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Three-toed sloth Photo Source: Unslpash sloth is extremely lazy, but its arms are strong, which is a survival strategy.

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"lazy" is not a beautiful word, but many countries will use it to refer to a kind of animal that always smiles and is loved by people-sloth (sloth). Sloths have the blood of "life lies in lying flat" in their bodies, so their lives are extremely simple. However, they also rely on "slow" to become an evolutionarily successful animal that has survived since 60 million years ago.

The sloth lives in the rainforests of South and North America. Compared with other animals here, the sloth hangs motionless from trees most of the time. If you move, moving about 4 meters per minute, the speed to the ground will be even slower, only 2 meters per minute-yes, they are the slowest mammals in the world.

Do not exercise, but the muscles are very strong according to our understanding, so lazy and motionless, even if there are muscles will degenerate. Sloths' muscles account for 23.6% of their total body weight, which is significantly lower than other arboreal mammals (average muscle mass is 33%) and surface mammals (48%). Compared with mammals of the same size, sloths have about 30% less muscle mass.

But paradoxically, they can easily hang their whole body from a branch with one arm for more than 10 minutes-a move that only a few people can do. In addition, people with exercise experience will know that the slower some movements are done, the more laborious they will be. But the slow motion that sloths can do can even make gymnasts jealous. Some scientists have found that special tendons on the sloths' hands and feet help them fix them, allowing them to hang upside down for a long time without wasting any energy.

Photo Source: Pixabay these have aroused the interest of some scientists who are sloth enthusiasts, who want to uncover the weak side beneath their seemingly weak appearance. In a 2017 study published in the Journal of Mammal Evolution (Journal of Mammalian Evolution), the scientists found that a three-toed sloth, Bradypus variegatus, has special muscle tissue in its forearm that gives them a strong grip. Recently, a study published in the Journal of Zoology (Journal of Zoology) revealed that such sloths have twice as strong a grip as all people.

Sloths can do this for an obvious reason: under their thick, fluffy fur, their bodies are surprisingly thin. In addition, some scientists have observed that they have particularly inconspicuous banded muscles, which are small but powerful and resistant to fatigue. How do you do that?

The three-toed sloth designed to save energy (with three toes on both forelimbs and hind legs) is usually a strict vegetarian. When they are well nourished, the food in their stomachs can account for 2 to 3 of their body weight. These vegetarians rely on symbiotic microbes in their stomachs for slow digestion, which can take up to a month or more. By contrast, their close relatives, the two-toed sloth (2 toes in the forelimb and 3 toes in the hindlimb) are omnivores and have more diverse ways to get energy.

The three-toed sloth's daily metabolic rate is very low, its body temperature is lower than that of other mammals (which is why the sloth likes to bask in the sun), and it is impossible to have extra muscles to waste energy. How to maximize the utilization of limited energy and lie flat is the life problem they face. Fortunately, they solve the problem very well.

In 2015 and the year after, researchers dissected the bodies of three adult three-toed sloths and one juvenile three-toed sloth, some of which died of natural causes and the rest euthanized for reasons other than the study.

They observed and recorded the muscles of the sloths' strongest forearms and found that they had 52 muscles on their forearms. The muscle weight of one forearm is equivalent to about 5% of body weight. Among them, the flexor muscle develops better than the extensor muscle, and the total mass accounts for 60% of the whole forearm. In other words, the muscles used by the three-toed sloth to grasp and pull objects are much more prominent than those that push objects. However, sloths rarely need to push things, so there is no need to waste energy to maintain this part of the muscle.

In addition, from their shoulders to the forelimb area and the hands, the muscles gradually become more feathered (arranged at an angle rather than along a straight line of the muscles). The increase of muscle fiber plume may partly make up for the defect of low muscle mass of the whole forelimb. When the fiber is more feathered, it can produce more force, in this case, it can promote the traction movement of the sloth and produce incredible grip strength.

The picture of the three-toed sloth scratching branches of different diameters comes from the paper. In the new study, the researchers also made some special findings: not all muscles work as expected. They found that muscle groups are paired in an unusual way: one muscle group allows the joint to rotate quickly, but the other muscle group working next to it produces a lot of slow and steady force. These muscles with opposite characteristics will work together in this unique way, weighing each other. As a result, the sloth can behave slowly and calmly, and can well control its own movements.

In a recent study, scientists also tested the grip strength of five three-toed sloths with a special device and found that their grip strength was about twice that of humans and many other primates. In addition, they found that such sloths can hold more than 100% of their body weight with one hand. And they all showed the characteristics of left-handed people in the experiment. On average, there is a 16% difference in grip strength between left and right hands.

The thrill of a slow life there are few special moments in the life of the sloth. However, when the speed is slow to a certain extent, some ordinary things will be full of excitement. There are two major threats to the life of the sloth: one comes from the predator eagle in the sky, and the other comes from its own stubborn habit of defecating at a fixed point (one defecation is equivalent to 1 inch of body weight). The latter is even more harmful than the former.

The three-toed tree sloth will climb from the tree to the ground to defecate every 8 days or so, will go to a fixed position, and will pay more attention to bury the feces after the discharge is completed. Because of their weak hind legs, they travel at a speed of only 2 meters per minute on the ground. As long as their natural enemies understand this habit, it is easy to wait and prey on them. Scientists have a few guesses about their persistence: return the trees that give them food, or help a moth that lives in its fur lay eggs. (to avoid these dangers, their close relative, the two-toed sloth, will try other options, such as defecating directly in the tree like a bird. )

Two-toed sloth Photo Source: Unslpash only talks about excitement and has to mention the sloth's "passionate" sex life. When a sexually mature female three-toed sloth is in estrus, it makes a loud cry, which attracts many male three-toed sloths around it. In order to gain the right to mate, these male three-toed sloths fight each other, conceivably violently. Then, successful males have a chance to mate with females, but only for a short time, only one minute. But according to the video, they may hang upside down, face to face or dangle from branches to mate-in any position they like.

Within a few days, successful males will guard the females, mate with them many times, and repel other males. Losers make loud cries like female sloths to express their pain. But as long as they are willing to wait, they still have a chance. Because after this strenuous activity, successful males need to take a nap from time to time to regain their energy, and then the female sloth will mate with other males. "promiscuity" is common in sloths, with both females and males mating with the opposite sex as much as possible.

But even if they try so hard, the female three-toed sloth will eventually give birth to only one child (one after each conception).

Behind most male three-toed sloths is a unique "speculum" coat with black stripes and bright (yellow or orange) fur. It is also easy for humans to distinguish them. However, the male and female two-toed sloth look almost the same, and the female pregnancy is basically asymptomatic, so there is the story of raising "the same" sex sloth together and giving birth to a baby sloth.

Sloth: I am not slow, I am calm under precise control.

Reference link:

Https://slothconservation.org/think-stronger-sloth/

Https://slothconservation.org/10-incredible-facts-about-the-sloth/

Https://link.springer.com/article/10.1007/s10914-017-9411-z

Https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/jzo.13041

This article comes from the official account of Wechat: global Science (ID:huanqiukexue)

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