Scavenging Behaviors of Cottonmouth Snakes at ... - ResearchGate

Herpetological Review, 2002, 33(x), xx–xx.
© 2002 by Society for the Study of Amphibians and Reptiles
Scavenging Behaviors of Cottonmouth Snakes at
Island Bird Rookeries
HARVEY B. LILLYWHITE *
COLEMAN M. SHEEHY III
and
MARSHALL D. MCCUE
Seahorse Key Marine Laboratory, Department of Zoology
The University of Florida, Gainesville, Florida 32611-8525, USA
e-mail [HBL]: hbl@zoo.ufl.edu
*
To whom correspondence should be sent concerning this manuscript.
Cottonmouth snakes, Agkistrodon piscivorus, are well known
as generalist and opportunistic feeders consuming fish, amphibians,
rodents, birds, various reptiles including snakes and conspecifics,
and carrion (Gloyd and Conant 1990; Savitzky 1992). The
Florida cottonmouth, A. p. conanti, is an abundant snake throughout
much of Florida, with well-established populations on some
of the offshore islands. The success of this species in varied habitats
is related, in part, to the wide range of prey items that provide
a potential energy base for populations (see also Fritts 1988).
Insular populations of cottonmouths inhabiting the Cedar Keys
of Florida have been studied by Wharton (1966, 1969) who focused
his investigations on an unusually dense population of snakes
inhabiting Seahorse Key. This island is approximately 64 ha in
size and was estimated to support 600 cottonmouths during the
period when Wharton conducted his studies. The island has no
permanent fresh water, and the cottonmouths inhabit a mixed hardwood
forest where they are entirely terrestrial. These snakes subsist
largely or entirely on fish that are dropped or regurgitated by
colonial wading birds that nest on the island in large numbers from
March through September or October. At present, the colonial
nesting birds are largely represented by Brown Pelicans (Pelecanus
occidentaliscarolinensis), Double-crested Cormorants
(Phalacrocorax auritus), White Ibis (Eudocimus albus), and several
species of herons and egrets.
One of us (HL) has confirmed and extended the observations of
cottonmouth behaviors recorded by Wharton. At Seahorse Key
these snakes characteristically forage at night on substrate directly
beneath bird rookeries where avian excreta alter the vegetation
and humus layers of soil. Foraging snakes methodically investigate
these environments where numerous objects become splattered
with fluids that are regurgitated or excreted by birds and
presumably contain fish odors. The snakes largely consume fish,
although occasional young birds fallen from nests might also be
ingested. It is not clear, however, whether birds are eaten for their
own sake or because they bear fish odors. Dead birds that are ignored
by snakes are frequently found in or near the rookeries. Both
their shape and size appear to render them too difficult to swallow.
Foraging snakes are generally quite placid and can be approached
and observed at close distances. They readily eat fish
when these are presented to, or thrown near, a foraging snake.
Snakes are also observed to investigate and grasp objects such as
plant materials, bones, or bird feathers that have contacted avian
excreta or fish fluids. These are usually released following contact
with the internal tissues of the mouth.
Recently we observed some unusual scavenging behaviors in a
foraging female cottonmouth estimated to be about 110 cm in total
body length. The snake was first seen at 1810 h EDST and was
observed for 20 min while it foraged in an open area beneath a
Brown Pelican rookery at Seahorse Key, 2 June 2001. The substrate
was comprised of sandy soil covered with many fallen twigs,
dried leaves, and other scattered debris. Most of these objects were
white or mottled due to urates that were excreted by the pelicans
nesting overhead. The area was in full sun, and the ambient temperature
and humidity at the time of observation were 31.4°C and
61%, respectively. The snake appeared to be in good health and
was somewhat distended posteriorad, suggesting possible pregnancy.
The snake crawled very slowly and deliberately over the substrate
while investigating various objects with frequent tongue
flicks. We first observed the animal through binoculars from a
distance of about 8 m. Eventually, we advanced closer to the snake,
which continued to forage and eventually approached us within
0.5 m distance. Although the snake appeared to be aware of our
presence at this point, it continued to forage as described.
Soon after we first saw the snake, it investigated the ground
intensely at one location and positioned its head nearly vertically
with the mouth directed downward as it grasped a dark object.
When the snake lifted its head, the grasped item was seen to be
soil as half of it broke away from that being held in the mouth and
fell to the ground. The snake then lifted its head upward at an
angle of about 30° and swallowed the soil, which was seen to be
damp and loosely held together with moisture. Later inspection of
the ground on which the snake foraged revealed several locations
where the soil was wetted, due either to excreta or regurgitated
fluids from birds overhead.
