Do you think you are stronger than a sloth?

Do you think you are stronger than a sloth? Probably. Sloths are often perceived as being simple, lazy animals that do little other than sleep all day. In fact, you might find it downright insulting if somebody suggested that a sloth could beat you in an arm-wrestle. However, you may be surprised to know that sloths are, in fact, incredibly strong. Despite having 30% less muscle mass compared to other similar-sized mammals, we assume that sloths are weak, and that is just another example of humans underestimating what it really means to be a sloth.

Brown-throated Three-toed Sloth Bradypus variegatus Mother with newborn baby (less than 1 week old) climbing tree Aviarios Sloth Sanctuary, Costa Rica
© Suzi Eszterhas

If you have ever watched a sloth move through the trees, you will know that they can execute extraordinary slow-motion acrobatics that would make any gymnast jealous. A sloth can easily suspend its entire body weight from a single limb, holding it at a 90-degree angle for over ten minutes. They can hold the crucifix position, suspended between two tree branches for extraordinary lengths of time and their grip strength can withstand the force of a harpy eagle trying to rip them from the tree. However, underneath that shaggy coat of fur, sloths are surprisingly skinny with particularly unimpressive ribbon-like muscles (kind of like a junior high student in a track suit in the weight room). So how, then, do they manage to perform such great feats of strength and stamina?

To answer this question, we collaborated with muscle physiology expert Dr Michael Butcher and his team from Youngstown State University and the Sloth Sanctuary of Costa Rica. The resultant research paper has just been published in the Journal of Mammalian Evolution. By meticulously dissecting, mapping, weighing, measuring and sampling each of the 52 muscles that can be found in the arm of a sloth, we were able to shed some light on the sloth’s mysterious movement. As the technical terminology in the research paper can be difficult to understand or interpret for anyone who hasn’t been trained in physiology, we will simplify and summarise the main findings below.

Brown-throated Three-toed Sloth Bradypus variegatus Male Aviarios Sloth Sanctuary, Costa Rica *Digitally removed highlight in background
© Suzi Eszterhas

The muscles that sloths use to grip and produce a pulling motion are much more prominent than those that produce a pushing motion. Muscle tissue requires a lot of energy to produce and maintain, and this arrangement means that the sloths muscle mass is concentrated and limited only to the areas where it is most important. This simply means that sloths primarily use their arms to pull themselves upwards / along a tree branch, or to pull a branch towards the body for feeding or traversing a gap. They rarely have any need to push anything, and so maintaining muscles for this purpose would simply be a waste of energy. The arrangement of the individual fibres within the muscle also support this function, with the fibres becoming progressively more pennate (i.e. arranged at an angle rather than along the length of the muscle) towards the hand. When the fibres are more pennate, it means that they can produce more force, and in this instance, it would facilitate the strong pulling motion and incredible grip strength seen in sloths.

However, this research also threw out some surprises as not all muscles show the expected architecture. Instead, what appears to be happening is that the muscle groups responsible for movements are paired in unusual way so that muscles working together have opposing functions. I.e. one muscle group has properties that allow the joint to rotate quickly, while another muscle group working alongside it has properties to produce a lot of slow and steady strength. Muscles with opposing features working together in this unique way act as a trade-off and produces in the slow, deliberate and controlled movements shown by sloths.

Brown-throated Three-toed Sloth Wild sloth climbing trees in forest at Sanctuary Aviarios Sloth Sanctuary, Costa Rica
© Suzi Eszterhas

While these findings explain how the sloths manage to move in the way they do, it does not explain how their muscles are able to resist fatigue or how they are able to support the body weight for such prolonged periods. It does tell you, however, to never get in a pull-up or arm wrestling competition with a sloth. The sloths surprising stamina is in fact linked to special metabolic properties of sloth muscles and is the topic of our next research paper due to be published this month!

~ Rebecca Cliffe 

Founder and Executive Director
** All sloths used for the completion of this research died of natural causes