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90 IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005 SAJDAK AND BERG

Timber Rattlesnakes (Crotalus horridus) reach the northernmost extent of their range on the bluff prairies in Wisconsin.



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Rattlesnakes are an essential bit of Americana, confined solely
to the New World. The “Belled Serpent” was one of the

most distinctive and frightening members of a fauna that con-
fronted the pilgrims after their arrival in Massachusetts. Man
and the rattlesnake have always had a checkered relationship —
on one hand hated, feared, bountied, on the other, used as a
sports logo to adorn baseball caps and once hoisted aloft by colo-
nial militiamen over the motto, “Don’t tread on me.” Even
today, in parts of the Appalachians, handling rattlesnakes as
proof of faith is practiced in a few fundamentalist churches.
Shamefully, in several parts of the country, rattlesnake roundups
still attest to the uglier emotions the snakes elicit.

The Timber Rattlesnake (Crotalus horridus) is one of the
largest and most widely ranging species of rattler. The Timber
Rattlesnake ranged from southern Maine south through the
Appalachians, along the coastal plain from Virginia to northern
Florida, and westward to east Texas, Oklahoma, Kansas, and
extreme southeastern Nebraska. In the Mississippi River Valley,
the snake meanders north with the river into Wisconsin and
Minnesota, making minor excursions along some of the larger
tributaries. In Wisconsin, the Timber Rattler reaches the north-
ernmost edge of its range, just barely below 45° N latitude, per-
haps 20 miles north of historical records from Vermont and over
a hundred miles farther north than current New York and
Vermont populations.

Declining Populations
Ever since the arrival of Europeans, humans have been in inces-
sant advance, the Timber in constant retreat. Originally known
from 32 states and the province of Ontario, the snake has been
extirpated in Delaware, Rhode Island, Maine, and Canada. It
remains only in a bare handful of sites in New England. For
another example, in Ohio, historical records are known from at
least 20 counties, whereas current populations are known from
only nine.

Beginning in the 1970s, some effort was made in various
states to repeal bounties and enact protective legislation. In
Wisconsin, a bounty was paid until 1973. Wisconsin bounty

records themselves highlight the rattler’s decline. In the county
where our study was conducted, the number of bounties paid
dropped from approximately 10,000 in 1965 to only 2,000 in
1972. Across the river in one Minnesota county, the number of
bounties declined from 4,955 in 1980 to 191 in 1987.

Both of us have played a part in gaining protection for this
snake. In 1973, Rich was part of the bounty repeal effort,
attending public meetings and contacting people. In 1997,
Craig, with David Sorensen from the Milwaukee County Zoo
and Gary Casper and Robert Henderson from the Milwaukee
Public Museum, petitioned the State of Wisconsin to list the
snake as a “Threatened Species.” However, politics being poli-
tics, listing a venomous snake as “Threatened” was not possible,
but “Protected Wild Animal” was. Discretion being the better
part of valor, in 1998, the Timber Rattlesnake became a pro-
tected species in Wisconsin.

During the 1980s and 90s, one of our rite-of-spring tradi-
tions was to drive across the state from Milwaukee one day each
year in mid-May, climb the bluffs overlooking the river, and
revel in the beauty of the place and the returning warmth and
sun after a long winter, and to see if the rattlers were still at
home. Like many other rattlesnake admirers, we were aware of

Rattlesnakes on the Bluffs: 
Wisconsin Timber Rattlesnakes

Richard Sajdak1 and Craig Berg2

1Pittsford, NY 14534
2Milwaukee County Zoo, Milwaukee, WI 53226

Photographs by R. Sajdak except where indicated.

The beeping kept getting stronger as I moved in, and inside my boots my toes were beginning to curl. It was my third day of radio tracking
Timber Rattlesnakes, and my largest snake, a four foot male later to become famous as the Swamp Fox because of his love for a soggy patch of
creek bottom, was almost certainly coiled up somewhere in the tangle of branches of a huge fallen oak tree. Over the ensuing weeks I would
grow confident (and occasionally overconfident) in approaching venomous snakes hidden in tangles, but today I was nervous. — Rich Sajdak 

WISCONSIN TIMBER RATTLESNAKES

�

Rich Sajdak searching for a Timber Rattlesnake.



92 IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005

the snakes’ decline, and of new roads and new building
encroaching and splintering their habitat. Over the years, the vis-
its were slowly transformed into a very low energy effort to study
the snakes.

