This mammal has the world’s slowest metabolism | Science News for Students

This mammal has the world’s slowest metabolism

Three-toed sloths compensate by moving little and using the environment for heat
Jul 6, 2016 — 7:00 am EST
three-toed sloth

The brown-throated sloth is a type of three-toed sloth. It has the lowest rate of daily energy use of any mammal, a new study finds.

Christian Mehlführer, User:Chmehl/Wikimedia Commons (CC-BY 2.5)

There are degrees of slothfulness, even when it comes to sloths. And three-toed sloths may be the most slothful of all, new data show.

Researchers studied two species of sloth in Costa Rica. They measured the rate at which these animals’ bodies operate, converting food to fuel and growth. And this metabolic rate in one species of three-toed sloth was the lowest ever recorded — not just for a sloth, but for any mammal.

Six species make up the category of animals that most people call sloths. All fall into one of two families — either the two-toed or three-toed sloths. Both families live in trees throughout Central and South America where they eat leaves. But millions of years of evolution separate the families. Three-toed sloths tend to have smaller ranges and eat a more restricted diet than do their two-toed cousins. That means they prefer to dine on fewer species of trees. And they’ll usually eat from only a few individual trees.

two-toed slothJonathan Pauli is an ecologist at the University of Wisconsin-Madison. He got interested in sloths not because they’re adorable, he explains, but because “other things eat them.” And Pauli has kept his interest in these slow moving animals because he also finds them “biologically fascinating.”

Studies had shown that three-toed sloths have a very slow metabolic rate. But how slow? To find out, Pauli and his colleagues captured 10 brown-throated sloths. They are a three-toed species. The scientists also collected 12 Hoffmann’s sloths, which are a two-toed type. All came from a study site in northeastern Costa Rica. Here, the sloths live among a variety of habitats. These range from pristine forest and cacao (Ka-KOW) agroforest to fields of banana and pineapple.

“It’s really a quilt of different habitat types,” Pauli says. And it’s one that allowed the researchers not only to study many habitats at once but also to more easily capture and track sloths than if they were in dense jungle.

Many elements come in more than one form, or isotope (EYE-so-toap). The researchers injected the sloths with water labeled with specific isotopes of oxygen and hydrogen, then released the animals back to the wild. After 7 to 10 days, the scientists again captured the sloths and sampled their blood. By seeing how much of the isotope labels remained, they could calculate the sloths’ field metabolic rate. This is the energy that an organism uses throughout the day.

The field metabolic rate for the three-toed sloths was 31 percent lower than that for two-toed sloths. It also was lower than that found in any mammal that was not hibernating. The researchers reported this May 25 in the American Naturalist.

tree sloth“There seems to be kind of a cool combination of behavior and physiological characteristics that lead to these tremendous cost savings for three-toed sloths,” Pauli says. (By physiological traits, he means those relating to the animals' bodies.) Three-toed sloths spend a lot of time in the forest canopy eating and sleeping. They don’t move much. Their two-toed cousins “are much more mobile,” he notes. “They’re moving around a lot more.”

But there’s more to it than that, Pauli observes. “Three-toed sloths have the capacity to fluctuate their body temperature,” he points out. People need to keep their temperature within a few degrees of normal to stay healthy. But not sloths. They can let theirs rise and fall with the outdoor temperature. This is a bit like how a lizard or snake might regulate its body temperature. “Those are big cost savings to let your body change with your surroundings.”

Arboreal folivores (AR-bo-REE-ul FO-li-vors) are vertebrates that live in trees and eat only leaves. The new data help to explain why there aren’t more types of sloths and other arboreal folivores, Pauli and his colleagues argue. More than one-third of Earth’s land is forested. That means there is lots of treetop space for these critters. Yet few vertebrate species choose to subsist on tree leaves. In contrast, other types of animals have heavily diversified throughout habitats that take up much less space globally. For instance, there are 15 species of finch on just the Galapagos Islands. And there are hundreds of species of cichlid fish in Africa.

But there are constraints on being a leaf eater that lives in a tree. Leaf eaters tend to be big. The elephant and giraffe are good examples. They need a body big enough to accommodate a large digestive system that can process all of the leaf matter they need to survive. But an animal that lives in the trees can’t be too big. It needs lots of special adaptations for an arboreal life. And that might prevent the rapid diversification seen among other groups, such as Darwin’s finches, Pauli says.  

Indeed, this might be why arboreal folivory is one of the world’s rarest lifestyles, says Pauli. It “is really tough living.”


Power Words

(for more about Power Words, click here)

agroforestry    A type of agriculture that incorporates trees into the landscape.

arboreal    Living in the trees. Often this term refers to an animal.

canopy    (in botany) The top layer of a forest, where the branches of the tallest trees overlap.

cichlids    A freshwater fish that has become popular in the aquarium trade. This animal’s family is large and diverse. It includes at least 1,650 species, many of which are eaten. Although found all over the world, they are most diverse in Africa and South America.

cocoa    A powder derived from the solids (not the fats) of beans that grow on the Theobroma cacao plant, also known as the cocoa tree or cacao. Cocoa is also the name of a hot beverage made from cocoa powder (and sometimes other materials) mixed with water or milk.

diverse    (in biology) A range of species that differ broadly. The breadth of their differences may occur in a limited region (such as a local ecosystem) or as measured overa wide zone (such as across the planet).

diversify    (n. diversification) To make or become more diverse. In the environment, it would refer to an ecosystem that has a wider array of species.

ecology   A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

evolution   (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

family    A taxonomic group consisting of at least one genus of organisms.

fluctuate     Conditions that can change dramatically over a fairly short period of time, usually due to outside influences. For instance, temperatures fluctuate (rise and fall) throughout the course of a day. Average rainfall or snow rates can fluctuate from year to year.

folivore    A plant eater that specializes in eating leaves.

folivory     A term for a lifestyle that includes or depends on leaf-eating.

habitat   The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

isotopes    Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus. As a result, they also differ in mass.

mammal    A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

habitat   The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

hibernation    A state of inactivity that some animals enter to save energy at certain times of year. Bears and bats, for example, may hibernate through the winter. During this time, the animal does not move very much, and the use of energy by its body slows down. This eliminates the need to feed for months at a time.

hydrogen    The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

isotopes    Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus. As a result, they also differ in mass.

metabolism    The set of life-sustaining chemical reactions that take place inside cells and bigger structures, such as organs. These reactions enable organisms to grow, reproduce, move and otherwise respond to their environments.

oxygen    A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

physiology    The branch of biology that deals with the everyday functions of living organisms and how their parts function. Scientists who work in this field are known as physiologists.

range   The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

sloth    A slow moving, plant-eating mammal that lives in tropical rainforests in the Western Hemisphere. Most of these tree dwellers sleep all but four to nine hours a day.  

species    A group of organisms that share similar traits and ancestry, and can usually breed to produce fertile offspring. It is also the basic rank in a classification system called taxonomy. A species name (such as sapiens)  is usually given with the next highest rank, the genus (such as Homo).

vertebrate    The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.


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J.N. Pauli et al. Arboreal folivores limit their energetic output, all the way to slothfulness. American Naturalist. Published online May 25, 2016. doi: 10.1086/687032. 

Further Reading

S. Ornes. “Jet lag slows hamster brains.” Science News for Students. November 29, 2010.