'Herman', a two-toed sloth at the Detroit zoo.
Credit: Wikimedia
John Nyakatura feeds 'Julius', a two-toed sloth. Nyakatura used X-ray video to observe the sloth's locomotion and anatomy, gaining new insight into sloth movement and behaviour.
Credit: John Nyakatura / Friedrich Schiller University Jena
X-rays indicate that the sloth's shoulder is quite 'loose', able to dislocate freely to achieve a wide range of motion with little energy exertion.
Credit: John Nyakatura / Friedrich Schiller University Jena
CAMBRIDGE: The key to the sloth's 'slothfulness' has been discovered - they've evolved to use gravity to their advantage.
Researchers in Germany have investigated the movement of two-toed sloths using X-ray video to find that despite spending most of their lives upside-down, the sloth's locomotion and movement patterns are more typical than previously thought.
The findings, published in the Journal of Anatomy, also suggest that the sloth's specialised anatomy led to their characteristic energy-conserving behaviour.
"The locomotion of sloths is basically not so different from the locomotion of other mammals, like monkeys," said study researcher John Nyakatura, a biologist at the University of Jena in Germany. "The adaptation to the slow, energy-saving way of movement occurred solely through [the sloth's] anatomy."
Evolving upside-down
Modern two- and three-toed sloths are unique in that they both evolved to live upside-down independently. Due to their low metabolism, sloths could rely on low energy foods - such as leaves - when they started living in trees.
And since leaves are so abundant in this environment, it was unnecessary to search for or exploit other food sources, so there was no need to evolve physiological features for high-energy exertion. As a result, sloths evolved a lifestyle similar to Australian koalas - long resting hours and slow movements.
Yet, unlike the koala, sloths were able to solve the 'expensive' energy problem of having to balance on small branches through an ingenious adaptation - hanging upside-down.
With hooked limbs and certain muscular specialisations, sloths were able to use gravity to their advantage, their flexed legs positioned in a naturally 'resting' phase, similar to sitting, when they are hooked upside-down.
'Walking' right-side-up
While sloths have evolved to hang upside-down, they still share much of their morphology with creatures that live right-side-up. According to Nyakatura, this is an unexplained phenomenon in sloth evolution. Nyakatura used X-ray video to record the sloths' motion as they climbed across a branch, hoping to understand the different postural consequences of the species' hybrid morphology.
What he found was surprising: the movement of individual limbs and joints is barely different from right-side-up ancestors. The sloths, it seemed, were 'walking' upside-down. A closer look at their anatomy suggested that sloths were able to retain the muscles that moved limbs forward and back in order to adapt to their upside-down lifestyle.
The videos also showed that sloths are able to swing their body weight with little energy, due to fluid dislocation of certain muscle contact points, as well as small shoulder blades.
It was previously known that sloths - by virtue of having extra vertebrae and a long reach - require less motion than mammals of similar size. So it appears that they don't need to move as much as other mammals, and when they do, can save more energy doing the same movement. This suggests that the sloth's movement patterns are more efficient than 'lazy'.
Specialised, not maladapted
Though focused on the sloth's functional morphology, Nyakatura's research is helping to unravel the mysteries of this complex organism.
Sloths remain a largely enigmatic species, particularly the three-toed sloths, said Bryson Voirin, a PhD student at the Max Planck Institute for Ornithology in Germany. For instance, similar to their locomotion, their defecating habits are still quite comparable to their ground-dwelling ancestors. Sloths spend their life in the treetops, but venture to the forest floor to defecate, making them highly susceptible to prey.
Though only one piece of the puzzle, Voirin sees Nyakatura's research as helping elucidate the logic behind the sloth's seemingly bizarre life habits. "The research at Uni Jena helps shed light on the strange life strategy that sloths have adapted," said Voirin. "Once thought to be a unfortunately maldeveloped species, hardly fit for the world, we now are realising how amazingly specialised and well adapted they are.
The sloth's slowness decoded
"Sloths spend their life in the treetops, but venture to the forest floor to defecate..."
Have you ever tried to do it upside down?
sloth can remain hanging on branch even after death ?
I am curious , is the sloth's arms and claws made up similar to an eagles talons and muscle structure ?
supposedly when the eagles claw clutches tight it can not let go until the weight is reduced , such as depositing it's prey in the nest .
some animals drown while trying to return to the nest with a fish that is too large to carry , not being able to release prey that is too large in flight .
i have read that sloths have very low muscle mass , but can hang from their toes for many hours at a time .
to me this seems a contradiction , and for some to remain hanging after death even more mystifying .
does anyone have a good explanation , or is this all nonsense ?