is necessary to rely on balls to slow down the dispersion of water in the body, so as to adapt to life after landing.” ”

For the ball turtle, resistance to predators is also a very important factor. The researchers selected the ball golden turtle and two other kinds of golden turtles, the camel golden turtle and the gill golden turtle, carried out mechanical analysis, and found that in the specific scene of being directly attacked by natural enemies, the ball golden turtle curled into a ball can resist greater bite force without deformation, showing more “anti-beating” characteristics.

“The ‘anti-beating’ characteristics of these bulbous golden turtles are due, on the one hand, to the spherical structure itself, and on the other hand, to the thicker outer body wall, as well as the strong mechanical properties of the body wall itself.” Another co-corresponding author, Gu Qi, a researcher at the Institute of Zoology of the Chinese Academy of Sciences, said.

Interestingly, the loose spherical species of the bulbous golden turtle mainly live in the deciduous layer of dead branches and the compact spherical species mainly live in termite nests, which led to the “attacker pressure” hypothesis – the threat from termites drives the complication of the ball-forming behavior of the ball golden turtle.

It has also been found that when some of the bulbous golden turtles that live in the canopy are disturbed, they will quickly form balls and fall and disappear. “For those who live in open environments, the spherical shape helps it to escape quickly by rolling. However, we believe that this is more likely to be the secondary adaptive evolution of the glomerular behavior of compact spherical individuals in re-adapting to the open environment. Bai Ming said. (Source: China Science Daily Li Chenyang)

Related paper information:https://doi.org/10.1038/s42003-022-03685-2



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