Sloths are known to spend a significant amount of their time hanging vertically upside down, yet no one has ever discovered how they manage this. If we were to stand on our heads for several hours a day, we would probably end up with a lot of problems. In this publication, we solve part of the mystery!
Three-fingered sloths have multiple unique fibrinous adhesions that effectively anchor their abdominal organs against the lower ribs. These evolutionary ‘coat hangers’ support the weight of the sloths stomach and bowel whilst the animal is hanging inverted, preventing the lungs from being squashed and facilitating breathing. Offering further support to this hypothesis, we discovered that the kidneys were bound by connective tissue against the pelvic girdle with absolutely no mobility. This unusual configuration means that they too are exempt from contributing to the abdominal weight.
We used ‘Daily Diary’ data loggers on both wild and captive sloths to determine the proportion of time spent inverted. Combining the hang-angle preference information with ventilation rate data, we use an energetics based model to estimate that the presence of the adhesions could reduce a sloths energy expenditure by 7- 13% when hanging upside down!
Of course, these adhesions don’t solve all of a sloths upside-down problems. Indeed, we are discovering that both two and three-fingered sloths have a whole array of fascinating anatomical features that allow them to hang inverted for up to 90% of the day. These include circulatory adaptations that prevent the blood rushing to the head, and a highly specialised oesophagus that allows them to swallow whilst upside down – just to name a few!
How did you first discover the adhesions?
Discovering the internal adhesions was actually quite a slow process. The Sloth Sanctuary has been rescuing sloths for over 20 years and during that time, they have had to perform many necropsies on sloths that didn’t survive. At first, because sloth anatomy was still poorly understood, the adhesions were assumed to be scar tissue from previous injuries. It was only when we began to document the exact location of the attachments that we discovered they were a general feature of sloth anatomy. It was then a case of working backwards and figuring out what the adaptive benefit could be. Interestingly, two-fingered sloths (Choloepus) actually have more of these internal adhesions than the three-fingered sloths – probably because they spend a higher proportion of time hanging upside down.
Why do you think the complications of hanging upside down have never been considered previously?
I am sure that someone somewhere has briefly pondered this issue in the past, but it has never been followed up or solved. I think this is partly due to the difficulties of studying sloths in the wild and the lack of reliable scientific data documenting their natural behaviours. The typical sloth hanging position (horizontal) and sleeping position (sitting vertically) don’t result in any gravity based problems. It takes a good amount of time spent observing them in the wild to see that they do hang vertically upside down, and more importantly, they need to. Sloths need to eat the young, tender leaves that grow on the outer tips of branches because they are easier to digest and often have a lower toxin content. In order to reach these new leaves, sloths frequently have to hang inverted.
Why are these adhesions so important for sloths?
For many mammals, an energy saving of 7-13% wouldn’t be particularly game changing. In fact, there are several groups of mammals (howler monkeys, for example) that do regularly hang inverted, yet don’t have any internal adhesions to support the organs. The difference with sloths however is a very tight energy budget. Because of their extremely slow metabolic rate and unusually low body temperature, it can take a sloth up to 30 days to digest a single leaf. Having a constantly full stomach means that they really can’t ingest very much on a daily basis and so their caloric intake is very low. Sloths have virtually no flexibility in their energy budget: they generate just about enough energy from their diet to move when and where required, but there is not much left in the tank afterwards. To a sloth, an energy saving of 7-13% is quite a big deal.
In addition, sloths can store up to a third of their body weight in urine and faeces before defecating – this means that the stomach and bowel contents make up an unusually large proportion of their body mass. With their limited energy supply, it would be energetically very expensive, if not completely impossible, for a sloth to lift this extra weight with each breath were it not for the adhesions.
You can view the paper online here.