The Sloth's Secret Is Not Laziness, It Is Arithmetic

Leaves are abundant, safe to approach and almost worthless. They are mostly cellulose, they are low in calories, and many are laced with chemicals designed to make a herbivore regret its choices. An animal that decides to live on canopy leaves has to solve a budget problem, and the sloth's solution is to spend almost nothing.

Its metabolic rate runs at roughly 40 to 45 percent of what you would predict for a mammal of its size. It lets its body temperature drift with the air rather than paying to hold it steady, which is why sloths climb to sunlit branches on cool mornings. Its muscle mass is about 25 percent of body weight, against 40 percent or so in most mammals, and what muscle it has is built for slow sustained holding rather than quick force.

Digestion takes weeks. A single leaf meal can spend more than a fortnight in a multi-chambered stomach that occupies a third of the animal's body. At any moment, a sizeable share of a sloth's weight is lunch in progress.

Then there is the descent. Roughly once a week a sloth climbs all the way down to the forest floor to defecate, which is the single most dangerous thing it ever does. Most sloth predation happens on those trips. Biologists still argue about why: fertilising its favourite tree, keeping the moths in its fur supplied with somewhere to lay eggs, or communicating with other sloths. Whatever the reason, the animal has judged it worth the risk, weekly, for millions of years.

None of this is laziness. It is an animal that has read the terms and conditions of eating leaves and has priced everything correctly.