If you spent a lot of your life hanging upside-down, you’ve got a problem. Your liver, kidneys, stomach and other internal organs would press down upon your lungs and diaphragm, making it marginally harder to breathe. This won’t matter in the short term but in the long run, you’d spend a lot more energy on the simple act of inflating your lungs.
So, how do sloths cope? They live life in the slow lane, and they don’t have a lot of spare energy to play with. They also spend a lot of time hanging upside-down from branches. Cartoon drawings usually have them hanging horizontally, which isn’t a problem. But if you spend enough time watching wild sloths, as Rebecca Cliffe from Swansea University has done, you’ll see that they often hang vertically upside-down, to reach the youngest and most tender leaves on the furthest, smallest branches.
Cliffe knows their secret. She has discovered that sloths effectively tape their internal organs to their ribs and hips, preventing them from pressing down on the lungs.
Cliffe spends much of her time in Costa Rica’s Sloth Sanctuary, where she tries to understand the lives of these little-studied but adorable animals. The sanctuary has been caring for rescued sloths for more than 20 years, and the staff often have to dissect individuals that die of natural causes. In the process, they noticed thin fibrous sheets attached to the sloths’ internal organs, rather like the membrane that connects your tongue to the bottom of your mouth. “They’re stringy, tough and a little rubbery,” says Cliffe.
At first, the team assumed that these “fibrinous adhesions” were scar tissues from previous injuries. But slowly, they noticed that the adhesions were always in the same place. They anchor the liver and stomach to the lower ribs, and the kidneys to the hip bones. They’re not scars. They’re normal parts of the sloths’ bodies, and they ensure that the ribs always bear the weight of the viscera.
The team also measured the lung capacity and breathing rates of their sloths, and made them wear small data-loggers to work out how much time they spend upside-down. Based on their data, they estimated that an inverted sloth would spend between 7 and 13 percent more energy on breathing if it wasn’t for its adhesions.
“For many mammals, an energy saving of 7 to 13 percent wouldn’t be particularly game-changing,” says Cliffe. In fact, some mammals like howler monkeys do regularly hang inverted without any organ-anchors. The difference is that “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 to 13 percent is quite a big deal.”
They also take a very long time to digest their food, and they only urinate and defecate once a week. Most of the time, a sloth is carrying a third of their body weight in waste matter, which means that its stomach and bowels are very heavy. “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,” says Cliffe.
So why don’t other mammals, like the howler monkeys, have similar anchors? Saved energy is still saved energy, after all. One answer might be that anchored organs make an animal less flexible. That would be a deal-breaker for a mobile monkey, but it’s a non-problem for a slothful sloth.
Reference: Cliffe, Avey-Arroyo, Arroyo, Holton & Wilson. 2014. Mitigating the squash effect: sloths breathe easily upside down. Biology Letters http://dx.doi.org/10.1098/rsbl.2014.0172
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