In the locality of my study racers spend approximately half the year
in winter dormancy. Earliest spring records and latest fall records
for 13 years are shown in Table 4. The spring records in nearly all
instances pertain to snakes found in the open or beneath flat rocks
warmed by sunshine, usually at or near the rock ledges where
hibernation occurs. Juveniles are especially well represented in these
earliest spring records, and it seems that they tend to emerge a
little earlier, on the average, than the adults, either because they
have hibernated in more superficial and less well insulated situations
or because their lesser body mass permits more rapid warming to
activity than can occur in the adults. The latest fall records all
pertain to racers trapped along the rock outcrops, and none was a
young of the year.
Table 4. Earliest and Latest Recorded Annual Dates When Blue Racers
Were Active on the Reservation or Rockefeller Tract
==========+==========+=============++==========+==========+============
| Earliest | Latest || | Earliest | Latest
Year | spring | fall || Year | spring | fall
| record | record || | record | record
----------+----------+-------------++----------+----------+------------
1950 | April 16 | November 4 || 1957 | April 30 | November 2
1951 | April 19 | November 13 || 1958 | April 25 | November 20
1952 | April 23 | November 12 || 1959 | April 6 | November 10
1953 | April 8 | November 12 || 1960 | April 22 | November 6
1954 | April 20 | November 12 || 1961 | April 19 | November 1
1955 | April 15 | November 2 || 1962 | April 23 | October 27
1956 | May 11 | November 14 || | |
----------+----------+-------------++----------+----------+------------
Most of the population undoubtedly emerged somewhat later than the
average date of April 16 indicated by the records in Table 4, and
retired somewhat earlier than the average date of November 8. However,
a small percentage of the population probably emerged even earlier
each year than my records indicate, and retired into hibernation later
than my records indicate. In a typical year, temperatures in April and
early May are only occasionally above the level at which racers are
able to become active, but are below this threshold most of the time.
The same statement applies to an autumn period of late October and
November. Most racers are dormant in their hibernacula during these
transitional periods of spring and autumn, but some--those that have
emerged early in spring, or those that have not yet retired (in
fall)--retreat to temporary shelters and revert to a semi-torpid state
when temperatures fall below the critical level.
Fig. 3 shows the relative extent of activity along the hilltop
outcrops, as reflected by numbers of racers caught at different times
during the autumn. Data from 14 years are combined, and the large
composite sample indicates that in an average year there is relatively
little activity along the hilltop outcrops in early September, but
that activity rapidly increases to a peak in mid-October and then
tapers off rapidly, usually ending in mid-November, but occasionally
ending as early as late October or as late as late November.
The racers recorded in traps had, in many instances, been confined in
them for from one to three days before they were found. For any one
year records are not sufficiently numerous to show the trend as well
as Fig. 3, but Fig. 4 shows year-to-year differences; 1958 was a
fairly typical year, and also was the year in which the largest sample
was obtained; in 1949 the largest catches were made earlier than
usual, and the racers retired early into hibernation; in 1954 warm
weather persisted until unusually late in autumn, and racers remained
active beyond the time when they ordinarily would have been
hibernating; in 1955 and 1961 the most concentrated activity along the
outcrops, as reflected by day to day catches, came later than usual,
but unseasonably cold weather ended all activity abruptly, earlier
than usual.
[Illustration: Fig. 3. Records of blue racers trapped along hilltop
limestone outcrops in autumn, a composite sample of 14 years (1949
through 1962) from the Reservation and Rockefeller Tract, showing the
catch grouped in ten-day intervals, beginning with September 1 to 10
and ending with November 20 to 29. Averages of the maxima, means, and
minima of daily temperatures for each period are shown.]
The racer's annual cycle of activity is, of course, controlled
primarily by the weather, and is much delayed or accelerated in some
years. But certain stabilizing factors cause the racer's annual cycle
to be less variable than that of the weather. For example, in spring
when persistently cool weather delays emergence from hibernation
beyond the normal time, the increasing azimuth of the sun, and more
intense sunshine cause the soil to warm, despite low air temperature,
until emergence is finally triggered. Having once emerged, the racer
is able to control its bodily temperature to a large extent by basking
in sunshine to gain warmth, or by seeking shade or underground shelter
to escape overheating. By such behavioral thermoregulation extremes of
weather are neutralized, or at least buffered to some degree.
[Illustration: Fig. 4. Yearly variation in catch of blue racers along
hilltop outcrop in autumn on the Reservation and Rockefeller Tract,
grouped in same ten-day intervals indicated for Fig. 3.]
In the course of my study no racers were found in their actual
hibernacula. Insofar as known, these were always in deep crevices in
strata of limestone near hilltops, and were inaccessible except by
removal of the solid rock. The crevices where racers hibernate are
known to be several feet deep in some instances, extending well below
the frost line. Periodic temperature readings in two such crevices at
depths of 12 inches and 30 inches, taken in the winter of 1954 (Fitch,
1956:471) showed that temperatures during dormancy are usually well
within the range 0°C to 10°. Whether the racers congregate in
hibernating masses in regular "dens" on the Reservation has not been
definitely determined, but if so, most of the hibernating groups must
be small, because those trapped along the ledges are well scattered,
and, in fact, give the impression of being rather uniformly
distributed along them. However, ledges of northward exposure are not
used as hibernation sites, so far as known, and those of full
southward exposure are perhaps preferred, especially where the hilltop
has a southward projecting spur, and the exposed rock face is
extensive, with many cracks and fissures. I have been unable to detect
differences in types of hibernation sites preferred between the racer
and the copperhead, which is somewhat more numerous on the same area.
