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Herpetological Review, 2002, 33(x), xxxx.

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, amphib-

ians, rodents, birds, various reptiles including snakes and conspe-

cifics, and carrion (Gloyd and Conant 1990; Savitzky 1992). The

Florida cottonmouth,A. p. conanti, is an abundant snake through-

out much of Florida, with well-established populations on some

of the offshore islands. The success of this species in varied habi-

tats is related, in part, to the wide range of prey items that providea 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 fo-

cused 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 hard-

wood forest where they are entirely terrestrial. These snakes sub-

sist 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 (Pelecanusoccidentaliscarolinensis ), Double-crested Cormorants

(Phalacrocorax auritus), White Ibis (Eudocimus albus), and sev-

eral 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 investi-

gate these environments where numerous objects become splat-

tered 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 ig-

nored by snakes are frequently found in or near the rookeries. Both

their shape and size appear to render them too difficult to swal-

low. Foraging snakes are generally quite placid and can be ap-

proached 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 con-

tact 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 to-

tal 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 sub-

strate 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 tem-

perature and humidity at the time of observation were 31.4C and61%, respectively. The snake appeared to be in good health and

was somewhat distended posteriorad, suggesting possible preg-

nancy.

The snake crawled very slowly and deliberately over the sub-

strate 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 30and 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 theeffort 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 x4

x1.5 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 swal-

lowed, but we disturbed the snake while advancing to take photo-

graphs. The snake crawled forward and disappeared within the

forest edge that bordered the rookery site.

These and other observations of cottonmouth foraging behav-

iors 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 some-

times contain leaves that are ingested inadvertently as adhering

parts of carrion. In general, Agkistrodonspecies 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

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feeding system and use of a strongly proteolytic venom, coupled

with predominance of chemosensory information during forag-ing, have likely conferred a predilection for carrion in the cotton-

mouth (Savitzky 1992). However, the significance of such behav-

ior 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 peri-

ods lasting from April/May to October/November (late fall nest-

ing 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 JulyOctober. 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 adher-

ence 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 dis-

tances unless the locations of food resources change (Wharton

1969). Thus, the scavenging of diverse objects that yield poten-

tially useful energy during foraging might have significant selec-

tive advantages in island environments where alternative prey items

are scarce or limited. The intensive scavenging behaviors of cot-

tonmouth snakes at Seahorse Key perhaps have become incorpo-

rated to be part of an island syndrome of behavioral and physi-

ological modifications that have evolved in response to resource

limitations on islands (Adler and Levins 1994; McNab 1994;

Wikelski and Trillmich 1997).

Acknowledgments.We owe special thanks to Kenneth Litzenberge

of the Lower Suwannee and Cedar Keys National Wildlife Refuge man

agement 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.

LITERATURECITED

ADLER, G. H., ANDR. LEVINS.1994. The island syndrome in rodent popu

lations. Quart. Rev. Biol. 69:473487.

CHISZAR, D., L. SIMONSEN, C. W. RADCLIFFE, ANDH. M. SMITH.1979. Rate

of tongue flicking by cottonmouths (Agkistrodon piscivorus) during

prolonged exposure to various food odors, and strike-induced

chemosensory searching by the cantil (Agkistrodon bilineatus). Trans

Kansas Acad. Sci. 82:4954.

COCKBUNING, T. DE, R. C. GORIS, ANDS. TERASHIMA.1981.The role o

thermosensitivity in the feeding behavior of the pit vipersAgkistrodon

blomhoffii brevicaudatus. Jap. J. Herpetol. 9:727.

FRITTS, T. H. 1988. The brown tree snake, Boiga irregularis, a threat to

Pacific islands. U.S. Fish Wildl. Serv., Biol. Rep. 88(31). 36 pp.

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A Monographic Review. Society for the Study of Amphibians and Rep

tiles.MCNAB, B. K.1994. Resource use and the survival of land and freshwater

vertebrates on oceanic islands. Amer. Nat. 144:643660.

SAVITZKY, B. A. C. 1992. Laboratory studies on piscivory in an opportu-

nistic pit viper, the cottonmouth, Agkistrodon piscivorus. In J. A

Campbell, and E.D. Brodie, Jr. (eds.), Biology of the Pitvipers, pp

347368. Selva, Tyler, Texas.

WHARTON, C. H. 1966. Reproduction and growth in the cottonmouth

Agkistrodon piscivorus Lacpde, of Cedar Keys, Florida. Copeia

1966:149-161.

______. 1969. The cottonmouth moccasin on Sea Horse Key, Florida

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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.