How Concerning is “Least Concern”? Sloths and the IUCN Red List

How Concerning is “Least Concern”? Sloths and the IUCN Red List

What is the IUCN Red List?

Since 1964 the International Union for Conservation of Nature (IUCN) has kept a list of the world’s most threatened species. This Red List of threatened species has since become the go-to authority for the status of Earth’s flora and fauna, serving as a critical indicator of our world’s biodiversity. The status of a species on the IUCN Red List is how governments, NGOs, and conservation groups allocate resources and determine the policy that affects the natural resources needed for species to survive.

IUCN Red List Categories.

The most important part of protecting any species is knowing what needs protecting. The IUCN Red List information concerning species range, population size, habitat, ecology, exploitation, threats, and ongoing conservation actions are critical for species survival.

Sloths species by the IUCN Red List criteria

There are six living species of sloths on the planet. One of them is “critically endangered” (the pygmy sloth), one of them is “vulnerable” (the maned sloth), and the other four are listed as “least concern”. What, exactly, does that mean?

The problem with sloths

“Least concern”, according to the IUCN Red List, means that a species population is healthy and does not require any intervention on the part of humanity to ensure its continuation.

Wildlife rescue centers in Costa Rica receive two or three sloths per day, representing almost half of all admitted mammals. Authorities such as the Ministry of Environment and the National System of Protected Areas (SINAC) have declared that both sloth species (B. Variegatus and C. hoffmanni) living in the country are threatened and their populations declining.

In the past 13 years, we have seen first-hand how habitat loss and urbanization are causing the decline of sloth populations. This downturn has so far been witnessed but not yet measured, and is not confined to Costa Rica, but is likely at least as prevalent in other countries across South and Central America.


sloth on a powerline


Currently, there is no accurate data available on the status of sloth populations. We don’t know how many sloths there are, how many can or should inhabit any given area, or how fast populations are declining. There is not a single population estimate available for the majority of the sloth species, population trends are not being monitored and the geographical distributions of the different species are still unknown.


sloth on the road
Sloths crawling on roads are vulnerable to dog attacks, roadkills, or tourists’ harassment. / Photo: Rafacdul


If we don’t know how many sloths there are (or how many there should be), or even where they live, how can we make an accurate assessment of the state of the populations?


So, why are they listed as “least concern” if there is no data?

The IUCN Red List represents a global risk of imminent extinction; the chance that an entire sloth species could disappear from the planet tomorrow. This is a very valuable measurement, but can often be misunderstood with disastrous consequences. Some species can fall through the cracks entirely.

For example, the four “least concern” sloth species all have large population ranges covering many countries in South and Central America, as well as the Amazon rainforest. This means that there are a lot of individual sloths out there–and therefore the risk of all of these individuals becoming extinct overnight is relatively low.



maned sloth brazil
Maned sloths (Bradypus torquatus) are listed “vulnerable” / Photo: Cecilia Pamich


Unfortunately, the rate of population decline isn’t taken into account (because there is no data available to measure) and there hasn’t been an IUCN population assessment since 2013. In the case of sloths, the “least concern” status is based entirely on their wide distribution, as explained on the IUCN website:

“Bradypus variegatus is listed as Least Concern in view of its wide distribution including a large part of the Amazon forest, presumed large population, its occurrence in a number of protected areas, and because it is unlikely to be declining fast enough to qualify for listing in a threatened category.”


IUCN red list sloths least concern map
Population trend: UNKNOW  |  Source: IUCN Red List


Sloths move slower, reproduce slower, and disperse slower than other animals. If sloth populations become fragmented, isolated, or spread thin, they may not be able to recover in the same way as other mammals, and we have no idea what this critical population density might be or whether or not sloths are heading in that direction.

The case of the pygmy sloths

By comparison, the pygmy sloths in Panama are listed as “critically endangered”. They are found exclusively on one tiny island off the coast of Panama, and there are as few as 500 to 2000 pygmy sloths in existence. In fact, they are considered to be one of the most critically endangered mammals on the planet.


Pygmy three-fingered sloth, Isla Escudo de Veraguas, Panama. Photo: Suzi Eszterhas.

While that sounds alarming, it is also important to note that there have only EVER been a few pygmy sloths in existence. The size of the island constrains the population, and the sloth population there is actually stable.

So long as they are left alone on their uninhabited island, this species, while rare, is threatened more by global challenges such as global warming and would benefit more from broad rather than specific conservation measures.


pygmy sloth swimming
Pygmy sloth swimming, Isla Escudo, Panama. Just like most coastal zones, their habitat is threatened by climate change and sea levels. Photo: Suzi Eszterhas.


Despite this, pygmy sloths are eligible for a vast range of wildlife conservation and research grants and funding because of their “critically endangered” status.

Why absolute numbers do not tell the whole story

Meanwhile, sloth populations on the mainland are in decline, yet are burdened with the “least concern” status because of the very nature of their ecology. This means that it is virtually impossible to secure any funding for conservation or research.


sloth backpack research
Fitting a “backpack” with data loggers to a three-fingered sloth. Photo: Suzi Eszterhas


Without research, we cannot make a more accurate assessment of the population status, which means that we will not know when declining sloth populations reach a critical threshold until it’s too late.

The Great Sloth Census

Risk assessment for sloth populations hinges upon being able to take an accurate population census-a project that, before now, has defeated researchers by the nature of its impracticality. Sloths are very hard to find, and therefore to count.

For this reason, The Sloth Conservation Foundation has launched a groundbreaking new project to measure sloths in an area by measuring their scat, which is more reliably found than the sloths themselves, and training a Sloth Scat Detection Dog to help.


The dog will lead researchers to the sloth scat, the scat will tell us how many sloths are in an area, and long-term studies of the scat will yield information about population genetic diversity, sloth movements, territory sizes, and whether or not these populations are thriving or declining.

With this information, authorities such as the IUCN will have the data they need to decide if sloths are truly safe, stable, and of “least concern”.


-Dr. Rebecca Cliffe

Founder and Executive Director.