Journal of Mammalogy Advance Access published June 1, 2015
Journal of Mammalogy, xx(x):1–5, 2015
DOI:10.1093/jmammal/gyv078
Biology and conservation of the pygmy sloth, Bradypus pygmaeus
Bryson Voirin*
The Max Planck Institute for Ornithology—Seewiesen, Eberhard-Gwinner Strasse, 82319 Seewiesen, Germany
California Academy of Sciences, 55 Music Concourse Drive, San Francisco, CA 94118, USA
* Correspondent: [email protected]
The pygmy three-toed sloth (Bradypus pygmaeus) has garnered much interest since being described in 2001 as
a new species, which occurs on a single island, Escudo de Veraguas, Panama. Recent work has found that the
species has a highly diverse eukaryotic community in its hair, activity, and sleep patterns markedly different
from other three-toed sloths in the region, and that some individuals live in nonmangrove areas as well as in
the mangroves on the island. This critically endangered species is being threatened by several factors, including
habitat degradation due to timber harvesting, increased development, and collecting. An accurate understanding
of the ecological needs of pygmy sloths is imperative to forming a comprehensive and successful conservation
strategy.
El perezoso de tres dedos (Bradypus pygmaeus) es un especie ha ganado mucho interés desde su descripción
como una nueva especie de perezoso Bradypus existiendo en una única isla. Trabajos recientes de investigación
y conservacion en Escudo de Veraguas ha conducido a importantes y nuevos conocimientos acerca del
B. pygmaeus. La comprensión exacta de las necesidades ecológicas de los perezosos pigmeos es imprescindible
para la formación de una estrategia de conservación, especialmente por las amenazas del desarrollo futuro y por
la continua degradación del hábitat en la isla.
Key words: conservation, Escudo de Veraguas, Panama, three-toed sloth
© 2015 American Society of Mammalogists, www.mammalogy.org
hair, physiology, and population size. I also report findings
from my recent tracking study that call into question earlier
estimates of population size. Lastly, I describe the conservation
issues facing the species and the outlook for B. pygmaeus in
the future.
The pygmy three-toed sloth (Bradypus pygmaeus) is a
dwarf sloth species endemic to the island of Escudo de
Veraguas, Panama (9°5ʹ58.78ʺN, 81°33ʹ33.60ʺW). The species is 1 of 2 extant members of the family Bradypodidae
found in Panama and the sole sloth species occurring on
the island. B. pygmaeus was first described as a species in
2001, when it was separated from its sibling species, the
brown-throated three-toed sloth (Bradypus variegatus).
In the species description, Anderson and Handley (2001)
report that pygmy sloths are only found in red mangrove
trees (Rhizophora mangle).
The island of Escudo de Veraguas is 4.3 km2 in area and is
located 17.6 km offshore from the nearest point of the Valiente
peninsula, Rio Caña, Panama. It has been separated from
the mainland for approximately 8,900 years (Anderson and
Handley 2002). In addition to B. pygmaeus, other vertebrate species endemic to the island include the solitary fruit-eating bat
(Artibeus incomitatus) and the Escudo hummingbird (Amazilia
handleyi). Overall, relatively few mammal species are found on
the island.
Herein, I review the known facts about the species, including morphology, genetics, diversity of eukaryotes living in their
Morphology
Outwardly, B. pygmaeus is strikingly similar to B. variegatus in
appearance, with only minor differences immediately apparent
(B. Viorin, pers. obs.). For example, B. pygmaeus often appear
within the normal size range of B. variegatus, especially in the
Bocas del Toro archipelago. However, on average, B. pygmaeus
is smaller, approximately 40% lighter in body mass and 15%
shorter in overall body length than B. variegatus (Anderson and
Handley 2001). The average body mass reported by Anderson
and Handley (2001) was 2.9 kg, (n = 7; range 2.5‒3.5 kg), and
average body mass we found was 3.0 kg (n = 5; range 2.7–
3.5 kg—Voirin et al. 2014b).
In addition to smaller overall body size, the skull of
B. pygmaeus is 12–16% smaller than B. variegatus (Fig. 1).
Cranial characteristics distinguishing B. pygmaeus include
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JOURNAL OF MAMMALOGY
the absence of an external carotid foramen, large external
auditory meatus, smaller stylomastoid foramen, more curved
and thin coronoid processes on the mandible, and a smoother,
more concave ventral edge on the stylohyoid (Anderson and
Handley 2001).
