The Western Rattlesnake (Crotalus viridis)
The western rattlesnake, Crotalus viridis,
is the most widely distributed rattlesnake in the western United States and
Canada, and also the most variable in North America, with nine subspecies.
It is also Oregon's only truly venomous snake. Two subspecies are found within
Oregon's borders; C. v. oreganus, the Northern Pacific Rattlesnake,
and C. v. lutosus, the Great Basin Rattlesnake. By studying the ecology
of these potentially dangerous snakes I hope to demonstrate the wonderous
diversity of these New World snakes that we threaten to eliminate as a result
of unsubstantiated fear.
There are no other species of snakes
that can be readily confused with a rattlesnake, but intraspecific recognition
becomes difficult due to the many variations in color and pattern. The snakes
do have a unique relatively robust body, broad, diamond-shaped head, infamous
rattles, and hidden fangs making them easily identifiable as dangerous. C.
viridis varies in ground color from gray, olive, greenish gray, greenish
brown, brown, yellowish brown, tan, salmon, and reddish to black. The dorsal
body pattern consists of a series of 20 to 50 hexagonal or circular blotches,
which may become more like crossbands near the tail. It has sharply outlined
head marks with 2 light diagonal stripes occurring on the side of the face.
One line extends from in front of the eye along the mouth's labial scales and
the other from in back of the eye to the back corner of the mouth. Narrow, light
transverse stripes may pass across the face over the supraocular scales. The
underside is gray, cream, or white with no dark markings and lateral secondary
blotches run the length of the body. Adult snakes typically reach 24-60 in.
in total length with males averaging larger than females. They can attain the
larger lengths over an estimated lifespan of 16-20 years in the wild (Fitch,
1949). The longevity record for C. viridis is 27 years and 9 months in
captivity. The growth rate of individual snakes is quite variable, depending
on foraging success and climatic conditions. It is known, however, that growth
slows with age and length.
Rattlesnakes add a rattle to the string each
time the skin is shed. The rattle is in fact composed of hardened keratin.
At birth each rattlesnake has a prebutton on the tip of its tail. This prebutton
is replaced at the first shedding with the button, and subsequent rattles
are added with each shed skin. C. viridis typically sheds 3-5 times
in its first summer and 1-3 times in subsequent years. Since growth is variable
and the string of rattles seldom remains intact, it is inaccurate to use the
number of rattles as justification of age.
The western rattlesnake ranges from south-central British Columbia, southeastern
Alberta, and southwestern Saskatchewan southeastward through the United States
to extreme western Iowa, Nebraska, and Kansas, and south to northern Baja California
and northern Mexico. Elevations occupied range from near sea level to over 12,000
ft. (Wright, and Wright 1947). In Oregon C. v. oreganus occurs east of
the cascades and in valleys west of the Cascades south of Salem while C.
v. lutosus occurs only in the southeastern corner of the state.
The northern Pacific rattlesnake is one of the most broadly tolerant of all
rattlers in its choice of habitats. It occurs in arid plains, desert margins,
fertile valleys, prairie grasslands, chaparral-covered foothills, on rocky ridges,
in mountain meadows, and forests. Northern Pacific rattlesnakes are often found
around watercourses in the summer, but too much water can be a limiting factor.
They are unable to maintain stable populations in coastal coniferous forests
probably due to excessive moisture and notably low summer temperatures. C.
v. lutosus occurs nearly exclusively in the arid plains and desert areas
of southeastern Oregon. All rattlesnakes are careful to keep usually south-facing
rocky outcroppings with deep crevices or prairie dog towns within migratory
C. viridis is generally only active from April through September. The
pivotal body temperature for both arousal and dormancy is 10°C (Jacob and
Painter, 1980), but a body temperature of 16°C may be necessary to bring
them out of the den (Woodbury, 1951). Rattlesnakes conserve their energy during
the coldest winter days and nights by entering a state of torpor. Males and
females emerge from the hibernacula at about the same date. The dens of these
snakes may include several other species of snakes as well. As spring progresses,
they disperse onto their summer range, usually after remaining near the hibernaculum
for several days or weeks. The mean size of these summer activity ranges was
found to be only 2.9 ha. for males and 1.8 ha. for females, but ranges most
likely are larger than these figures indicate (Macartney et al, 1988). They
migrate long distances to and from the den in the spring and fall. These long
migrations which may measure kilometers in distance are not usually direct.
Snakes may stay in an area rich in food supply for quite some time before moving
on. Field observations seem to indicate that snakes use a fixed-angle, sun-compass
orientation in homing to the den and their summer range (Duvall et al, 1985).
Watercourses provide no barrier to C. viridis as they have been shown
to swim very well (Klauber, 1972).
