Here is a species that lives up to its namesake. Sloths are known for being extraordinarily slow, with a maximum speed of around 250 meters per hour moving amongst the trees . For comparison, humans tend to walk around 5000 meters per hour. Have you ever wondered why these animals are so, well, slothful? Let’s look into their history and physiology to find the answer!
There are 4 known species of three-toed sloths (genus: Bradypus): the maned, brown-throated, pale-throated and pygmy. All live in trees (i.e., they are “arboreal”), inhabiting tropical regions in Central and South America . These trees are thick with greenery, helping to keep the sloths camouflaged to hide from predators. Sloths love the treetops. In fact, they’ll only leave their trees once per week to poop. It is thought that sloths evolved to do this because pooping at the base of their trees aids in nutrient cycling, benefitting the trees that sloths depend on for habitat . Whatever the reason, it’s a dangerous journey. A grounded sloth is easily caught by cat predators such as jaguars, ocelots, and pumas to name a few.
Sloths have distinct claws on all 4 limbs that are super long and curved, optimal for gripping tightly onto tree branches. The muscles in their arms are adapted for suspending on branches, as opposed to walking on the ground. Their front arms are also much longer than their back arms . Between the impressive claws and gangly arms, their body is built perfectly for hanging upside down all day. They can also swim quite well!
If sloths stay in the trees by their lonesome, how are they able to reproduce? Ironically enough, sloths are speedy when it comes to the mating process. Female sloths will produce a vocalization that has been described as a “scream” to attract males to their tree . Males then embark on a dangerous quest, slowly braving the perils of the forest floor and following their lover’s voice until they finally reach her home, with copulation ranging from just a handful of seconds to a few minutes. Pregnancy lasts around 170 days, and the moms give birth while hanging from their branches! Baby sloths cling onto their mother for 5–9 months after birth, and they often establish territories nearby their mom as adults .
Sloths primarily eat—you guessed it—tree parts, including leaves, fruits and even twigs. Sloths’ digestion is just as slow as their movement speed. The slow fermentation of plants in the foregut, as well as the leisurely rate of passage through the intestines, means it can take multiple days to digest a single meal .
Overall, the unhurried nature of the three-toed sloth can be attributed to its easygoing, tree-dwelling behavior, slow digestion, and unique morphology, adapted perfectly to hanging from the trees above. If you ever get to visit the tropical rainforests in South America, be sure to look up and try to spot these well camouflaged creatures! If you are lucky enough to spot one, you can bet it won’t be running out of view anytime soon.
Isabelle McDonald-Gilmartin (she/her) is an animal behavior PhD candidate and a part of the Center for Animal Welfare at UC Davis. She currently studies the abnormal behaviors of dairy cattle, and how abnormal behaviors are related to animal welfare. Isabelle is also interested in teaching and mentorship of students of all ages. When she’s not working with her cows, she enjoys spending time with her dog, reading, and watching scary movies.
 Young, M. W., McKamy, A. J., Dickinson, E., Yarbro, J., Ragupathi, A., Guru, N., Avey-Arroyo, J. A., Butcher, M. T., & Granatosky, M. C. 2023. Three toes and three modes: Dynamics of terrestrial, suspensory, and vertical locomotion in brown-throated three-toed sloths (Bradypodidae, Xenarthra). Journal of Experimental Zoology Part A: Ecological and Integrative Physiology. https://doi.org/10.1002/jez.2684
 Moraes-Barros, N., Silva, J., Miyaki, C. Y., & Morgante, J. S. (2005). Comparative Phylogeography of the Atlantic Forest Endemic Sloth (Bradypus torquatus) and the Widespread Three-toed Sloth (Bradypus variegatus) (Bradypodidae, Xenarthra). Genetica, 126(1–2), 189–198. https://doi.org/10.1007/s10709-005-1448-x
 Voirin, B., Kays, R., Wikelski, M., & Lowman, M. D. (2012). Why Do Sloths Poop on the Ground? Springer New York EBooks, 195–199. https://doi.org/10.1007/978-1-4614-7161-5_19
 Olson, R. E., Glenn, Z. D., Cliffe, R. N., & Butcher, M. T. (2018). Architectural Properties of Sloth Forelimb Muscles (Pilosa: Bradypodidae). Journal of Mammalian Evolution, 25(4), 573–588. https://doi.org/10.1007/s10914-017-9411-z
 Gilmore, D., Da-Costa, C. G. A., & Duarte, D. (2000). An update on the physiology of two- and three-toed sloths. Brazilian Journal of Medical and Biological Research, 33(2), 129–146. https://doi.org/10.1590/s0100-879×2000000200001
 Taube, E., Keravec, J., Vie, J., & Duplantier, J. (2001). Reproductive biology and postnatal development in sloths, Bradypus and Choloepus: review with original data from the field (French Guiana) and from captivity. Mammal Review, 31(3 & 4), 173–188. https://doi.org/10.1046/j.1365-2907.2001.00085.x
 Foley, W. J., Engelhardt, W. V., & Charles-Dominique, P. (1995). The passage of digesta, particle size, and in vitro fermentation rate in the three‐toed sloth Bradypus tridactylus (Edentata: Bradypodidae). Journal of Zoology, 236(4), 681–696. https://doi.org/10.1111/j.1469-7998.1995.tb02739.x
[Edited by Jessica Schaefer & Jacob Johnson]