In the initial species description (and repeated in subsequent
literature), B. pygmaeus is characterized as having a “hood”
of hair, which projects over the brow, similar to the hood of
a monk. However, despite numerous field trips to the island
and observing over 75 pygmy sloths, my colleagues and I have
never encountered a pygmy sloth with hair folding over the
brow or forehead. B. pygmaeus, as well as some B. variegatus
found in certain parts of Panama, have long neck hair, which
begins at the ventral portion of the skull and hangs off the
neck (Fig. 2). Consequently, a more systematic assessment
is needed to determine whether B. pygmaeus can be distinguished from B. variegatus living on nearby islands using pelage characteristics.
Similar algae have been found on other species of sloths;
however, Trichophilus found on samples from Choloepus
and Bradypus (including B. pygmaeus) are genetically different in the 2 genera (Suuatri et al. 2010). Although the
exact ecological relationship between sloths and sloth algae
is debated, recent work by Pauli et al. (2014) found evidence
that sloths may supplement their diet by ingesting the algae.
Genetics and Natural History
Preliminary studies of B. pygmaeus genetics indicate a very low
level of diversity (N. de Moraes-Barros, University of Porto,
Portugal, in litt.). However, these initial analyses included only
samples of B. pygmaeus located within the mangroves on the
northeast side of the island (Voirin et al. 2014a). Continued
research in population genetics, using samples from B. pygmaeus from around the island, as well as from B. variegatus
from nearby islands, may yield important insights into population size and its genetic diversity.
Like other Bradypus sloths (Aiello 1985), B. pygmaeus
hosts algae inside its fur, often causing it to have a greenish
appearance (Figs. 2 and 3). The hairs are thick and spongy
and have irregular transverse cracks that increase with age
(Aiello 1985). The green algae are most apparent on the top
of their head and neck, dorsal portions of the forelimbs, and
upper backside. Suuatri et al. (2010) examined hair from all
6 extant species of Bradypus and Choloepus sloths, and analyzed genetic diversity of the eukaryotic community found
on each. Only a single species of algae was found in the
fur of B. pygmaeus, thought to be Trichophilus welckeri.
Fig. 1.—Skull comparison of adult Bradypus pygmaeus (left) from
Escudo de Veraguas and B. variegatus (right) from central Panama.
Note size difference, as well as differences in dorsal skull architecture.
Skulls were found during fieldwork and left in Panama.
Fig. 2.—A female pygmy three-toed sloth (Bradypus pygmaeus) with
baby, climbing on a red mangrove branch on Escudo de Veraguas,
Panama. Photograph by Bryson Voirin.
Fig. 3.—Microphotograph of green algae living in the fur from
Bradypus pygmaeus. Hair diameter is 0.05 mm. Photograph by Bryson
Voirin.
SPECIAL FEATURE—BIOLOGY AND CONSERVATION OF PYGMY SLOTHS3
In addition to the green algae, B. pygmaeus was found to
host a diverse eukaryotic community, including ciliates, apicomplexans, dinoflagellates, and fungi representing at least 22
taxa, with an estimated total of 38 taxa present (Suuatri et al.
2010). Interestingly, B. pygmaeus was found to host the largest
number of eukaryotic taxa of all sloth species studied.
My colleagues and I recently compared activity patterns
and sleep behavior of wild B. pygmaeus and B. variegatus
in Panama (Voirin et al. 2014b). We found that although the
total time spent asleep did not differ between B. pygmaeus and
B. variegatus from the nearby mainland, the timing of sleep
differs between the species. B. pygmaeus, which has no natural
predators on Escudo de Veraguas, showed no significant daytime or nighttime preference for sleeping and was awake for
periods throughout the day and night. In contrast, B. variegatus, which experiences high levels of nocturnal predation by
felines on the mainland, was largely diurnal. Consequently, the
differences in the times of sleep and waking behavior may be
related to differences in predation pressure.
The amount of sleep spent in rapid-eye-movement (REM)
sleep was significantly lower as a percentage of total sleep time
in B. pygmaeus than in B. variegatus. This difference in REM
sleep and differences in brainwave intensity observed during
non-REM sleep mimic those observed following benzodiazepine consumption in other mammals (Borbély et al. 1985;
Kopp et al. 2004). Interestingly, benzodiazepine-like compounds have been found in certain plants, including mangroves
(Xia et al. 2007; Nicoletti et al. 2009). These differences could
reflect dietary differences between B. pygmaeus and B. variegatus (Voirin et al. 2014b).
Population Size and Distribution
The total population size of B. pygmaeus remains unclear,
as no unbiased estimate exists. The International Union for
Conservation of Nature Red List assessment estimated the population to be < 500 individuals (Voirin et al. 2014a), but this
is a rough estimate based on anecdotal evidence. In the initial
species description, B. pygmaeus was characterized as living
only in the mangrove patches around the edges of the island
(Anderson and Handley 2001). However, no systematic efforts
have been made to find pygmy sloths in the island’s interior.