During the spring and fall, C. viridis often prowls during the middle of the
day. With the onset of summer and temperatures averaging more than 25°C,
the snake's activity patterns shift to mainly in the morning and late afternoon.
Snakes may be active nocturnally if climate permits. The hours in between are
spent either under cover, coiled in the shade, or basking depending on the air
temperature. Adult snakes prefer to maintain body temperatures of 20-35°C.
The critical thermal maximum for C. viridis is 38°C, and body temperatures
of 41-42°C are lethal (Brattstrom, 1965).
Food and Feeding
Pit vipers like C. viridis have several adaptations making them well
suited for both seeking and catching prey. C. viridis uses 2 predatory
strategies: either that of an actively searching forager, or that of sit and
wait ambush (Diller, 1990). The first is used when prey are scattered or often
during migration to or from the den. The second is used more often when colonies
of rodents have been identified and the snake can lie near the opening of burrows
or rodent runs.
Prey may be detected by vision, infrared emissions, or olfaction. Movement
of the prey seems to be the primary visual component that brings on further
exploration (Scudder and Chiszar, 1977). It has been shown, however, that
vision is not necessary for successful hunting. A congenitally blind, captive
C. v. oreganus accurately directed its strikes to the vulnerable
anterior region of mice (Kardong, 1991). In general, the vision of the
rattlesnake is moderately good, but with narrow spatial limitations.
A curious snake will usually increase the rate of its tongue flicking obtaining
more detailed olfactory information about its surroundings (Chiszar, 1981).
The tongue is an accessory to the sense of smell. It picks up minute particles
in the air and transfers them to 2 pits, known as Jacobsen's (or vomeronasal)
organs in the roof of the mouth. The organs relay the chemosensory information
resulting from the airborne particles, through the vomeronasal nerve, to the
brain where the information is combined with other sensations (Duvall et al,
1990). Jacobsen's organs, however, are not the sole source of the snake's olfactory
knowledge. Odors can also be detected by the nose and its accompanying organs
of scent. This unique combination of sensory input gives rattlesnakes a keen
sense of smell that allows them to seek out and methodically trail bitten prey
without unnecessary retaliation (Diller, 1990).
As important as olfactory cues are to seeking and recovering prey, it appears
that they may be unnecessary for predatory strikes. Free ranging C. viridis
were exposed to warm models of deer mice (Peromyscus) devoid of odors.
Snakes always struck the models with core temperatures 1.5-4.5°C warmer
than the background air temperatures (Hayes and Duvall, 1991). C. viridis utilize
a pit organ located in the loreal scales of the head to detect prey body heat
as it relates to background temperatures. Experiments by Noble and Schmidt (1937)
apparently show that the thermoreceptive function of the pit is essentially
of short-range value, but the acuteness of the pit to the thermal fluctuations
within its range of approximately 350mm is astounding. Very sensitive nerves
in the mouth and an auxiliary infrared sensitive system, nociceptors, may also
contribute to thermal recognition of prey (Dickman, 1987).
Once prey is found, rattlesnakes use efficient and deadly venom to subdue
their prey. Members of Crotalus have the longest fangs of any snake species
(Klauber, 1972). They are able to keep such large fangs hidden within their
mouth because of a unique hinge mechanism. The fangs are individually hinged
so that they can easily rotate from a passive position to the active biting
position. In the resting position, they are folded back against the upper jaw,
with the base and point at about the same level, and the bulge of the fang's
curve fitting into a hollow in the lower jaw. To assume their striking position,
the fangs are rotated downward until the proximal part of the fang is substantially
perpendicular to the upper jaw (Klauber, 1972). These fangs are sequentially
replaced at intervals not exceeding 8 weeks by a series of replacement fangs
lining the upper jaw. At the base of each hollow fang is a duct that leads to
the venom gland. The snakes do have the ability to control the release of venom
in both or individual fangs by controlling the pressure placed on a sphincter
located by the individual venom glands.
Venom has 2 major purposes: it is used to poison the prey and prevent retaliatory
injury to the snake and aids in digestion. The venom of C. viridis is
predominantly hemorrhagic in its effects (Russel, 1983), but hemorrhagic, neurologic,
and proteolytic activities may all result during a single bite. Snakes farther
north have higher hemorrhagic capabilities (Adame et al, 1990) and larger snakes
have increased venom yields. The mean mass of venom expended in a single predatory
strike is 14 mg with 89% of the venom being injected into the tissues (Hayes
and Duvall, 1991).