Following this act, the snake continued to investigate objects
that were on the ground. Next it investigated the blunt end of a
wooden branch about 1 cm in diameter and bearing several stains
of bird excreta. The snake grasped this branch and attempted to
advance the jaws over it, but the object was soon dropped and the
effort abandoned. The snake continued to crawl slowly over the
substrate using rectilinear locomotion while investigating objects
with searching movements of the head and tongue.
Finally, the snake stopped to investigate an object roughly 4 x 4
x 1.5 cm in overall size and comprised of a partial fish skull with
other bones, debris and several dry leaves adhering to it. The snake
grasped this object and advanced its jaws to hold the bulk of the
item firmly within its mouth (Fig. 1). Then it elevated its head
further off the ground, crawled forward about 3 m and stopped
with the head elevated. We believe the item would have been swallowed,
but we disturbed the snake while advancing to take photographs.
The snake crawled forward and disappeared within the
forest edge that bordered the rookery site.
These and other observations of cottonmouth foraging behaviors
at Seahorse Key suggest that numerous, diverse objects of
appropriate size, shape, and odor might be ingested by foraging
snakes. Thus, feces that are defecated by wild-caught snakes sometimes
contain leaves that are ingested inadvertently as adhering
parts of carrion. In general, Agkistrodon species appear to rely
more on chemosensory information during foraging than do other
crotalines (Chiszar et al. 1979; Cock Buning et al. 1981). It has
been suggested that evolutionary reliance on an envenomation

Wikelski and Trillmich 1997).
Acknowledgments.—We owe special thanks to Kenneth Litzenberger
of the Lower Suwannee and Cedar Keys National Wildlife Refuge management
office for a special use permit to study cottonmouth snakes at
Seahorse Key. Logistical support was provided by the University of Florida
Seahorse Key Marine Laboratory.
LITERATURE CITED
FIG. 1. A female Florida cottonmouth, Agkistrodon piscivorus conanti,
ingesting a dry mass of fish skull with adhering bones, dry leaves, sticks,
and soil debris at a pelican rookery on Seahorse Key. Note the generally
white appearance of the ground and numerous white spots on the leaves
and other objects, including the snake. These are dried urates excreted by
pelicans from the overhead tree canopy.
feeding system and use of a strongly proteolytic venom, coupled
with predominance of chemosensory information during foraging,
have likely conferred a predilection for carrion in the cottonmouth
(Savitzky 1992). However, the significance of such behavior
in energetic contexts has not previously been emphasized.
At Seahorse Key, fish dropped by nesting birds can be a “boom
or bust” situation. In the 1970s, as many as 200,000 colonial birds
occupied the Cedar Keys, and 95% of all nesting occurred on
Seahorse Key (U.S. Fish & Wildlife Service, unpublished records).
Such numbers produce abundant and concentrated energy resources
for cottonmouths, which literally gorge themselves during periods
lasting from April/May to October/November (late fall nesting
attributable to pelicans; HBL, unpublished observations). In
recent years, however, drought has severely impacted wetlands
on which many of the nesting birds depend for feeding their chicks.
Consequently, the number of nesting birds has diminished greatly.
Recent counts have been as low as 10,000 or fewer nesting birds.
During the severe 2000 drought in Florida (especially at Seahorse
Key), avian nesting was greatly curtailed at Seahorse Key, and the
snake foraging period was shortened to July–October. Moreover,
cottonmouths do not forage during drought, even though dropped
fish might be available at the rookeries (unpublished observations).
Consequently, foraging success can vary greatly from year to year.
It seems reasonable to infer that carrion degraded by age or adherence
of indigestible materials might be significant resources when
snakes employ foraging behaviors that maximize the intake of
available energy. Snakes at Seahorse Key remain beneath or very
near the rookeries, and they do not usually move extensive distances
unless the locations of food resources change (Wharton
1969). Thus, the scavenging of diverse objects that yield potentially
useful energy during foraging might have significant selective
advantages in island environments where alternative prey items
are scarce or limited. The intensive scavenging behaviors of cottonmouth
snakes at Seahorse Key perhaps have become incorporated
to be part of an “island syndrome” of behavioral and physiological
modifications that have evolved in response to resource
limitations on islands (Adler and Levins 1994; McNab 1994;
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