Defensive Behavior of Cottonmouths (Agkistrodon piscivorus)
ENOMOUS snakes have a reputation
sive when approached by humans (Klauber,
1972). Attitudes about potential harm from hos-
tile behavior of venomous snakes extend to
some biologists, including even a suggestion
that natural selection has operated through
‘‘biocultural evolution’’ in developing an innate
predisposition by humans to learn to fear
snakes (Wilson, 1993). A pervasive belief in the
southeastern United States is that the cotton-
mouth (Agkistrodon piscivorus), a common ven-
omous snake around many aquatic areas, is dan-
gerous not only because of its venom and vio-
lent temper (Ernst and Barbour, 1989) but be-
cause it will bite whenever possible and even
attack or chase people (Blythe, 1979).
In contrast to the attitudes of anxiety and fear
toward venomous snakes by most people, some
herpetologists have maintained, beginning al-
most a century ago (Ditmars, 1907), that the
image of some or all venomous snakes as ag-
gressors toward humans is greatly overstated,
with escape or efforts to go undetected being
the most likely behavioral responses (Shine,
1991; Greene, 1997). New Guinea natives have
been suggested to show a learned response in
distinguishing between dangerous and harmless
snakes but not to possess ‘‘an irrational innate
fear’’ (Diamond, 1993). Despite the contention
by some herpetologists that cottonmouths are
not aggressive when encountered in the ﬁeld
(Wright and Wright, 1957; Gloyd and Conant,
1990) and that the primary purpose of venom
is to subdue prey (Campbell and Lamar, 1989),
misgivings about the species persist among the
general public and many scientists.
Humans have been used to represent gener-
alized threat stimuli by simulating a predator to
elicit defensive behaviors in snakes (Scudder
and Chiszar, 1977; Goode and Duvall, 1989;
Greene, 1989), and tests of the effects of tem-
perature and other factors on defensive re-
sponses by snakes have been conducted (Scud-
der and Chiszar, 1977; Goode and Duvall,
1989). Nonetheless, quantitative measurements
for purposes of determining the response of
venomous snakes to humans are limited (Whi-
taker and Shine, 1999), and the suite of behav-
ioral responses of cottonmouths to direct phys-
ical contact with humans has not been reported.
The objective of our study was to test the de-
fensive responses of free-ranging cottonmouths
confronted by a human aggressor. We use this
test to determine whether conventional wisdom
about aggressive behavior in this species is war-
ranted. Information about the validity of real
versus perceived threats by venomous snakes is
necessary to address irrational negative atti-
tudes and to help quell the recently document-
ed global decline in reptiles (Gibbons et al.,
of behavioral responses to encounters by hu-
mans because (1) they can be found in high
densities in many areas of the southeastern
United States (Ernst and Barbour 1989), (2)
preconceived notions exist that suggest that cot-
tonmouths are among the most aggressive of
North American venomous snakes (Blythe,
1979; Ernst, 1992; Rubio, 1998), and (3) they
exhibit a suite of measurable defensive behav-
iors suitable for tests in the ﬁeld.
We searched the Savannah River ﬂoodplain
swamp on the U.S. Department of Energy’s Sa-
vannah River Site (Gibbons et al., 1997) in
South Carolina for cottonmouths during spring
COPEIA, 2002, NO. 1
Defensive responses of wild cottonmouths
of individuals that responded in a particular manner
for each of the three different treatments: stand be-
ϭ 13), step on (n ϭ 22), and pick up (n ϭ
summer of 2000. We examined defensive be-
havior of wild cottonmouths in response to a
human aggressor by subjecting them to one or
more of three different treatments. When we
encountered a cottonmouth in the ﬁeld, we ap-
proached the snake and either (1) stood beside
it with a ‘‘snakeproof’’ boot touching its body,
(2) stepped on the snake at midbody with
enough force to restrain but not injure it, or (3)
picked up the snake at midbody with a pair of
1-m snake tongs (Whitney Tongs) with a grasp-
ing handle that was modiﬁed to resemble a hu-
man arm and hand. A leather glove was ﬁtted
over the end of the tongs, with one extension
covered by the thumb and the other by the mid-
dle ﬁnger. Hence, the glove could be closed
around the snake’s body. A padded shirt sleeve
was used to cover the remainder of the rod up
to the handle. Each treatment was carried out
for 20 sec, and the behavior of each snake was
For each treatment on each snake tested, we
recorded whether it attempted to escape by
crawling or swimming away, exhibited other de-
fensive behaviors (vibrated tail, released musk,
gaped, feigned a bite by striking but not closing
the mouth), or bit the boot or model hand. An
audio tape recorder was used to describe events
during each encounter, and the majority of en-
counters were videotaped. After testing, the sex,
size (snout–vent length to nearest centimeter),
and body temperature (to nearest C using a clo-
acal thermometer) were determined for most
counters of 45 snakes (12 females, 16 males, 17
of undetermined sex) in the ﬁeld. Of these,
nine escaped (immediately entered water)
when ﬁrst observed or approached and were
not available for further testing. Of the remain-
ing 36 snakes, we initially stood beside 13,
stepped on 12, and picked up 11. Of those that
we stood beside, 10 were then stepped on, and
of those stepped on initially or secondarily, 15
were picked up, resulting in total sample sizes
of 13 (stood beside), 22 (stepped on), and 36
Body size ranged from 20–101 cm (mean
59 cm, n
ranged from 17–27 C (mean
ϭ 24 C, n ϭ 25).
