There are two dictionary descriptions of the word ‘sloth’- first: ‘reluctance to work, laziness’ - the second: ‘a slow moving American mammal that hangs upside down from the branches of trees using its long limbs and hooked claws.’

Scientists still don’t know a lot about these creatures, who are the slowest moving mammals on the planet with a top speed of 1 metre in 1.5 seconds and are found in the tropical rain forests of Central and South America. Their slowness works in their favour by not drawing attention to themselves – they can appear motionless, and blend into their surroundings very well.

Their fur is a dull colour, with the coarser outer coat giving camouflage, which grows in the opposite direction to every other mammal - parting at the stomach and pointing towards their back to an area called the ‘drip tip’ which allows rain to fall away. Weirdly, their fur contains micro-cracks which trap moisture for over 80 different kinds of algae and fungi, which turns their fur green over time and helps them blend into the rainforest canopy, with the sloth becoming a miniature ecosystem. Just as the forest is home to sloths, each sloth is home to a mutually benefitting variety of plants and animals.

Hanging around in trees has some disadvantages – females have to go to lower levels to give birth - still upside down – so the baby doesn’t fall far. They go to the ground to poo too - they don’t go very often (maybe once a week) and is the only time they stand upright - and, of course, it is a slow process, leaving them vulnerable to predators.

The word “sloth” comes from the word slouthe or slewthe, meaning indolence, laziness, inertness or torpor, and has led to centuries of stereotyping sloths as creatures lacking in motivation. A naturalist in 1749 said their stupidity and slowness resulted in a bungled conformation; that they were the lowest form of existence, and that one more defect would have made their lives impossible. Rather harsh I thought.

Sloth on a tree

A sad fact is that two of the six species of sloths rate high on the list of endangered animals. The Pygmy Three-Toed Sloth is ‘Critically Endangered’ and the Maned Three-Toed Sloth is considered ‘Vulnerable’, apparently due to habitat deconstruction, making it hard for them to survive. It seems there are less than 100 Pygmy Three-Toed Sloths left in the world. Urbanisation is robbing them of their natural habitat, and in Costa Rica many are electrocuted by swinging on overhead wires where their forest used to be.

A sloth’s stomach has four chambers, same as a cow, with the partially digested food it contains accounting for up to 37% of its body weight. In addition, its oesophagus does not go in a straight line from mouth to stomach but has a loop in it, which enables the sloth to eat while hanging upside down without having gravity pull the food back out – which also prevents vomiting.


Another fact - Two-fingered sloths have 46 ribs - more than any other mammal. For comparison, humans have 24, and whales only have 18. These extra ribs help support their stomachs when hanging upside down, and are very flexible, making them hard to break. Sloths have been known to survive falls from nearly 30 metres up in the rainforest canopy.

Both the three-fingered and two-fingered tree sloths that we see today evolved from giant ground sloths, of which there were thought to be over 80 different kinds, and although they are not closely related to each other, they are examples of convergent evolution, where different animals evolve similar traits to adapt to their environment.

The largest was Megatherium, which weighed as much as an elephant, and stood over 6 metres high. Several colossal ancient sloth burrow networks have been discovered in Brazil, the largest of which measures over 2000 feet long and is thought to have been dug out by many generations of now extinct sloths. Giant ground sloths were covered in bony armoured plates, much like armadillos, while others adapted to marine life and lived in the ocean.