The odd couple: interactions between a sloth

NATURAL HISTORY NOTES NATURAL HISTORY NOTES
The odd couple: interactions between
a sloth and a brown jay
170
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African oxpeckers (Buphagus spp) perch on the backs of
large mammals, such as zebras and rhinoceroses, and feed
on ticks, botfly larvae, and other parasites (Dean and
MacDonald 1981). Several South American bird species
have been observed foraging for ectoparasites on the skin
of capybaras (Hydrochoerus hydrochaeris) (Tomazzoni et al.
2005). Even close relatives of the brown jay, including
magpies (Pica pica), have been seen perching on ungulates and pecking insects off the mammals’ bodies (Bent
1946). But this is the first known feeding association documented between a brown jay and any mammal.
Brown jays feed opportunistically on a diverse assortment of prey including insects, spiders, small lizards, and
frogs, as well as many wild fruits (Stiles and Skutch
1989). Now, you may be wondering why a brown jay
would want anything to do with a mouthful of sloth fur.
Three-toed sloths harbor a diverse and highly specific
assemblage of algae, fungi, and coprophagous arthropods,
including pyralid moths (Cryptoses spp) and scarabaeid
beetles (Waage 1979), in their pelage. These arthropods
depend on the sloth for phoresis, a special form of commensalism in which one organism attaches to another,
primarily as a dispersal mechanism (Waage and Best
1985). However, phoresis can also facilitate reproduction,
as in the case of the pyralid moths. Each week when a
sloth descends from the canopy to the forest floor to defecate, gravid female moths leave their sloth host and
oviposit in the fresh dung. After hatching, moth larvae
feed on and pupate within the sloth dung, then emerge as
adults and fly to the forest canopy to locate potential
mates within the fur of a sloth (Waage 1979). These
moths depend entirely on sloths to complete their life
J Yu
hen we speak of biodiversity loss, it is almost
always in terms of the extinction of individual
species. Forty years ago, however, Janzen (1974) wrote of
“a much more insidious kind of extinction: the extinction
of ecological interactions”. Ecological interactions are
critical elements of biodiversity and the Earth’s ability to
generate more biodiversity (Bøhn and Amundsen 2004).
As conservation research is shifting toward a landscape
perspective with an increasing emphasis on the structure
and dynamics of species interactions, Janzen’s concerns
are finally being addressed.
In May 2014, I traveled to the cloud forests of Costa Rica
to study the spatial ecology of three-toed sloths (Bradypus
variegatus) for my thesis research. On one particular cloudy
afternoon, I was searching for sloths on a 15-ha tree plantation near the town of San Isidro de Peñas Blancas. As I
walked a transect, I spotted a sloth feeding on leaves in a
Cecropia tree while a flock of brown jays (Psilorhinus morio,
Corvidae) caused a commotion nearby. Although the raucous calls of the brown jays caught my attention, the sloth
appeared to be unfazed. Three brown jays, perched on
Cecropia branches, exhibited a keen interest in the sloth.
What exactly the birds were so interested in was not initially apparent. As one brown jay audaciously inched closer
(Figure 1), tilted its head, and drove its beak deep into the
sloth’s fur, I began to better understand the bird’s behavior.
While the sloth was busy feeding, the brown jay appeared
to be searching for a meal of its own (Figure 2). Video
recordings of this unusual interaction are available on
YouTube (http://youtu.be/OwvvNKRFvIY).
Feeding associations between birds and mammals are
widespread, both taxonomically and geographically.
Figure 1. A brown jay (Psilorhinus morio) approaches a brown-throated three-toed sloth (Bradypus variegatus) in San Isidro de
Peñas Blancas, Costa Rica.
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KD Neam
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171
J Yu
cycle. As a result of this phoretic relationship, the pelage
of sloths contains a concentrated reservoir of moths and
other insects that would certainly be appealing to a
brown jay seeking a meal.
