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A study on sloths a slow moving mammal

Messenger Conventional wisdom has it that sloths are simple, lazy creatures that do very little other than sleep all day. It seems astonishing that such an animal survives in the wild at all.

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In 1749, French naturalist Georges Buffon was the first to describe the creature in his encyclopedia of life sciences, saying: Slowness, habitual pain, and stupidity are the results of this strange and bungled conformation. These sloths are the lowest form of existence. One more defect would have made their lives impossible. Given such a precedent, it is of little surprise that sloths are subject to such profound speculation and misinterpretation, ranging from the benign — that they sleep all day — to the creative anecdotes I regularly hear, such as: The truth is that sloths are incredibly slow movers, but for a very simple reason: The fact that slow sloths have been on this planet for almost 64m years shows that they have a winning strategy.

But in order to understand exactly what it is that makes them such slow movers, and why this works so well, we have to look at the biology of these unusual animals in more detail.

Plenty of time to hang around for this little sloth. They do move, but very slowly and always at the same, almost measured, pace. Sloths are not the only creatures in the animal kingdom to adopt a slow pace.

Cold-blooded ectotherms such as frogs and snakes, are commonly subject to enforced slow movement when faced with cold temperatures, due to their inability to regulate their own temperature independently of the environment. Just like any chemical reaction, cold muscles are slow muscles so cold reptiles are slow reptiles.

This is in stark contrast to most homeothermic mammals which maintain a stable, high core temperature via a process of adaptive thermogenesisand are consequently able to move fast and effectively regardless of the ambient conditions.

But this athletic ability comes at a cost: So where do sloths fit into this dichotomy?

  • The truth is that sloths are incredibly slow movers, but for a very simple reason;
  • But the truth about sloths is that humans have done a very bad job at figuring out why they do what they do.

They move slowly at all temperatures and, unsurprisingly, deviate from the typical homeothermic mammalian plan by operating at lower body temperatures than most mammals, while apparently having a reduced ability to thermoregulate. The average temperature of the three-toed sloth is around 32.

Both two and three-fingered sloths have a predominantly folivorous leaf-based diet, consuming material with a notably low caloric content. There are plenty of other mammals which specialise on a leaf-based diet, but usually these animals compensate for their low-calorie diet by consuming relatively large quantities of food.

Fellow leaf-eating howler monkeys move at a normal pace but consume three times as many leaves per kilogram of body mass as sloths, digesting their foodstuff comparatively quickly.

Therein lies another sloth peculiarity: Sloths appear to break this rule to an unprecedented extent. The exact rate of digestion remains unclear, but current estimations for the passage of food from ingestion to excretion range from 157 hours to a staggering 50 days 1,200 hours.

Food intake and, critically, energy expenditure are likely limited by digestion rate and room in the stomach. A sloth sports its backpack tracker. Author provided All this points to an extraordinary lifestyle, with sloths living on a metabolic knife edge where minimal energy expenditure is finely balanced with minimal energy intake.

The truth about sloths

And with this, they do not have the capacity to defend themselves or run away from predators, as a monkey might. Instead, their survival is entirely dependent upon camouflage — a factor aided by their symbiotic relationship with algae growing on their fur. They are energy-saving mammals taking life at a slow pace to avoid the rush and tumble for food, while subscribing the movement patterns that help them avoid being identified as prey.

There must be a lesson somewhere in that for all of us.