Censusing a cryptic animal such as a sloth is challenging.
Standard methods such as footprint tracks and camera traps are
ineffective, and visual searches likely miss an overwhelming
majority of sloths, especially in the dense rainforest. Indeed, in
previous studies tracking two- and three-toed sloths in Panama,
radiocollared sloths that were tracked with a handheld antenna
to a specific tree were only visually seen < 5% of the time
(Montgomery et al. 1973).
The mangrove forests on Escudo de Veraguas are generally
low and relatively open, making spotting a sloth much easier
than in a high-canopy rainforest. Kaviar et al. (2012) systematically searched the entire mangrove habitat on the island
and located 79 sloths. They also mapped all of the mangrove
patches and calculated that this habitat covers 10.67 ha (Kaviar
et al. 2012, 2013). The island is 430 ha in area (Anderson and
Handley 2002), and mangroves occur on about 2.5% of the
island (incorrectly reported as 0.024% by Kaviar et al. 2012,
2013). Thus, density of sloths was 7.4 sloths per ha in the
mangroves.
Radiotracking Study
In 2012, I began a tracking study of B. pygmaeus to determine
if mangrove-dwelling pygmy sloths venture into interior forests. I caught 10 pygmy sloths in mangrove patches around
the island and outfitted them with radiocollars (Model M1940,
ATS Tracking, Isanti, Minnesota). I checked their locations at
3–6 month intervals with a 3-element Yagi antenna and marked
their location with a handheld GPS (Garmin 60Cx, Garmin,
Inc., Olathe, Kansas).
Initial data show that at least some pygmy sloths move from
the mangroves into the interior of the island. Four sloths were
found far (≥ 200 m) outside the mangroves in the interior rainforest of the island, and 5 sloths were found in nonmangrove
trees near (≤ 10 m) the edge of the mangroves (Fig. 4). Three
sloths were found only in mangroves. The average minimum
area used by an individual was 0.25 ± 0.13 ha (± SD; range
0.08–0.49 ha), as calculated by the minimum convex polygon
for location data. However, the present data are insufficient to
calculate an accurate home range because of the infrequency of
locations. Because all sloths were initially caught in the mangroves, it remains unknown if all B. pygmaeus on the island are
at least partially dependent on mangroves for their diet. It is possible that there are other individuals living in the interior forests
of Escudo de Veraguas that do not come to the mangroves.
Although further research on the density of sloths in the
island’s interior is needed, my findings suggest that the most
recent population size estimate of 79 individuals is inaccurate.
Escudo de Veraguas is 430 ha in total area, with 10 ha of mangroves, and ~400 ha of mixed tropical forest (Fig. 4). Kaviar
et al. (2012) estimated there to be ~7.4 sloths per hectare of
mangrove forest. This density is similar to that observed by
Montgomery et al. (1973) for B. variegatus living in a central
Panamanian rainforest (8.5 individuals per hectare). Although
the plant diversity and canopy height are greater in the forests
where Montgomery and Sunquist worked (Croat 1978), the
interior forests on Escudo de Veraguas appear somewhat similar (although a botanical species list of the island has not been
published). Therefore, it is possible that there is a similar density of B. pygmaeus on the interior of the island. If that is so,
the number of sloths on the island could be several thousand
(430 ha × 7.4 individuals/ha = 3,182 individuals). The actual
population size is most likely somewhere between these 2 estimates (perhaps ~500–1,500 individuals), and in any case, is
extremely small for an entire species.
Conservation Status and Problems
Threats.—The primary threat to B. pygmaeus is human interference, with habitat loss and degradation being the most pressing.
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JOURNAL OF MAMMALOGY
Fig. 4.—Isla Escudo de Veraguas, Panama, showing rainforest cover (light shading) and mangrove cover (dark shading). Representative tracks of
10 radiocollared sloths shown as black lines. Tracks show 4 sloths ventured far outside the mangroves, while 3 sloths were always found in the
mangroves. Map reproduced and edited with permission from Fundación Almanaque Azul (Lombarfo 2006).
Short-term visitors from Ngäbe-Buglé Comarca, including
indigenous families, fishermen, and divers seasonally inhabit
Escudo de Veraguas. At times, the island’s population can
reach several hundred, concentrated around several privately
established campsites with basic wooden structures. Timber is
harvested from the island’s forests for charcoal and building
materials (Voirin et al. 2014a), and remnants of logging can be
found around the island. B. pygmaeus is reliant on various trees
for food—I have observed them eating leaves from red mangroves, various species of Cecropia, and flowers from guava
(Myrtaceae) trees—but we do not know which species are necessary to sustain them. It is possible that timber harvesting on
such a small island could be a threat to the species.
