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Systematics
Pigmy rattlesnakes are vipers (family
Viperidae). All vipers are venomous with hinged fangs
(68kb). The Viperidae is divided into two subfamilies, the
Viperinae and Crotalinae (the pitvipers). Pitvipers evolved
from a viperine ancestor. In addition to the traits
characteristic of the Viperinae they also have facial
pits that detect heat energy (infrared light). Vipers are
apparently a recent addition to the fauna of the new world.
About 20-30 million years ago it appears a pitviper crossed
over between Siberia and Alaska and thereby colonized this
hemisphere (Greene,
1992). In the years since the new world was colonized, the
pitvipers have undergone an adaptive radiation (evolved into
many species with diverse niches).
One advanced group of pitvipers evolved a rattle
(68kb) attached to the tip of the tail. There are two
genera of rattlesnakes--Sistrurus with three species
and Crotalus with 23 species (Campbell
and Lamar, 1989). Members of the genus Sistrurus
appear to be similar to the ancestral rattlesnakes since they
have nine
large plates on the top of the head like most other
pitvipers instead of many small scales (as in Crotalus).
Some aspects of the circulatory system of Sistrurus are
also intermediate between other pitvipers and the genus Crotalus
(Lillywhite,
1992). The genus Sistrurus consists of three extant
species, the pigmy rattlesnake (Sistrurus miliarius),
the massasauga
(217kb) (Sistrurus catenatus), and the Mexican
pigmy rattlesnake (Sistrurus ravus). The massasauga and
pigmy rattlesnakes are clearly more closely related to each
other than they are to other rattlesnakes. The relationship of
the Mexican pigmy rattlesnake to the other species of Sistrurus
and Crotalus is not clear at this time (Knight
et al., 1993). There are three subspecies of S.
miliarius, the Carolina Pigmy rattlesnake (S. m.
streckeri), the dusky pigmy rattlesnake (S. m. barbouri),
and the western pigmy rattlesnake (S. m. streckeri). (TOC)
Description
Pigmy rattlesnakes are short, relatively
thick-bodied snakes. They have a dark line through the eye on
each side of the face and a series of dark, roughly circular
spots running down the center of the back. These dorsal spots
interrupt a thin reddish-orange stripe that runs along the
midbody line. This line may be weakly formed, especially in
the western subspecies. A row of dark blotches that are
slightly smaller than the dorsal spots is located along each
side of the body. The tail of young snakes is yellow, but
darkens (88kb) as the snakes reach maturity. Compared to
other species of rattlesnakes, the rattle of S. miliarius
is extremely small and is often broken
(77kb) back to only one or two segments. The background
color of pigmy rattlesnakes varies both geographically and
within populations. In S. m. barbouri the background
varies from a pale gray to almost black, it is often stippled
with many small light dots. In S.
m. miliarius (245kb) the background is often a light
gray or brown, but individuals may also have pinkish or
reddish background coloration--particularly in eastern North
Carolina (Conant
and Collins, 1991). In the western subspecies, S. m.
streckeri the background is light gray or light brown. In
all subspecies the belly is white with many dark spots. (TOC)
Distribution
The genus Sistrurus is found in
North American and Mexico. It consists of three extant
species, the pigmy rattlesnake (Sistrurus miliarius),
the massasauga (Sistrurus catenatus), and the Mexican
pigmy rattlesnake (Sistrurus ravus). The massasauga is
found in a wide band across the United States, ranging from
Arizona in the southwest to New York and Ontario in the
northeast (Campbell
and Lamar, 1989). The pigmy rattlesnake is found in the
southern quarter of the United States (North Carolina, South
Carolina, Georgia, Florida, Alabama, Mississippi, Kentucky,
Tennessee, Louisiana, Arkansas, Texas, Oklahoma, Missouri) (TOC)
Methods of Study
Our study of pigmies in central Florida has
relied on mark-recapture techniques in which
individually-marked animals in four somewhat separate
populations are censused on a regular basis. At the Hog Island
site, we have been conducting an average of 2-3 population
censuses per week over the last six years, involving over 1000
individually-marked snakes. Mark-recapture studies of snakes
are a bit more difficult than those of most vertebrates, as
many of the marking techniques used to identify individuals of
other species (ear clips, toe clips, drilling shells,
attaching tags, etc.) are either not possible or not practical
on snakes. We use PIT-tags
(105kb) (Passive Integrated Transponders) to individually
identify our animals that are a year of age or over; these
small glass-encapsulated microchips are implanted
hypodermically into the snakes' body cavities, and when
scanned with a reading device, transmit a signal containing an
unique 8-digit number displayed by the scanner. Animals
younger than one year of age are individually marked with
unique color combinations of spots of nail polish; however,
this technique is not permanent as the snakes lose the color
combination when they shed. Prior to about a year of age (and
approximately 20g in weight), we feel that injection of a
PIT-tag has too high a potential for permanent damage to the
snake, and we therefore withhold that procedure until they are
clearly capable of receiving a PIT-tag with minimal effect (Jemison
et al., 1996).
