Field study of the blue racer was carried on in several localities in
Kansas, but chiefly at the University of Kansas Natural History
Reservation (the northeasternmost section of land in Douglas County),
and the adjacent 160-acre Rockefeller Experimental Tract in Jefferson
County. By October 26, 1962, after 14 years of field work, a total of
1423 racers had been captured some 2197 times.
The locale of the present study was near the geographic center of the
blue racer's range. The range, chiefly in the Mississippi Valley and
Great Plains region, is centrally situated with respect to the other
seven subspecies. An extensive but scattered literature concerning the
ecology of the species as a whole, and its several geographic races,
has been reviewed and utilized for comparison with my own field data.
Blue racers were caught in wire funnel traps set in prairie and
pastureland habitat in summer, and along hilltop limestone outcrops in
woodland in autumn. The autumn trapping along rock outcrops was
carried on each year from 1949 to 1962, but effective summer trapping
was carried on only in the last six years of the study. Each racer
caught was individually and permanently marked by scale clipping. More
than half were caught only once, but many were recorded repeatedly,
with a maximum of 16 captures.
The racer occurs throughout most of the United States, and its
populations are subject to much geographic variation. The snakes are
largest in the northeastern part of the range, with clines of
decreasing size toward the southeastern, southern and western parts of
the United States. There are somewhat parallel trends in coloration;
the black racer of the northeastern states grades into paler, gray or
light brown subspecies in southern Florida, Texas, and the far western
states. Accompanying these changes in color and size are minor
morphological changes and major ecological changes. The black racers
of the eastern states often inhabit forest or forest-edge habitats
while the paler and smaller snakes of more southern or western areas
typically inhabit scrub, chaparral, or prairie. The large,
dark-colored racers of the eastern and northeastern states are
especially inclined to attack larger prey including small
vertebrates, even weasels, rabbits, and chipmunks, whereas the
smaller and paler racers of more southern and more western areas take
a higher proportion of insects and rarely attack vertebrates other
than small reptiles.
On the area where field work was carried on in northeastern Kansas,
tall-grass prairie habitat is preferred, but fields of grain or
alfalfa, grazed pasture, brush, woodland edge, groves or open
woodland, and weedy fields are all utilized to some extent. The racer
is strictly diurnal and largely terrestrial but it may climb through
bushes or small trees in foraging or escaping.
The blue racer is a typical colubrine snake of slender build, with
large eyes, and vision plays an important role in finding prey and
detecting enemies. In the adult blue racer the dorsal color is
variable, pale brown or gray, bluish, greenish or slaty. In the
hatchling, however, there is a distinct pattern of a type widespread
among colubrines and also among snakes of other groups--a series of
middorsal blotches on an olive ground color, with alternating rows of
smaller spots on each side. The ventral surface is pale, with dark
speckling. The pattern is sharply defined on the anterior part of the
body, but markings become progressively more obscure posteriorly and
are scarcely discernible on the tail.
The juvenal pattern fades gradually as growth proceeds, and there is
much individual variation in the rate of its loss. Some racers still
retain the juvenal pattern faintly discernible after attainment of
sexual maturity. There are also striking ontogenetic changes in the
proportions of the head, body and tail. The diameter of the eye is
approximately one per cent of the snout-vent length in hatchlings, but
is only a little more than half that relative size in the largest
adults. In the course of allometric growth other parts of the head
also enlarge less rapidly than the body, but more rapidly than the
eye. In hatchlings there is a slight average difference between the
sexes in relative tail length, with males' tails the longer. Relative
tail length increases slightly in both sexes up to the time of sexual
maturity, and then decreases slightly with advancing age.
Racers in northeastern Kansas spend nearly half the year in
hibernation, with average recorded emergence date April 16, and
average date of retirement into hibernation November 8. Hibernacula
are usually in crevices in hilltop limestone outcrops with south
exposures. Winter temperatures within the hibernacula are usually well
within the range 0 degrees to 10 degrees Centigrade. Spring emergence
has been recorded at an air temperature of only 12.5 degrees
Centigrade. Racers bask in sunshine frequently even in warm weather,
and the temperature preferendum is several degrees higher than in most
other kinds of snakes. Bodily temperatures obtained from blue racers
that were fully active, either under natural conditions or in a large
outdoor enclosure, were concentrated in the neighborhood of 34 and 35
degrees Centigrade. For short periods racers can survive temperatures
up to 45 degrees without damage, but more prolonged exposure to
temperatures of slightly less than 40 degrees can be fatal. In
hibernation, racers can withstand temperatures slightly below
freezing, but they cannot survive being frozen solid.