Before making an attempt to tease apart the precise
nature of the sloth–brown jay interaction, it is important
to first discuss the types of ecological relationships that
exist between sloths and their arthropods. Most research
on sloth–arthropod relationships indicates that there is no
apparent gain for the sloth host, and thus the interaction is
a commensal one (Ratcliffe 1980; Waage and Best 1985),
but recent work by Pauli et al. (2014) suggests that pyralid
moths are “phoretic symbionts”, whereby sloths also benefit from the association through a series of linked mutualisms. Specifically, Pauli et al. postulated that the sloths’
precarious and energetically costly journey to defecate on
the forest floor is not solely for the benefit of the moth;
rather it is driven by a cryptic, yet critical, nutritional
input. Whenever the adult moths die on their sloth hosts,
fungi living within the cracks of the sloth’s hair decompose
and mineralize their remains. This process increases the
concentration of inorganic nutrients within the sloth’s
ecosystem, thus fueling algal growth. Pauli et al. (2014)
suggested that sloths consume these lipid-rich green algae
(Trichophilus spp) in their fur to augment their nutritionally limited diet. In addition to nutrient supplementation,
these fur-borne algal communities provide camouflage
against aerial predators (Vizcaino et al. 2008). Under this
scenario, it is a complex mutualism, rather than a commensalism, that drives the sloth–moth association.
The implications of the sloth–brown jay association for
the sloth depend on the nature of the relationship between
arthropods and their sloth hosts. If it is commensal, then
the observed consumption of arthropods by the brown jay
may also be a form of commensalism, in which the brown
jay benefits and the sloth is seemingly not affected.
Alternatively, if the sloth–arthropod relationship is a
form of mutualism from which the sloth also derives
resource benefits, as suggested by Pauli et al. (2014), then
the brown jay’s feeding behavior may represent parasitism.
The removal of adult moths from the sloth’s fur could
potentially cause indirect negative effects; decreased availability of organic nutrients would limit the release of inorganic nutrients by fungi, ultimately resulting in lower algal
biomass and nutrient intake by the sloth. The loss of moths,
and consequently algae, could also negatively affect the
sloths by making them more visible to predators.
Sloths also carry an assortment of hematophagous
ectoparasites, including several genera of ticks, mites, biting flies, and triatomine bugs (Waage and Best 1985). If
the brown jay was foraging for ectoparasites rather than
moths, then they would be providing a beneficial service
by removing these parasites. Sloths could further benefit
from having brown jays in close proximity if the former
are alerted to the presence of predators in the area by the
birds. More research should be conducted to pinpoint the
true ecological relationship between sloths and their
Natural History Notes
Figure 2. The brown jay feeds on pyralid moths and other
arthropods hidden in the sloth’s fur.
associated arthropods before any inferences can be made
about the sloth–brown jay feeding association.
The unforeseen events that transpired during my threemonth-long investigation show the advantage of spending
an extended period of time in the field. Had I conducted a
study using existing data on sloths rather than traveling to
Costa Rica to collect data of my own, I would not have had
the chance to witness this interaction. Field-based
approaches to scientific research provide an opportunity
for building a solid understanding of species distributions
and activity patterns, as well as biotic interactions and
their importance in maintaining ecosystem processes.
Such new discoveries from the field offer important
insights into the functioning of ecosystems and communities. Discoveries of new information about organisms often
reset the research cycles of hypothesis testing and theory
refinement that underlie progressive science (Greene
2005). Understanding how organisms interact through
natural history and descriptive ecological research can
help us make important scientific advancements and practice conservation more effectively.
n Acknowledgements
Financial support for this research was provided by the
Department of Wildlife and Fisheries Sciences at Texas
A&M University. This field work would not have been
possible without the generosity of numerous landowners.
I thank J Yu for permission to use her photographs, and T
Lacher Jr for valuable comments and suggestions.
n References
Please see WebReferences
Kelsey D Neam
Department of Wildlife and Fisheries Sciences,
Texas A&M University, College Station, TX
([email protected])
www.frontiersinecology.org