Surprisingly, C. viridis is not immune to its own venom, however,
the venom must enter the bloodstream to be effective. The overall toxicity
of the venom is slightly higher than that of the slightly larger species
C. atrox, the western diamondback rattlesnake, and the potency coupled
with the high irritability of many individuals makes C. viridis
a very dangerous snake (Russel, 1983). Symptoms recorded by Hutchinson
(1929) resulting from human envenomation include swelling, pain, weakness,
giddiness, breathing difficulty, hemorrhage, weak pulse, heart failure,
nausea, vomiting, secondary gangrene infection, ecchymosis, paralysis,
unconsciousness, nervousness, and excitability. Luckily, bites are relatively
infrequent in occurrence and rarely lead to death in humans. In smaller
prey, envenomation leads to certain death within minutes.
C. viridis has the most variable diet of any rattlesnake in North America.
Warm blooded prey seem to be preferred, but reptiles, amphibians, and bird eggs
are also taken. Prey selection is limited by the size that can easily be swallowed,
and prey size increases as the snakes grow. The chief prey of C. viridis
in Oregon are ground squirrels (Spermophilus), kangaroo rats (Dipodomys),
cottontails (Sylvilagus), white footed mice (Peromyscus), pocket
gophers (Thomomys), and voles (Microtus) (Diller and Johnson,
1988; Wallace and Diller, 1990).
C. viridis may also be forced to use its harmful venom on 1 of many predators
known to prey on rattlesnakes. Hawks (Buteo), golden eagles (Aquila),
and coyotes (Canis latrans) may be the worst predators of rattlesnakes,
but their diet will rarely consist of more than 1% rattlesnake (Fitch, 1949).
Accidents involving large animals such as deer (Odocoileus), horses (Equus),
and members of the Bovidae family trampling snakes are often fatal and may account
for more deaths than natural predators. However, C. viridis has the most
to fear from humans who usually kill them on sight.
Mating in C. viridis is a science of confusion. It is believed that
female rattlesnakes are on a biennial reproductive cycle, but annual cycles
have been noted (Charland, 1989). Male rattlesnakes are capable of mating at
any time, except when in torpor, with varying success, depending on climatic
conditions. Females however, have the ability to hold mature spermatazoa within
their oviducts for years or until ovulation in the spring (Hayes, 1986). For
a long time a so called "combat dance" was thought to be 2 snakes
mating. In reality the dance involves 2 male snakes that may entwine their bodies
together and lunge at each other with their heads. The ritual affirms the dominance
of 1 snake over the other yet results in no harm to either snake. Although this
curious act demonstrates dominance and strength, desired evolutionary attributes,
no females need be present to witness the act. To further complicate our understanding
of breeding, climatic and geographic variations lead to variations in reproduction
among members of the same subspecies.
Several aspects of the reproductive process are definitively known though. Rattlesnakes
are viviparous. They keep their fertilized eggs within their bodies until maturation
and then give live birth to the young. A normal clutch of young contains anywhere
from 1 to 14 rattlers with an equal ratio of males to females. The birthing
season lasts from late summer, around August, through early fall, around October
(Diller and Wallace, 1984). At the time of birth the young snakes average 251
mm in length. Number of young is proportional to female body length, and species
farther north generally have fewer young (Macartney and Gregory, 1988). Mating
usually takes place in the late spring and summer, coinciding with the peak
of male spermatogenic activity. Males must take the initiative to find the females
while still searching for a productive home range, whereas females concentrate
solely on reaching their home range and finding food (Duvall et al, 1990). Males
find females following olfactory cues such as pheromone trails (Scudder et al,
1988) and breed in a polygamous fashion.
A lack of knowledge and the resulting fear threatens the existence of these
misunderstood pit vipers. If the habitat is proper, C. viridis may be
the most common local snake, but in many areas populations are disappearing
due to human intervention. Generally the largest existing populations occur
in isolated areas. Countless rattlesnakes have been murdered in our efforts
to control their populations. Of course our idea of control is complete irradication
of all rattlesnakes. Many methods of control have been used in the past including
campaigns for killing at dens (extremely effective), traps, fences, gases and
poisons, encouragement of competitive predators and enemies, elimination of
food supply and cover, bounties, and fires. Rattlesnakes must also contend with
floods, freezes, disease, speeding cars, and loss of habitat due to settlement
and agriculture. However, their value as small mammal predators, which can cause
great harm to agricultural and grazing fields and may spread disease, is rarely
acknowledged. The threat of a painful and possibly lethal bite dominates our
view of these snakes even though they have a defensive temperament, avoiding
larger animals including us whenever possible. Nearly all defensive strikes
are precluded by a buzzing rattle, and many are in fact "dry bites"
with no venom injected. Although envenomation may cause serious tissue damage
if not treated within 18 hours (Klauber, 1972), it rarely causes death. Oregon's
rattlesnakes should be treated with caution, but they need not be feared.
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