Our population censuses involve a group of
3-8 observers thoroughly combing the habitat for snakes that
are visible without disruption of cover. Once located, these
snakes typically remain in their hunting coil, or become
immobile if they were moving when found, and can be identified
by scanning for a PIT-tag or looking for a color combination
on a younger snake. Once a snake is located, it is only
handled for data collection if that individual has not been
measured within the previous month. If it has been recently
handled, its location in the habitat, substrate type, and
activity (moving or coiled) are noted and the snake is left
undisturbed where originally found. Most individuals, either
coiled or moving, will allow an observer to approach very
closely without fleeing. Most individuals that are not handled
remain in the spot where first sighted after we determine
their identity and leave them. For individuals that have not
been recently sighted, we pick them up using a combination of
pinning with a hook and handling with heavy leather gloves,
and collect a variety of morphological, behavioral and
physiological data, which include sex, length, weight, body
temperature, presence of recently ingested prey, rattle
characteristics, reproductive status, and defensive responses
(rattle, flee and/or strike). Along with these regular
censuses of marked individuals in our field-populations, we
have also conducted a variety of studies on other aspects of
pigmy biology; these studies are often individual projects
that participating Stetson University students are conducting
for classes, independent research, or senior projects.
Another useful technique for studying
snakes is radiotelemetry (Reinert,
1992). In this technique, radiotransmitters are surgically
implanted into the body cavity of a snake. The transmitted
signal allows a researcher to find the snake even when they
are hiding beneath cover or in dense vegetation. Some
transmitters have a pulse frequency that is a function of
temperature. With these transmitters biologists can study the
thermal biology of the snake without bothering it each time
body temperature data is collected. There has been very little
work done on radiotelemetry of pigmies. The only two studies
that used radiotelemetry on Sistrurus miliarius were
preliminary, involving a few snakes for short periods of time
(Jacob,
1981; Holder,
1988). (TOC)
Foraging Behavior
Like most pitvipers, pigmy rattlesnakes are
sit-and-wait (or ambush) predators. These snakes coil
(189kb) in spot and wait for their prey to come to them.
In Florida over 90% of the snakes we find are coiled and
immobile when located, and individual snakes have been
observed to remain in the same location for as long as 2-3
weeks (May
et al., 1996). At our study sites, pigmy rattlesnakes feed
nearly exclusively on other herps, primarily Anolis
lizards and several species of frog, including Hyla spp.
(tree frogs) and Rana utricularia (leopard frogs). We
have observed pigmies envenomating prey, tracking recently
envenomated prey, or consuming prey on over a dozen occasions,
and the prey in question has consisted of the species named
above, as well as one ribbon
snake (63kb) (Thamnophis sauritus). The proportion
of snakes with prey
in the gut (95kb) at the time of capture varies from less
than 10% during the winter months to 20-25% in the late spring
and fall. Though many pitvipers feed primarily on mammals as
adults, the snakes at our sites apparently do so relatively
rarely. Our dissections of museum specimens from the
University of Florida Museum of Natural History indicate pigmy
rattlesnakes in other habitats or areas may rely more heavily
on other prey, such as small mammals as adults, and
surprisingly, centipedes
(119kb) as juveniles.
There doesn't seem to be a great deal of
variety in the prey taken by pigmies in our populations, but
there is quite a bit of variation in foraging postures and
techniques used by individual snakes. Although the most
frequent foraging location is coiled on the ground in some
type of leaf litters, some snakes forage from "arboreal"
(75kb) positions, elevated several inches to several feet
above the ground on branches, fallen logs, or palm fronds. At
times, especially during flood periods, some snakes move as
high as 8-12 feet up into palm trees or saplings. Some
individuals, in fact, seem to prefer feeding on a particular
substrate type, so that we have some individuals that we know
as "log
pigs" (69kb) who are consistently found elevated on
logs in a foraging coil. As has been observed in other
pitvipers, we have seen a number of pigmies using the "Reinert
posture"(Reinert
et al., 1984), with the body positioned at the side of a
fallen log, and the head oriented upwards towards the top of
the log, presumably waiting for a prey animal to use the log
as a runway. In grassy areas of our study sites, pigmies
typically position themselves at the edge of a small clearing
in the grass, backed up to a grass clump, with their head
orienting towards the middle of the opening or"
arena".
