He doesn't exercise for even a second, but his grip strength is twice that of anyone else. Who is he?

Three-toed sloth Image credit: Unslpash

Sloths are extremely lazy, but have very strong arms, which is a survival strategy.

"Laziness" is not a beautiful word, but many countries use it to refer to a kind of animal that always smiles and is loved by people - sloths. The blood of sloths flows in their bodies, and their lives are extremely simple. However, they have also become a very successful animal in evolution by being "slow", and have survived from 60 million years ago to the present day.

Sloths live in the tropical rain forests of South and North America. Compared with other animals here, they spend most of their time hanging on trees without moving. If they move, they move about 4 meters per minute, and the speed on the ground will be even slower, only 2 meters per minute - yes, they are the slowest mammals in the world.

Part 1

No exercise, but strong muscles

As we understand it, if they are so lazy and motionless, even if they have muscles, they will degenerate. Sloths have muscles that account for 23.6% of their total body weight, which is significantly lower than other tree-dwelling 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 entire body on a branch with just one arm for more than 10 minutes - a move that only very few people can do. In addition, people who have exercised will know that the slower some movements are done, the more strenuous they will be. However, the slow movements that sloths can do would even make gymnasts jealous. Some scientists have discovered that special tendons in the hands and feet of sloths help them stay fixed, allowing them to hang upside down for a long time without wasting any energy.

Image source: Pixabay

All of this has aroused the interest of some scientists who are sloth enthusiasts. Scientists want to uncover the weak side of their seemingly fragile appearance. In a study published in the Journal of Mammalian Evolution in 2017, these scientists discovered that a special muscle tissue in the forearm of a three-toed sloth, Bradypus variegatus, allows them to generate great grip strength. Recently, a study published in the Journal of Zoology revealed that the grip strength of this type of sloth can be twice that of a human.

There's an obvious reason sloths can do this: beneath their thick, shaggy fur, they're surprisingly thin. In addition, some scientists have observed that they have unusually small girdle muscles that are few, powerful, and resistant to fatigue. How do they do this?

Part 2

A body designed for energy conservation

Three-toed sloths (three toes on both forelimbs and hind limbs) are usually strict vegetarians. When well-nourished, they can fill their stomachs with food that accounts for two-thirds of their body weight. These vegetarian foods need to be slowly digested by the symbiotic microorganisms in their stomachs, which can take up to a month or even longer. In contrast, their close relatives, two-toed sloths (two toes on the forelimbs and three toes on the hind limbs), are omnivores and have more diverse ways of obtaining energy.

The daily metabolic rate of three-toed sloths is very low, and their body temperature is lower than that of other mammals (this is why sloths like to bask in the sun), and they are unlikely to have extra muscles to waste energy. How to maximize the utilization of limited energy while still being able to lie flat is the life problem they face. Fortunately, they have solved this problem very well.

In 2015 and the following year, the researchers dissected the bodies of three adult three-toed sloths and one juvenile three-toed sloth. Some of the sloths died of natural causes, while the rest were euthanized for reasons other than research.

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

Additionally, the muscles gradually become more feathery (arranged at an angle rather than in a straight line) from their shoulders to the forelimb region and hands. This increase in muscle fiber feathering may partially compensate for the low overall muscle mass in the forelimbs. When the fibers are more feathery, they can generate more force, which in this case facilitates the sloth's pulling movement and incredible grip strength.

Three-toed sloths grasp branches of different diameters. Image from the paper

In this new study, the researchers also made some special discoveries: not all muscles work the same way as expected. They found that muscle groups are paired in an unusual way: one muscle group allows the joint to rotate quickly, but another muscle group working next to it can produce a lot of slow and stable force. These muscles with opposite characteristics work together in this unique way, playing a role of mutual trade-offs. Therefore, sloths can be slow, leisurely, and well controlled in their movements.

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

Part 3

The thrill of slow living

There are almost no special moments in the life of a sloth. However, when the speed slows down to a certain extent, some ordinary things will also be full of excitement. There are two major threats to the life of a sloth: one is the predator eagle in the sky, and the other is its own stubborn habit - it will defecate at a fixed location (the amount of defecation is equivalent to 1/3 of its body weight at a time). The latter is even more harmful than the former.

Three-toed sloths will crawl from trees to the ground to defecate every 8 days or so. They will go to a fixed location, and after defecation, they will be more particular about burying the feces. Because their hind legs are weak, their speed on the ground is only 2 meters per minute. As long as their natural enemies understand this habit, it is easy for them to sit back and wait and prey on them. Scientists have several guesses about their persistence: to reward the trees that give them food, or to help a moth that lives in their fur to lay eggs. (In order to avoid these dangers, their close relatives, two-toed sloths, will try other options, such as defecating directly in trees like birds.)

Two-toed sloth Image credit: Unslpash

But when it comes to excitement, we have to mention the "passionate" sex life of sloths. When a sexually mature female three-toed sloth is in heat, it will make a loud call, and this sound will attract many male three-toed sloths around. In order to obtain the right to mate, these male three-toed sloths will fight with each other, and as you can imagine, it will be very fierce. Afterwards, the successful male will have the opportunity to mate with the female, but the mating time is very short, only one minute. However, according to the video footage, they may mate upside down, face to face, or hanging precariously on the branches-in any posture they like.

For days, the successful male will stay by the female's side, mate with her multiple times, and fight off other males. The losers will make loud noises (crying) similar to the female sloths to express their pain. However, as long as they are willing to wait, they still have a chance. Because after this strenuous activity, the successful male needs to rest from time to time to recover his energy, and then the female sloth will mate with other males. "Promiscuity" is common among sloths, and both females and males will mate with as many opposite sexes as possible.

However, even if they try so hard, female three-toed sloths will only give birth to one litter (one litter after one conception).

Most male three-toed sloths have a distinctive "speculum" of fur on their backs, with black stripes and brightly colored (yellow or orange) fur. It's easy for humans to tell them apart. However, male and female two-toed sloths look almost identical, and female pregnancy is asymptomatic, so there are stories of "same" sex sloths being raised together and giving birth to baby sloths.

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

Reference Links:

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

Source: Global Science-Popular Science China-Starry Sky Project (Creation and Cultivation)

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