No relationships were observed among the be-
havioral responses of individuals and their sex,
size, or body temperature; hence these variables
were omitted from further analyses.
Of the 13 individual cottonmouths that we
initially stood beside (Fig. 1), four attempted to
escape, ﬁve gave some form of defensive display,
and none tried to bite, although one individual
feigned a bite during a strike. Only two of the
individuals performed more than one defensive
or secondarily, 15 gave defensive displays, in-
cluding two that feigned bites. One bit the boot.
Nine of those stepped on were attempting to
escape by crawling away. Of the 36 individuals
that were picked up, 13 (36%) bit the artiﬁcial
hand near the point of contact with the snake’s
body. The probability of a cottonmouth biting,
regardless of the testing procedure, was less
than the probability of it not biting (chi-square
ϭ 15.05, df ϭ 2, P Ͻ 0.005).
tention of Pope (1958) that ‘‘snakes are ﬁrst
cowards, then bluffers, and last of all warriors.’’
Upon being seen, nine (20%) of the individuals
ﬂed into nearby water, apparently sensing that
immediate escape was possible and a safe route
was accessible. Such ﬂight behavior by cotton-
mouths was noted by Ditmars (1907), who stat-
ed that ‘‘snakes that observed us when some lit-
GIBBONS AND DORCAS—COTTONMOUTH DEFENSE TOWARD HUMANS
tle distance away, made for the water and es-
caped. . . .’’ Ernst (1992) likewise reported that
cottonmouths try to escape when ﬁrst dis-
which the name ‘‘cottonmouth’’ originates, was
observed by us in 64% of the individuals that
did not ﬂee. In addition, 33% vibrated the tail,
and 24% emitted a detectable musk. The open-
mouthed behavior is presumed to be a true
threat display to warn a predator that a bite is
imminent. Tail vibration has been regarded as
a warning signal in rattlesnakes (Crotalus; Sistru-
rus; Greene, 1988) and presumably serves the
same function in cottonmouths that it does in
many other snakes. The musky odor is pre-
sumed to be a means by which an individual
presents itself as a distasteful meal prior to at-
tack by a predator.
No relationship between body temperature of
the snake and a tendency to bite was apparent
in our study. Likewise, in a study of prairie rat-
tlesnakes (Crotalus viridis viridis), males and
non-gravid females showed no temperature-re-
lated change in defensive behavior, although a
negative relationship was observed between
temperature and defensive response in gravid
females (Goode and Duvall, 1989).
Most venomous snakebites in the United
States occur when someone picks up a snake or
attempts to kill it (Ernst and Zug, 1996). Our
results with cottonmouths quantify and support
this assertion. Such ‘‘illegitimate’’ snakebites
(‘‘those sustained by individuals who knowingly
place themselves at risk’’; Minton, 1987) may be
induced only after considerable harassment to
the snake. Of the 11 snakes in our study picked
up without previously being stepped on, only
one bit the glove, suggesting that the preceding
harassment (stepped beside or on) provoked
the highest incidence of biting.
The cost to a snake of biting and injecting
venom into a human antagonist has not been
quantiﬁed, but certain unfavorable consequenc-
es are obvious. Engaging in a ﬁght with a larger
animal constitutes unnecessary exposure that
could lead to injury or death for the snake.
Even if an effective bite were delivered, the time
to incapacitate an animal as large as a human
would still permit time to injure or kill the
snake. Therefore, once a snake perceives it has
been detected and has no ready escape route,
threats or other defensive displays designed to
ward off an attacker should be favored by nat-
ural selection over actual biting.
In the current study, little or no evidence of
venom was present on the glove following most
of the bites. Some venomous snakes can control
the amount of venom injected based on prey
size (Hayes, 1995), suggesting that snakes can
conserve venom when biting. Such control can
presumably be exercised during defensive bit-
ing, with the act of biting serving as a threat
display itself, without the injection of venom.
Such behavior may explain the high frequency
of ‘‘dry bites’’ in which little or no venom is
injected into human victims (Parrish et al.,
1966). However, we maintain that escape by
snakes from human confrontation by some
means other than biting is the most prudent.
er Ecology Laboratory, especially T. Mills, for
ﬁeld assistance and thank R. Semlitsch, J. Pech-
mann, K. Buhlmann, and D. Scott for helpful
comments on the manuscript. Testing was con-
ducted under South Carolina Department of
Natural Resources scientiﬁc collecting permits
issued to JWG and under auspices of the Uni-
versity of Georgia Animal Care Protocol pro-
gram. Research and manuscript preparation
were supported with Financial Assistance Award
DE-FC09–96SR18546 from the U.S. Department
of Energy to the University of Georgia Research
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Send reprint requests to JWG. Submitted: 9
Jan. 2001. Accepted: 10 June 2001. Section
editor: C. Guyer.