We soon realized from conversations and reading ‘the liter-
ature’ that next to nothing was known about these isolated
upper-Mississippi River Valley rattlesnakes. Anecdotal reports, a
few surveys, interviews with old bounty hunters, and extrapola-
tion from studies by Bill Brown and Howard Reinert in New
York and Pennsylvania were all we had. Bob Hay of the
Wisconsin Department of Natural Resources’ Bureau of
Endangered Resources made it clear that, although he and oth-
ers in the Bureau were concerned about the Timber Rattlesnake,
limited resources and a plate full of more severely threatened
species meant that funding for rattlesnake studies would remain
on the back burner.

Studying Rattlesnakes
While Rich was Reptile Curator at the Milwaukee County Zoo,
he put together a funding proposal to intensively study a rat-
tlesnake population using radiolocation and tagging. The goal

was to amass the basic ecological and behavioral data on
Wisconsin populations. These would be necessary for the devel-

SAJDAK AND BERG

The gray areas indicate the current known distribution of the Timber Rattlesnake (Crotalus horridus). The species occurs sparingly along some
streams on the Tifton Plateau in south-central Georgia. Also, notice the narrow corridor extending north along the Mississippi River, where prime
rattlesnake habitat is associated with prairie habitats on bluffs overlooking the river.

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The largest threats to Timber Rattlesnakes in the upper Midwest are
loss of bluff prairie habitat and development. Notice the thick growth
of Red Cedar overgrowing prairie on the point. In the last two years,
five roads and over one hundred vacation home lots have been cut
along the bluff tops within a mile of our study site.



WISCONSIN TIMBER RATTLESNAKES

opment of future conservation management plans. Although the
project was not funded by the time he left the zoo, Craig took
over the curatorial position, and continued to push for the study.
In 1998, the Zoological Society of Milwaukee County agreed to
fund the project. Craig administered the grant, and Rich became
the primary researcher. During the heat of the summer, Rich was
free to spend five months a year living in a ratty old trailer,
climbing 600-foot bluffs every day, and dodging swarms of mos-
quitoes, stinging nettles, and thunder storms. During the winter,
Craig had the pleasure of treading through snow, tobogganing
down hills on his backside, and fending off frostbite. Heaven! Of
course, as often as possible, we shared each other’s joys.

With Bob Hay’s help, we began our study with six snakes,
two large males, two gravid females, a juvenile male, and a juve-
nile female captured along rock ledges in open prairie habitat on
high bluffs overlooking the Mississippi River. In Wisconsin,
Iowa, Minnesota, and northern Illinois, these bluff prairies are
classic rattlesnake habitat, and locations favored by old time
bounty hunters. Many residents are surprised to find that the
rattlesnakes leave this habitat. Our study site, on an extensive
tract of state land, was relatively undisturbed, with huge ravines
forested with oak trees and a large area of swamp and marsh
habitat along a nearby creek.

As we followed the snakes through the first year, several
things soon became apparent. The two gravid females confined
themselves to relatively short movements within the prairie, mov-
ing between ledges and flat rocks. These open habitats were sunny,
hot, and dry, and the females used the heat to maintain body tem-
peratures over 5°C higher than those maintained by male snakes
(29.7°C versus 24.4°C). This pattern is well known in gravid
snakes and is thought to help embryonic development. 

IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005 93

Open bluff prairies are classic snake-hunter favorites. These prairies are not only critical as den and birthing locations, but reservoirs of rare prairie
plants as well.

Bob Hay, Armund Bartz, and Craig Berg processing a rattlesnake in
the field. The snake is restrained in a clear plastic tube for safety. Length
and weight are measured, the snake is sexed, rattles counted and
painted, tissue sample taken, PIT-tagged, and released.



While the females basked in the prairie, the big males
moved off the bluff into the wooded ravines to hunt. One snake
stayed in the ravines, while the “Swamp Fox” moved on into the
creek bottom. Both snakes preferred cooler, more shaded con-
ditions than the females, and, rather than rocks, they were most
often found touching or close to logs.

The juveniles seemed intermediate in habitat preference.
They were found in or near the prairie habitat, but in oak open-
ings: areas with more oak trees and more closed canopy. While
both males moved a mile or so from their capture location, the
juveniles and females moved only about a quarter mile.

As August arrived, the males became more active, and were
found courting females or copulating on several occasions. The
gravid snakes continued to move from rock to ledge, leaving me
to wonder each time whether this, finally, would be the birthing
site. At last, on the 30th, one female gave birth, followed by the
other two days later, each giving birth to six or eight foot-long
babies. After only four or five days, the females had abandoned
the neonates and the birth site, moving over the bluff top into
the wooded ravine, searching for a meal in the few weeks before
hibernation.

In succeeding years, we found that hunting females, unlike
gravid ones, behaved like males, moving into wooded ravines or
swamps to hunt. One female required two hunting seasons to
gain enough weight to reproduce again (a three-year cycle),
whereas the other needed four hunting seasons (a five-year
cycle). Over the years, we found that females average a three-year
reproductive cycle, although we have seen cycles as short as two
or as long as five years. In 2004, four of six females in our study
appeared to be gravid. One of the gravid snakes had not given
birth in five years. This observation suggests that reproductive
events may exhibit pulses that reflect variations in weather and
food supply. 