Pigmy rattlesnakes, like other ambush
predators, often have low prey capture rates since they must
literally wait for the prey to come to them. Pigmies use their
tail as a lure
(69kb) to increase the frequency that prey come within
striking distance (Rabatsky
and Farrell, 1996).
Luring behavior may have also influenced
the form and function of the rattle of pigmy rattlesnakes. In
contrast to their larger relatives in the genus Crotalus
and even S. catenatus, the massasauga, and S. ravus,
the Mexican pigmy, the rattle of pigmies is relatively small
and quite easily broken. An adult snake with a rattle string
of 6-8 segments produces a faint whirring sound when it
rattles (a colloquial name for pigmies is "buzzworms")
that can scarcely be heard from further than a few feet away broken
(77kb) their rattle back to the basal segment, so no noise
production at all is possible, since at least two interlocking
segments are needed to rattle against each other. Well over
half of the snakes have rattles of 3 segments or less, so
finding snakes by listening for them to rattle (which they do
less than 10% of the time when disturbed) is not a very
efficient technique. Patrick Cook and his co-workers at
Appalachian State University in Boone, N.C. have shown that
the rattles of pigmies are very small and the sounds produced
by pigmies rattling are far fainter than those produced by
other rattlesnakes, beyond what would be expected based on
their small body size alone (Cook
et al., 1994). They suggest that pigmies may have evolved
a smaller, less effective noise-producing rattle because
smaller, thinner rattles are more effective as luring devices.
One interesting result of feeding primarily on
cold-blooded as opposed to warm-blooded prey is the rapidity
with which the venom takes effect. Like many other pitvipers,
pigmies release their prey after the strike, and then
scent-track the prey after it has died. Feeding trials with
captive pigmies have shown that while a mouse struck in the
head or neck may be immobilized by the venom within 30-45
seconds, lizards and frogs may remain relatively mobile for
15-20 minutes after being struck. It is perhaps then no
coincidence that on numerous occasions we have found dead
Anolis (105kb) lizards and tree frogs at our study
sites with what appear to be fang marks and necrotic tissue,
but no sign of a snake nearby. Apparently some of these
ectothermic prey are able to escape beyond the range over
which a pigmy can scent-track its prey. In one case we
observed a pigmy orienting to a dead Anolis that had
climbed several feet up a tree trunk and died, hanging by one
claw. Though the snake apparently knew where the prey was, it
could not have reached it unless the lizard became dislodged
and fell. (TOC)
Defensive Behavior
Pigmy rattlesnakes first line of defense is
to remain motionless. Their color pattern makes them hard to
see in grass or leaf litter, especially when they are coiled.
Pigmy rattlesnakes almost never warn approaching people by
sounding their rattle. They are likely to remain motionless
until stepped on or over. When molested with a gloved hand
coiled pigmies usually remain motionless, but occasionally
flee (16.4% of encounters), strike (2.5%) or flee and strike
when grabbed (3.3%). Snakes that are stretched-out when
approached are far more likely to flee (39.5%), or flee and
then strike when grabbed (21.1%) than are coiled snakes.
Pigmies with high body temperature are far more likely to
strike than cool snakes (May
et al., 1996). Hudnall (1979)
also found pigmies were unlikely to strike when harrassed by a
moving boot. It would be interesting to see how pigmy
rattlesnakes react to more natural predators. The nasty
reputation of pigmy rattlesnakes (for example in Allen
and Neill, 1950) may be a result of people often finding
warm, moving pigmies in the road were they are likely to react
very aggressively to people. Pigmies are more like to warn
predators by rapid head twitches (side-to-side, with the head
moving only a centimeter or two) than by rattling. (TOC)
Thermal Biology
Our studies of the thermal biology of pigmy
rattlesnakes have shown these snakes are active over a wide
range of temperatures. They are active in air temperatures as
low as 13-14 degrees centigrade (with body temperatures only a
few degrees higher); it's a rare winter day in central Florida
when temperatures don't get at least that high for several
hours a day. We rarely find pigmies with body temperatures of
more than 35 degrees C. (May
et al, 1996). Pigmies held in the sun actively seek shade
(even ignoring threatening humans) when their body temperature
is over 37 degrees. In Tennessee, Jacob
(1981) found that pigmies placed in a thermal gradient
avoided body temperatures above 36 degrees C and preferred a
body temperature near 28 degrees C.