Several authors have contributed to knowledge of hibernation in the
racer. Boyer and Heinze in Missouri (1934:195) noted that blue racers
often were associated with copperheads in the vicinity of places
chosen for hibernation. Burt (1935:329) in Kansas found blue racers
emerging from dens among rocks on a prairie hillside, associating with
other snakes, _Diadophis punctatus_, _Elaphe guttata_, and _Pituophis
melanoleucus_. In the Chicago region, Pope (1944:173) reported scores
of blue racers aggregating in October on and around a sand dune with
oak woodland. In Ohio, Conant (1938:55) found three blue racers
hibernating together about three feet underground in a small hole. One
found at another locality had apparently hibernated in company with a
massasauga (_Sistrurus catenatus_). In Maryland Cohen (1939:137) saw
racers (_C. c. constrictor_) in the act of emerging from an old vole
burrow that was a communal hibernaculum, on April 6, 8, and 10. Air
temperatures at the time of emergence were 12.5° and 18.5° Centigrade.
In Illinois, Schroder (1950:1-2) found seven blue racers hibernating
in masses, intertwined with each other and with bull snakes at depths
of 36 inches and 42 inches in an abandoned mammal burrow in a sand
dune area in early February.
[Illustration: Fig. 5. Bodily temperatures of blue racers kept in a
large outdoor enclosure and checked from time to time when they were
active and the sun was shining, in June and July, 1962.]
In the course of routine field work I often carried a Schultheis
quick-reading thermometer, and from time to time I had opportunities
to take the body temperatures of blue racers newly captured by hand.
The trend of these records indicated the temperature range within
which the snakes normally limit their activity, and the preferred
temperature. In an earlier publication (Fitch, 1956:459-460) based on
a few more than half the number of records of temperature now
available, I discussed responses of the blue racer to temperature. The
newer data bear out the trends previously revealed; of 60 records, 39
are within the six-degree range from 29° to 35°, and records are most
concentrated in the one-degree range, 34° to 35°. Racers were found
active at air temperatures between 15.5° and 32.4°, with the greatest
concentration between 26° and 27°. Compared with most other kinds of
North American snakes, the racer is remarkably tolerant of heat, and
often is several degrees warmer than the level that those of other
genera will normally tolerate. Racers have been seen crawling in the
sunshine, or basking on days that were uncomfortably hot for humans.
For example, on August 28, a large female racer released from a trap
was followed and her behavior observed; after crawling some 50 feet
through the grass she climbed from a ditch bank onto sunflower stalks
and elm saplings, and came to rest among the stalks, in a spot
strategically situated for catching grasshoppers. More than half her
body was exposed to sunshine and air temperature was slightly above
34° Centigrade, yet the snake showed no sign of discomfort, and for
the several minutes that she was kept under observation, did not
attempt to withdraw into the shade.
[Illustration: Fig. 6. Bodily temperatures of blue racers captured by
hand in their natural surroundings. The preferred level is
approximately the same as indicated by Fig. 5 (between 29° and 36°
Centigrade), but some of the racers caught were not fully active and
had lower temperatures. Some bias results from the fact that those
having the lowest temperatures were the least active and hence were
most easily caught.]
[Illustration: Fig. 7. Air temperatures recorded at captures of the
racers whose records were used in Fig. 6. An active racer typically
maintains, by basking, a bodily temperature several degrees warmer
than the air.]
[Illustration: Fig. 8. Bodily temperatures of blue racers found in
live-traps at Harvey County Park. Opportunity to regulate temperature
by behavior was limited in these snakes in traps, which tended to
match ambient temperatures.]
At the Harvey County study area, bodily temperatures were recorded in
many of the racers that were caught in traps. These records are much
less significant than the records obtained from racers caught by hand
and promptly checked for bodily temperatures. The temperatures of the
trapped snakes may, to a large extent, reflect the temperatures of air
and soil at the time. However, despite their confinement, the trapped
racers probably were able to exercise some control over their
temperatures by shifting from shade to sunshine, or from the top of
the trap to its bottom, where they would be in contact with the
substrate. In most of the racers removed from traps, as in those
caught by hand, bodily temperatures were somewhat above air
temperatures, but the difference was less in the former group of
snakes. Figure 8 shows the bodily temperatures of these snakes removed
from traps. Occasionally racers died in the traps from overheating. On
July 2, 1960, each of two racers in traps had temperatures of 39.4°.
One of these was especially vicious and frantic in its attempts to
escape, but otherwise seemed unharmed. The second racer was dead,
seemingly having succumbed just before it was found. Probably
prolonged exposure to temperature in excess of 39° would always result
in death of the snake. Racers and other snakes that had become
overheated in the traps and were nearing exhaustion had a
characteristic limp feel when they were handled. In June, 1960, heat
tolerance of a halfgrown racer was compared with that of several other
snakes including a copperhead, garter snakes, and ringneck snakes.
Each snake in turn was enclosed in a plastic tube plugged with cotton
at one end, the snake having a quick-reading thermometer taped in
place for a rectal reading. The tube was then placed in sunshine. Over
periods of minutes the enclosed snake passed through a characteristic
cycle. Soon it would begin to register discomfort as its temperature
rose rapidly. Its struggles would become increasingly violent, then
would cease abruptly. The snake would suddenly collapse, its body
mostly limp, but knotted in slow contortions, its mouth gaping widely.
Within a few seconds all movements would cease, but in each instance
the seemingly dead snake was soon revived by holding it in cold
running water. The copperhead, garter snakes, and ring-necked snakes
all collapsed at temperatures near 41°. At this same temperature the
racer showed signs of acute discomfort, but did not collapse even
after many minutes of exposure. Probably more protracted exposure at
this level would have been fatal to the racer as well as to the other
kinds tested.