By early August, all hunting snakes had reached their maxi-
mum distances from the den, and had begun a slow trek back. By
early September, snakes had begun entering the den. The last to
enter were the two post-reproductive females, one entering during
the last week of September, and the other in early October.

Since that start in 1999, we have radio-tracked 19 rat-
tlesnakes, tagged over 100, and located seven dens. We found

that, like their cousins in the northern reaches of the
Appalachians, they have a very short activity period. Our snakes
began emerging from dens in late April and May, although we
have occasionally seen snakes out as early as 4 April or still in the

94 IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005 SAJDAK AND BERG

A large gravid female regulates her body temperature by adjusting how
much of her body is exposed to the sun.

One of our radio-equipped rattlesnakes in classic ambush posture.
Having picked up the odor of a chipmunk trail along the top of the
log, the snake settles in, ready to strike as the rodent passes by. Snakes
will maintain this position for days at a time.

A clutch of neonates lies basking at the edge of the birth rock. The
snakes are less than a day old, and have not yet shed.



IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005 95WISCONSIN TIMBER RATTLESNAKES

den in mid-June. Rattlesnakes begin entering the dens in
September or as late as 19 October. Oddly, Timber Rattlesnakes
leave the den weeks later and enter the den weeks before Black
Ratsnakes (Elaphe obsoleta) and Blue Racers (Coluber constrictor)
using the same dens. Rattlers moved as much as a mile or mile
and a half away from the dens. We found that, on our original
study site, an extensive tract of relatively undisturbed state land,
the snakes preferred hunting in oak woodland or swamp wood-
land habitats, and avoided entering agricultural fields or cross-
ing highways. However, in other locations, on private land in
areas closer to humans, Timbers utilized fields and even back-
yards, where they sometimes came to grief. 

The snakes grow very slowly, taking at least seven years to
reach maturity. In New York, Bill Brown has recaptured Timber
Rattlesnakes he originally tagged over twenty years earlier, and
believes they can approach a thirty-year life span. Our study is

not of that duration, but, based on our recaptures, one of our
big snakes like the “Swamp Fox” would have to be at least 15
years old. We suspect that we will eventually confirm a life span
of well over 20 years.

In years past, we had often wondered if the denning behav-
ior of Wisconsin snakes would prove to be very different from
the rattlesnakes that Bill Brown was studying in up-state New
York. The northernmost populations of Timber Rattlesnakes in
Wisconsin are probably reproductively isolated from those in
New England. One might expect to see differences in snakes
that are following separate evolutionary paths. On the other
hand, the severe winters in the north are likely to be an
extremely important evolutionary constraint. Similar environ-
mental challenges are likely to yield similar results. As we
learned, the denning behavior of Wisconsin snakes is almost
identical to those in New York.

A juvenile rattlesnake lying exposed in a rainstorm. Only extreme conditions, such as an unseasonable snowstorm, will make a hunting rattlesnake
take cover.

Table. Comparison of denning behavior in Wisconsin and New York Timber Rattlesnakes.

Wisconsin New York*

Earliest Emergence 6 April 8 April

General Emergence 7–21 May 7–21 May

General Ingress 14 September – 1 October 14 September – 1 October

Latest Ingress 19 October 16 October

Mean Body Temperature 10.8°C (51.4°F)(6 dens/1yr) 10.5°C (50.9°F)(1 den/3 yrs)

*From Brown (1992)



Although we do not know what the inside of a Timber
Rattlesnake den looks like, a recent observation does lead to
interesting speculation. Might Timber Rattlesnakes hibernate in
pools of water found in the rocky recesses of their dens? During
this past winter (2004–2005), we had to bring two large males
into the lab. During the month of November, when they would
normally be moving deeper into their dens, both animals spent
a considerable amount of effort trying to push their way through
the bottoms of their drinking bowls. This behavior had not been
seen in two previous months of captivity. Could they have been
trying to hibernate underwater?

Several tantalizing clues suggest that this might be the case.
In the Pine Barrens of New Jersey, Timber Rattlesnakes do
hibernate underwater. Also, Timber Rattlesnakes frequently
emerge from hibernation bearing fungal sores similar to those
found on Fox Snakes (Elaphe vulpina), another species known
to hibernate underwater.