Bill Richardson, a recent Stetson graduate,
found that pigmies in central Florida do not appear to use
microsite temperature to chose foraging sites. Few pigmies,
other than gravid females or some individuals that have
recently fed, appear to bask in warm microhabitats. (TOC)
Seasonal Activity
In the mild year-round climate of central
Florida, pigmy rattlesnakes are found active during all months
of the year, though densities in the middle of winter are on
average 2-3 times lower than in other months of the year (May
et al. 1996). Chamberlain
(1935) found pigmies year-round in South Carolina, but
reported a peak in activity from May-October. Similarly, Dalrymple
et al. (1991) and Hudnall
(1979) found pigmies throughout the year in south Florida
with a fall peak in activity. In southwest Missouri pigmies
are above ground from mid-April to early November (Holder,
1988). Peaks in activity in fall are also reported by Dodd
and Franz (1995) and Jacob
(1981). These peaks in fall activity are observed in
studies where authors depend on the snake moving to be
observed (by going into traps or crossing roads). The peak in
fall activity is probably a result of males hunting for
females in the fall mating season, and young snakes dispersing
from where they were born (Dalrymple
et al., 1991)
In central Florida the proportion of snakes
with prey in the gut at the time of capture varies from less
than 10% during the winter months to 20-25% in the late spring
and fall. Shedding also occurs throughout the year in this
population (May
et al., 1996). Unlike northern crotalids, pigmy
rattlesnakes in Florida that are not active during cold
weather do not migrate and congregate at hibernacula; rather,
they apparently simply go under shallow cover within their
active-season home range, and reemerge when temperatures are
warmer. (TOC)
Reproductive Behavior
Though the birth of nearly all pigmy
rattlesnakes at our sites begins in the month of August, the
courtship and mating behaviors leading to the production of
these offspring take place, surprisingly, nearly a year
earlier. In the six years of our study, all of the cases of
male-female reproductive interactions we have observed have
occurred between late August and January, with the bulk of
these observations taking place in October to December. Pigmy
rattlesnake mating behaviors consist of several distinct
components; like many pitvipers, male pigmies reportedly (Carpenter,
1972; Palmer
and Braswell, 1995) engage in male-male combat rituals, in
which a pair of competing males each attempts to physically
dominate the other by twining around his rival's body and
pinning the head and anterior trunk to the ground. We have not
yet observed this behavior in our Florida populations.
Male-female interactions have been observed on nearly 50
occasions, though, and consist of two phases. Pigmy
rattlesnake males, like many other crotalids, perform
mate-guarding behavior or prolonged courtship in which they
remain in close
(139kb) physical proximity to a reproductive female for
extended periods of time. These associated pairs may remain in
contact, often with one snake coiled on top of the other, for
several days at a time, without leaving their trysting spot.
Large males are significantly more likely to be in a pair with
a female than smaller adult males (Bishop
et al., 1996). The 40-plus observations we have made of
associated pairs have been between September and January. Less
frequently, copulating
pairs (130 kb) are observed. This process may take several
hours to complete; we have collected six pairs of snakes so
engaged for later data collection, and it can take up to
several hours for them to disengage. After this fall-winter
mating season, there is no further association between mated
pairs, and the female apparently stores the sperm of her mate
or mates until about April of the following year, when
fertilization of eggs occurs, and development of the young
begins. By early April, we begin detecting gravid females by
palpation of the abdominal cavity. As the development of
embryos proceeds, these gravid females frequently become more
obvious, as they often choose relatively exposed sites in
which to bask and elevate their body temperature, which
accelerates the development of embryos. Unlike many other
crotalid species, gravid female pigmies will feed until late
in the gestation process.
Like most crotalids and viperids, pigmy
rattlesnakes bear their young alive, and as parturition nears,
we begin collecting gravid females and housing them in field
cages until they give birth. In an average year, during the
last two weeks of July we collect 20-30 gravid females, who
are maintained in captivity until they give birth. Typically
females are in captivity two weeks or less before parturition;
we also normally find several recently-born litters in the
field, near their mothers, each season. Litters of neonate
pigmies (262kb) normally remain tightly clustered within a
few feet of their mother for several days after birth, until
they complete their first shed. After shedding, they disperse
to begin their mostly solitary lives. Though the mother is
often quite near her babies when we find them, we have never
seen active defense of the young when we gather up the litter
for data collection. We have collected data on over 100
litters thus far, and all but 4 or 5 were born in August, with
the remainder occurring in late July or early September (Farrell
et al., 1995, 1996).
