In the wild, sloths are no lazier than the average teenager.
The average day of the average sloth isn't so different from yours or mine, it seems. It goes something like this: 8 a.m.: wake up; 6 p.m.: dinner; 11 p.m.: bed.
Although that schedule doesn’t sound too hectic, it is a lot more activity than was previously expected from sloths. Studies of captive sloths had suggested that the animals slept for almost 16 hours a day. But the first recordings of brain activity from wild animals show that the actual figure is less than 10 hours.
“I was astonished — I expected minor differences, but six hours a day is a very big difference,” says the study’s lead author, Niels Rattenborg of the Max Planck Institute for Ornithology in Starnberg, Germany.
The finding shows that the amount that animals sleep in the lab might not reflect how much shut-eye they get in the wild. And it suggests that comparisons of the sleeping patterns of different species need to take into account many different behaviours and environmental factors.
Rattenborg and his colleagues fitted three adult female brown-throated three-toed sloths (Bradypus variegatus) living in the Panama rainforest with ten-gram devices to record their brain’s electrical activity. The sloths also wore radio tracking collars, and another recorder to measure how much they were moving.
Up and about
Between three and five days later, the researchers recaptured the sloths and removed the recorders. Downloading the data, they could easily tell when the animals had been awake, when they had been asleep and when they had been in REM sleep, which in humans is associated with dreaming. The results are published in Biology Letters1.
The movement sensor showed that the animals do most of their chewing after the Sun sets. And the brain recorder showed that sloths almost invariably sleep from 4 to 6 a.m.
Sloths’ total sleeping time — about one-fifth of which is spent in REM sleep — is roughly what you would expect for an animal of their size, says Rattenborg. “In terms of brain function, sloths may not be as slothful as was previously thought,” he says.
Wild sloths might be wakeful because they need to find food or avoid predators, Rattenborg suggests. Or there might be individual differences: the 16-hour figure comes from a study published in 1983, which looked at an unspecified mixture of adult animals and juveniles. Young animals tend to sleep more.
Horses and cows are known to sleep longer when in a barn than when in a field, says sleep researcher Jim Horne of Loughborough University, UK. Zoo animals are also dozy — perhaps because they are safer, or perhaps because they are bored. Domestic animals might sleep for longer simply to use up unproductive time, Horne suggests.
The amount that different species sleep varies greatly — the giant armadillo spends 18 hours a day unconscious, the giraffe less than two. To fully understand why animals sleep the amount they do, says Horne, we need to place sleep in a wider context, looking at animals’ time and energy budgets, and the threats they face.
It could be that urban humans' sleep patterns reflect our sheltered existences. “Is our sleep in its wild state, or are we in captivity?” Horne wonders. “I’d guess we are somewhere between the two extremes.”
Rattenborg, N. C. et al. Biol. Lett. doi:10.1098/rsbl.2008.0203 (2008).