Revealing the Secrets of Sloths

Sloths are unusual mammals from South America.

Sloths are unusual mammals from South America.


Since I was a child, I have always been fascinated with nature, particularly animal biodiversity. Not only because I come from the tropical South American country of Venezuela, with its astonishing biodiversity, but also because of the complexity of animal sizes, morphology, and behavior.

My fascination always asked the same questions: What is the make-up of a species? What makes species different? How are species maintained as independent units? During my college and graduate education, I had the opportunity to discover a way to understand wildlife diversity through a unique code that keeps the secrets of all organisms: DNA. I learned about those four letters A, C, G, T, which make the fundamental units of all species’ genetic code, and determine the fate of organism characteristics and survival.

Preserving Species
When I first came to work at San Diego Zoo Global as a postdoctoral associate in the Genetics Division, I immediately realized the importance of using genetics in preserving species in the wild and managing them in captivity. I was challenged to work on various projects including studying the evolutionary history of threatened mammalian species. One project in particular caught my attention; I was asked to identify genetic variation between two species of two-toed sloths, Hoffman’s and Linnaeus’s, and maybe create a simple genetic test for differentiating them in the wild and captivity.

Immediately, I was excited about working on sloths, not only because they belong to one of the most ancient living mammalian groups, but also because they originated in South America. I grew up watching sloths in trees near my home and around my university. Now, I had the chance to dig deeper and see part of the genetic make up of these species!

Toe-to-toe with Two-toed Sloths
Two-toed sloths are least concern species in the wild but frequent residents of zoo collections in North America, and both species have been bred and successfully maintained in captivity. However, species identification has always been problematic because of their large overlap in external morphology, particularly in body size and coat color. This represents an obstacle to the captive breeding program and a challenge in preserving species’ integrity in the confined population.

I started by decoding two genes in the sloths’ DNA, one commonly used for vertebrate species identification (COI) and the other with an interesting function in mammals: to produce the dental enamel necessary for teeth formation called “ENAM”. ENAM is a very conserved gene, meaning it is less variable among mammals, except for animals with lost or modified tooth structure, including baleen whales, pangolins, aardvarks, and sloths! Sloths have a reduced number of teeth and a modified version of ENAM characterized for being highly variable, making it an appropriate genetic marker for finding differences among species.

Decoding the DNA of the two species of sloth will benefit conservation of both species.

Decoding the DNA of the two species of sloth will benefit conservation of both species.


Retooling the Tools
I found high levels of genetic variation between both sloths in COI and ENAM genes, making species identification possible. Based on the genetic information, I designed a low-cost, easy to-use genetic tool to identify two-toed sloths and improve management of the captive population. This tool allows visualizing DNA differences between species in a polymer matrix, a procedure that can be implemented with non-sophisticated tools in simple laboratory settings. My job with sloths was done, and with it the satisfaction of learning more about this unique species and the validation of how powerful genetics can be in helping preserve species.

So what comes next? I really believe that deeper exploration of organisms’ DNA, through study of complete animal genomes, will unravel the still-obscure origin of species biodiversity.

Cynthia Steiner is a senior research associate in the Genetics Division, San Diego Zoo Institute for Conservation Research.

5 Responses to Revealing the Secrets of Sloths

  1. Can you please enlighten me? Do you actually have sloths in San Diego? I have spent the last five years in Costa Rica on the Caribbean side watching sloths in their natural environment. I am told that they should not be taken out of their environment or they will die. There are rescue centers trying to help their survival. I hope you are helping these groups – Avarios and Jaguar Rescue Center are two that I know of and I’m sure there are more. Two-toed and three-toed sloths are quite distinct. Not sure why you need a tool to identify them. Can you please give more information?

    • Hi, Joan.
      Thanks for your comment and your interest in sloth conservation. We don’t have sloths in San Diego indeed, but these two-toed sloth species are represented at the San Diego Zoo. Two-toed sloths are exclusively distributed in Central and South America. My work on sloths refers to two-toed sloths (not three-toed sloths), in particular Hoffman’s and Linnaeus’s species. These species are so similar in morphology that sometimes a genetic test is needed to verify species identification.

  2. Is the San Diego Zoo working on saving the pygmy three-toed sloth? It is only endemic on Isla Escudo de Veraguas, so it lives in a very specialized habitat.

  3. Hi, Tim.
    Thanks for your question and interest on sloth conservation. As far as I know, San Diego Zoo Global does not have a conservation program for the pygmy three-toed sloth. As you mentioned, this species has a restricted distribution exclusively in this island of Panama, and it is not a frequent resident of zoo collections.

  4. How interesting! This is a fascinating way to help conserve wildlife and very important, even though you don’t get to hang out in the forest watching the sloths.
    Thank you!