1. Why the Number of Fingers in the Sloths' Names Can Be Misleading
At first glance, the names suggest that two-toed sloths have two fingers and three-toed sloths have three. However, if you count carefully, both types have three fingers on their back feet [5][5]. The difference lies in the front feet. Two-toed sloths have two fingers on their front feet, while three-toed sloths have three fingers on both their front and back feet. Japanese encyclopedias also state that two-toed sloths have two fingers on the front and three on the back [5]. At the tips of these fingers, both groups have large, curved claws that serve as tools for hooking onto tree branches [5]. Two-toed sloths are slightly larger, with a weight of 4 to 8 kg (about 9 to 18 lbs) [5]. A species of three-toed sloth, the brown-throated sloth, is listed at about 3.5 to 5 kg (about 8 to 11 lbs) [5]. Beyond the number of fingers, are there other differences between the two groups? If you look inside the body, you will find differences in the neck bones.
2. How the Number of Neck Bones Differs Between the Two Sloth Groups
The number of neck bones (cervical vertebrae) is 7 in almost all mammals, including humans and giraffes. Sloths break this rule, and the number differs between the two groups. According to a 2010 paper, Hoffman's two-toed sloths have 5 to 6 neck bones, while Linnaeus's two-toed sloths have 7. Three-toed sloths have 8 to 10, with the number varying by species [5]. The extra neck bones in three-toed sloths do not have ribs. It is said that their development resembles the first rib in the chest of other mammals. One interpretation is that the bones from the chest moved into the neck area, rather than the neck simply becoming longer [5]. Differences are not limited to body structure; there are also differences in food.
3. What Two-Toed and Three-Toed Sloths Eat: Small Animals vs. Plants
Two-toed sloths are often active at night. They are said to eat mainly tree leaves, along with fruits, young shoots, nuts, bark, and even small vertebrates [5]. Zoo materials also describe their food as leaves, nuts, and shoots, noting that they hang from trees and sleep for about 15 to 18 hours a day [5]. In contrast, three-toed sloths eat plants, focusing on leaves but also eating flowers and fruits. For the brown-throated sloth, it is recorded that they mainly eat the Cecropia tree [5]. However, this description applies to one of the five species of three-toed sloths, and it is not known if it applies directly to the others. The difference between two-toed sloths, said to eat small animals, and three-toed sloths, which eat plants, seems to be connected to differences in how they move their bodies.
4. What Wild Sloth Measurements Show About Three-Toed Sloths' Energy Use
A study of wild sloths in Costa Rica shows the difference in lifestyle between three-toed and two-toed sloths in numbers. When 10 three-toed sloths and 12 two-toed sloths were measured, the energy consumption in the field (field metabolic rate) was reported to be 31% lower for three-toed sloths. Excluding animals in hibernation, this is said to be the lowest value recorded for mammals [5]. Researchers state that three-toed sloths use only about 460 kilojoules (about 110 kilocalories) per day [5]. Their movement is also different. Three-toed sloths often eat and sleep in the tree canopy and move very little. Two-toed sloths move around much more [5]. Researchers explain that the more an animal living in trees specializes in its environment, the smaller its daily energy consumption becomes [5]. So, why do two groups that are so different look so similar?
5. How Two Sloth Groups Evolved Separately for Tree Life but Look Alike
Today, there are only two genera of sloths, but there used to be many more relatives. Extinct ground-dwelling sloths include relatives of the Mylodon [5]. Analysis of ancient DNA showed that two-toed sloths are close to the extinct ground-dwelling Mylodon group, while three-toed sloths fall into a different extinct group (a lineage including Megatherium). It is thought that the two groups evolved life in the trees separately from different ancestors that lived on the ground [5]. The main eight families of sloths are said to have separated about 36 to 28 million years ago [5]. They followed separate paths but ended up with similar bodies. The interesting nature of evolution is clear when you compare the two.
6. How to Tell the Two Kinds of Sloths Apart by Fingers and Movement
At home, you can count the fingers on the front feet using photos or encyclopedias. Then count the back feet to check the difference between the names "two-toed" and "three-toed." If you see them at a zoo, compare the fingers on the front feet and how they move. If you can see them eating, observe what they eat. Few zoos keep sloths, so check the zoo's website in advance to confirm. If you want to read more, try opening explanations of ancient DNA [5] and the paper on neck bones [5].