Rattlers and Local Residents
One of our biggest surprises and unexpected rewards came not
from the snakes, but from local residents. We had initially

planned to keep as low a profile as possible, to avoid stirring up
trouble. We had visions of some of the angry opponents we
remembered from the bounty repeal and rattlesnake protection
meetings deciding that a visit to the study site would end this
nonsense. The problem, of course, was that our effort to keep
the work secret was impossible. Also, if people don’t know what’s
going on, they will imagine the worst — and the fact that the
“usual suspect ” drove a vehicle sporting New York license plates
didn’t help! Within a month, we heard of rumors that snakes
were being moved and introduced onto private lands. The vow
of silence was broken. Casual discussions revealed that, while
people didn’t exactly LIKE the snakes, they had a great deal of
curiosity about them.

Additionally, we had cut ourselves off from a lot of infor-
mation and help. We met several snake hunters from the bounty
days, who told us about other dens and things they had seen.
Likewise, they were surprised by many of the things we were dis-
covering. Most of their work was done at the dens or nearby
ledges and rocks. Rattlesnakes hunting in the woods are so scat-
tered and so well camouflaged that these hunters, despite years
of experience, knew almost nothing about the biggest parts of a
rattlesnake’s life. One man first said that the project was a waste
of time because “lots of Timbers” were still around. A week later,
he mentioned that he had been talking with some friends, and

96 IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005 SAJDAK AND BERG

A controlled burn: Without periodic burns and selective cutting, most
bluff prairies would become overgrown with Red Cedar and
Buckthorn, eliminating rare prairie plants and degrading rattlesnake
habitat.

Due to its slope and exposure to the winter sun, this den is mostly free
of snow even in February.

Implanting a transmitter into one of our anesthetized females at the
University of Wisconsin Veterinary Hospital.



IGUANA  •  VOLUME 12, NUMBER 2  •  JUNE 2005 97WISCONSIN TIMBER RATTLESNAKES

that, while a lot of snakes were still around, many places where
they used to find snakes don’t have them anymore.

Kirk soon became a friend. He showed us new locations, and
joined us in tracking Timbers in the swamps and woods. In 2000,
we put a transmitter into a rattlesnake that spent weeks at a time
in junk piles right behind his barn. The snake became “Kirk’s
snake,” and the rattlesnakes gained an advocate. Other locals would
call Kirk or us if they found a snake, so we could remove it.

We have come to believe that for any conservation effort to
succeed, especially those involving feared and despised species,
the education and support of local residents is an essential com-
ponent. Just last year, we heard about a school bus full of kids
that came upon a Timber Rattlesnake crossing the road, a sure
recipe for a dead snake in past years. The kids and driver got out
to see if it was one of our study snakes, which have painted rat-
tles. They let it crawl off to bask another day. Now that’s success!

Acknowledgments
Funding for the study was provided by the Zoological Society
of Milwaukee County and the Milwaukee County Zoo. Robert
Hay and the Bureau of Endangered Resources, Wisconsin
Department of Natural Resources gave invaluable support. John
Sealy and W. H. Martin provided the distribution map. Bob

Hay, John Kivikoski, Armund Bartz, and many others con-
tributed gallons of sweat and many days in the field. Lester Flick
provided hard-won experience. Kirk Holliday gave us his time,
sweat, good will, and home. Rich’s wife, Linn, was long-suffer-
ing and always supportive.

References
Brown, W. S. 1991. Female reproductive ecology in a northern population of

the Timber Rattlesnake, Crotalus horridus. Herpetologica 47:101–115.

Brown, W. S. 1992. Emergence, ingress, and seasonal captures at dens of north-
ern Timber Rattlesnakes, Crotalus horridus, pp. 251–258. In: J. A.
Campbell and E. D. Brodie, Jr. (eds.), Biology of the Pit Vipers. Selva, Tyler
Texas.

Brown, W. S. 1993. Biology, Status, and Management of the Timber Rattlesnake
(Crotalus horridus): A Guide for Conservation. Society for the Study of
Amphibians and Reptiles, Lawrence, Kansas.

Casper, G. and R. Hay 1998. Timber Rattlesnake Homepage. http://
www.mpm.edu/collect/vertzo/herp/timber/factshe1.html.

Reinert, H. K., D. Cundall, and L. M. Bushar. 1984. Foraging behavior of
the Timber Rattlesnake, Crotalus horridus. Copeia 1984:976–981.

Rubio, M. 1998. Rattlesnake: Portrait of a Predator. Smithsonian Institution
Press, Washington, DC.

Tyning, T. F. and D. W. Kimball (eds.). 1992. Conservation of the Timber
Rattlesnake in the Northeast and The Timber Rattlesnake in New England
– A Symposium. Massachusetts Audubon Society, Lincoln, Massachusetts.

Several of our radio-equipped snakes made forays into bushes and trees. Here, “Kirk’s snake” courts a pre-shed female. His head lies on top of hers.

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