Another potential human threat is the eating of pygmy sloths.
Together with an international team of scientists and conservation experts, we investigated the possibility that people are
or have been eating pygmy sloths. However, we found no evidence of humans eating pygmy sloths.
A previously unrecognized threat to pygmy sloths comes
from the pet trade, exotic species collectors, and zoos wanting to showcase this newly described and exceedingly rare
mammal in their collections. Recently, articles in the popular press on sloths has greatly increased their popularity and
led to a growing interest in keeping them as pets. In 2013, a
zoo in the United States collected 10 pygmy sloths, with the
reported aim of establishing a backup population in the United
States (Dimitrova 2013). Attempts to remove the sloths from
Panama were thwarted when locals learning of the plan protested. The animals were returned to Escudo de Veraguas, but at
least 2 individuals died shortly thereafter (D. Smith, Zoological
Society of London, United Kingdom, in litt.).
To date, nearly all attempts to maintain three-toed sloths in
captivity outside the tropics have resulted in mortality within
a short period of time, and even captive three-toed sloths
within the tropics rarely survive long-term (N. Moraes-Barros,
University of Porto, Portugal, in litt.; Superina et al. 2008).
Moreover, Bradypus sloths have never been bred in captivity,
even in the tropics (Superina et al. 2008). Given the pygmy
sloths’ unknown diet, their general vulnerability to changing
conditions, and the overwhelming failure to sustain or breed
any species of Bradypus in captivity under any conditions, it
seems ethically inappropriate for pygmy sloths to be removed
from their natural habitat for any purpose at this time.
Conservation status.—B. pygmaeus is classified as Critically
Endangered by the IUCN (Voirin et al. 2014a). Its high-risk
status is inferred, due to its extremely limited range and threat
of habitat loss. The United States Fish and Wildlife Service
(USFWS), which handles the United States Endangered
Species List, does not recognize the IUCN’s species threat classifications. At the time of this writing, the USFWS was reviewing an emergency petition to add the pygmy sloth to the United
States Endangered Species List.
Although B. variegatus has been listed in Appendix II of
the Convention on International Trade in Endangered Species
of Wild Fauna (CITES) since 1973 (initially under the name
B. boliviensis), because of an oversight B. pygmaeus was not
automatically listed when it was declared an independent species from B. variegatus. In November 2013, CITES corrected
this oversight and added B. pygmaeus to Appendix II (CITES
2013).
In 2009, the Autoridad Nacional del Ambiente, Panama’s
environmental protection authority, passed resolution AG-02992009, declaring Escudo de Veraguas, its flora and fauna, and the
surrounding waters a “landscape reserve” and protected area
(C. de Doens 2009). However, in 2012, Panama’s Law #11
(Aparicio 2012) reclassified Escudo de Veraguas as being open
SPECIAL FEATURE—BIOLOGY AND CONSERVATION OF PYGMY SLOTHS5
to development for “scientific, entertainment, tourist, and cultural purposes” by organized groups, communities, and individuals within Ngäbe-Buglé Comarca (indigenous territory).
Several development projects are currently being proposed for
the island, including an eco-lodge, casino, and marina (Ford
and Jeff 2013).
Future.—The fate of the pygmy sloth depends largely on
whether or not additional steps are taken to protect Escudo de
Veraguas. The island’s current protection status leaves open the
possibility of future development and is vague as to who may
develop it. Declaring the island a wildlife refuge or national
park would protect not only the pygmy sloths, but also the other
unique species found on the island. Similar conservation legislation has been successfully enacted in the past by the government
of Panama to protect vulnerable island ecosystems. For example,
Isla Iguana has been given the status of “wildlife refuge,” which
protects the island’s flora and fauna while also allowing tourism
and local use under the supervision of park rangers (C. de Doens
2007). A similar status as a protected national park (with park
rangers and a ranger station) for Isla Escudo de Veraguas would
not only protect the native wildlife, but could also regulate the
island’s usage by the local indigenous visitors and tourists.
Further scientific research on the pygmy three-toed sloth is
much needed, and we hope will continue to yield new insights
into their life history. Such research will not only help us to
better understand the species, but will enable us to ensure it
persists into the future.
Acknowledgments
I thank the Eddie Bauer clothing company, Zoological Society
of London, Smithsonian Tropical Research Institute, BBC
Natural History Unit, and Max Planck Society for supporting the conservation work on Escudo de Veraguas. Thank you
also to J. Lemire and “1,000 jours pro la planète,” D. Smith,
C. Turner, M. Manupella, and the numerous others who have
assisted with fieldwork. N. Rattenborg, W. J. Loughry, and
M. Superina provided valuable comments on the manuscript.
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Editor for Special Feature was Barbara H. Blake.