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Hanging around: why do sloths move so slowly?

Welcome to the strange, slowed-down world of sloths

By Adam Dorey

Some of us would enjoy the lifestyle of a sloth; no pesky commitments other than hanging from a branch all day – and imagine getting ten hours of sleep a night! It is common knowledge that sloths move slowly; the Spanish word for sloth, ‘perezoso’ literally translates as ‘lazy one’. But what is it about their biology that enables such sluggishness?

Sloths are not just slow in their behaviour, but their low metabolism means that their lethargy extends to a cellular level. All organisms have a basal metabolic rate (BMR), which is the rate at which a resting organism uses energy to keep their vital bodily processes functioning. What is unique about sloths is that their BMR is half the rate that would be expected of an average mammal of the same size. This explains why sloths are slower than the average mammal; it takes a long time for them to ‘turn over’ energy at the cellular level.

Another factor is how sloths respond to temperature changes in their environment. Sloths are heterothermic, meaning their core body temperature can fluctuate to about 5 degrees Celsius either side of their average. Initially, their temperature increases with the environment, but after reaching an optimum, the correlation stops and contrary to the rising environmental temperature, theirs decreases. No other mammal can actively decrease their core temperature in this way. Again, this enables the lowest possible energy expenditure, slowing down their cellular chemical reactions, which in turn slows down their behaviour. 

As sloths have evolved over time, one unique anatomic adaptation explains why 90% of their time is spent hanging upside-down. Scientists had wondered how sloths could spend so long upside-down – surely the weight of their abdominal organs would put too much pressure on their lungs for them to breathe easily. To mitigate this ‘squash effect’, researchers found that sloths have adhesions composed of fibrin (a fibrous protein involved in scab formation in humans), which attach a sloth’s abdominal organs to its lower ribs. When a sloth is upside-down, these organs are held in place above its lungs, letting sloths breathe a little more easily as they hang around.

One slightly gross consequence of slowness in sloths is that it allows a diverse variety of other organisms to live on their fur, including algae, moths and detritivorous fungi. One species of pyralid moth exclusively lives in sloth fur, even using it as their mating ground! (Imagine having insects getting freaky in your hair. Nasty.) Continuing with the grossness, as sloths descend from their tree to defecate, female moths lay their eggs in the sloth’s faeces, where their larvae can live a happy, if smelly, childhood. As these moths shuttle from the loo to their furry home, they act as nutrient portals, providing nutrients for commensal algae, which sloths use as a lipid-rich food source. An entire ecosystem functions in a sloth’s very own fur, resulting in a three-way mutualism where sloths can receive tasty nutrients from algae whilst little moth babies are being made – all enabled by physiological and anatomical features that slow these leisurely mammals right down.

From Issue 16

Photos courtesy of Lucy Cooke, Founder of Sloth Appreciation Society.

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