In central Florida, litter sizes range
between 2 and 12, with an average litter size of around six.
Pigmies in North Carolina have litter sizes that range from
3-9 with an average litter of 5.3 babies (Palmer
and Braswell, 1995). In Arkansas litters ranged from 6=14
offspring (or oviductal eggs) with an average of 10.0
offspring (Carpenter,
1960).
As in many other snakes, larger females
generally produce more offspring, but not bigger offspring. A
newborn pigmy rattlesnake is an amazingly miniaturized animal,
especially by crotalid standards. In central Florida they
average about 4-5 grams in weight, and when tightly
coiled (136kb), are scarcely larger in diameter than a
quarter (Farrell
et al, 1996) . In the western part of their range newborn
snakes may even be smaller than they are in Florida (Fleet
and Kroll, 1979) Young pigmies are so small and endearing
in appearance that we have to resist the temptation to handle
them without gloves.
Where pigmy rattlesnakes do differ from
other crotalids, however, is in the frequency of reproduction.
They can produce their first litter in as little as two years,
though three to four years is a more typical age of first
reproduction (Farrell
et al., 1996). A two-year old pitviper bearing young is
even more remarkable when one considers that such a female
would have engaged in courtship and copulation when just
slightly over one year of age. Not only can maturity come
relatively early, but in addition, some females (about 25%)
give birth in consecutive seasons, in some cases for at least
three years in a row. They are not able to produce two litters
in a single season, as they lose a significant fraction of
their body weight (40-60%) with each litter (Farrell
et al, 1995). Unlike most pitvipers, however, many females
are able to recover the resources lost in reproduction rapidly
enough to reproduce the following season. (TOC)
Growth
The growth of pigmy rattlesnakes has also
been studied extensively in central Florida. Newborn pigmy
rattlesnakes generally quadruple their weight within the first
year of life, and can reach mature sizes by two to three years
of age. The adult size of pigmies one Florida population
varies quite a bit - reproductive individuals have ranged in
weight from 50-150 grams, and 36-60 cm in length. Contrary to
the commonly held belief that snakes grow throughout life, our
snakes appear to do most of their growth in the first 2-3
years of life, after which growth slows appreciably, to less 1
cm per year (Farrell
et al., 1996). It appears that adult snakes allocate most
of their energy to reproduction, not continued growth. The
adult size of most snakes appears to be relatively stable, and
there are no differences in growth rates or body size between
male and female pigmy rattlesnakes Bishop
et al., 1996. There is, however, plenty of variation in
growth rates and adult size between individuals, both in wild
snakes and in captive snakes. A couple of pigmies that we have
kept in captivity since they were a couple of months old in
the fall of 1991 reached weights of 200-300 grams in less than
two years, larger than any snake of any age we have ever found
in the field. Sniffty, a captive male pigmy owned by Emmett
Snellings Jr. of South Carolina reached a maximum weight of
almost 660 grams (Snellings
and Collins, 1996). It had a total length of 83.2 cm (svl=73.8cm)
Clearly, growth rates of individuals are highly dependent on
their specific environmental conditions.
Population Biology
In Missouri pigmies appear to prefer
"south-facing, rocky and partially wooded hillsides"
(Johnson,
1987) , while in Tennessee pigmies appear to be found in
riparian habitats (Jacob,
1981). In South Carolina we have found pigmies in pine
woodlands. Palmer
and Braswell (1995) state that in North Carolina pigmies
are found in sandhill with pine and scrub oaks or in pine
flatwoods. In Florida pigmies are found in a very diverse set
of habitats. We have found exceptionally dense rattlesnake
populations in isolated patches of semi-evergreen broad-leaved
woodland (referred to as hammocks
(172kb) in Florida) that are surrounded by freshwater
floodplain marsh (May
et al., 1996). While pigmies in both Florida and other
parts of their range occur in a variety of habitat types,
including xeric uplands, our populations are found in moist
forest subject to extended flooding
(170kb) for sometimes several months at a time. The
specific characteristics of these floodplain habitats probably
strongly influences many aspects of the biology of pigmies
occurring there, so it is hard to know how the findings from
our study apply to populations of pigmies occurring in other
areas or habitat types.
Pigmy rattlesnakes are also one of the most
abundant snake species in the garden sections of Wal-Mart
stores It appears that pigmies are by far the most
abundant venomous snake in the state of Florida. Other
herpetologists have found them in areas far from permanent
bodies of water in sand pine scrub habitat (H. Tiebout,
personal communication--is this really a credible source?).(TOC)
Interactions With Other Species
(TOC)
Interactions With Humans
(TOC)
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