Blue racers tend to limit their activities to familiar areas or home
ranges; some individuals may live out their entire lives within the
same home range, but others shift from time to time. Average home
ranges of approximately 26 acres for males and 24 acres for females
were calculated. The racers' preference for hibernacula in a habitat
different from that to which summer activities are confined
necessitates spring and fall migrations between the limestone outcrops
where hibernation occurs and the grasslands where the snakes stay in
summer. The average spring or fall migration is approximately a
quarter of a mile, but an individual racer does not consistently
return to the same hibernaculum. Many racers were recorded to have
made movements of 2000 to nearly 4000 feet, involving shifts in home
range, but some later shifted back to their original areas. Some may
have made even longer shifts but their movements would not have been
recorded since they would have gotten beyond the limits of the study
area.
Blue racers hunt by various methods, often by coursing through dense
vegetation in active search in which vision is of primary importance
in locating the prey. Almost any small animal that moves nearby may be
overtaken and caught with a sudden dash. From analysis of scats and
prey from stomachs, a total of 1357 food items of more than 50 species
was compiled. Favorite prey species were the cricket (_Gryllus
assimilis_), grasshoppers (_Arphia simplex_, _Melanoplus
femur-rubrum_, _M. bivittatus_, _M. differentialis_), camel crickets
(_Ceuthophilus_ sp.), katydid (_Neoconocephalus robustus_), vole
(_Microtus ochrogaster_), white-footed mouse (_Peromyscus_ sp.),
racerunner, (_Cnemidophorus sexlineatus_), and leopard frog (_Rana
pipiens_). The insects taken greatly outnumbered the vertebrates, but
the vertebrates made up most of the actual bulk of prey eaten.
Crickets, grasshoppers, and katydids comprised most of the insect
prey. Beetles, moths and cicadas were rarely taken. Vertebrate prey
included miscellaneous small snakes (some of them juvenal racers),
mammals, and birds. Seasonal change in the composition of the food is
slight, but vertebrates figure more prominently in early summer, and
insects comprise increasing percentages later in the season.
Composition of the food differs according to size of the snake;
gryllid and ceuthophilid crickets are best represented in the food of
juveniles whereas small mammals, and grasshoppers of the genus
_Melanoplus_ are best represented in the food of large adults.
The breeding season is mainly in May. Mating is promiscuous and two or
more males may court the same female simultaneously. A courting male
lies on or alongside a receptive female, with spasmodic rippling
abdominal movements, and with his vent adpressed to hers. At intervals
in the courtship period the female moves swiftly for a few feet or a
few yards shifting to a new spot, and during her activity the male
strives to maintain contact with her. From time to time the male
leaves the female briefly and courses rapidly around her in a devious
route. Courtship is consummated when the female raises her tail in
acceptance of the male and intromission is effected. During coitus,
which lasts for periods of minutes, the female moves forward slowly,
dragging the passive male, tail-first behind her.
Ovulation normally is in late May. Eggs from 29 to 39 millimeters in
length and 14 to 21 millimeters in breadth are laid, from mid-June to
early August, usually in tunnels of fossorial mammals such as voles or
moles, at depths of five to 12 inches. Clutches of the blue racer
average 11.8 eggs but the number is correlated with age and size of
females; two-year-olds average only 9.2 eggs, whereas those females
that are six years old or more average 15.7 eggs. Also, there is
geographic variation in size of clutch, from only 5.8 eggs in _C. c.
mormon_ of the West Coast to 16.8 eggs in _C. c. constrictor_ of the
northeastern states. In each breeding season some females of adult
size do not produce clutches. Only about 13 per cent of the
two-year-olds in a small sample were fecund, but the ratio increased
to 80 per cent in old adults. Incubation averages 51 (43 to 63) days.
Hatchlings usually make several longitudinal slits in the eggshell
with the egg tooth before emerging, and often require a day or more to
emerge after the first slit is made. Hatchlings average 10-3/4 inches
and 4.16 grams. By late October when these young are ready to enter
hibernation, they have grown to 16-3/4 inches and 12.3 grams. Typical
October lengths (overall) in inches for males and females,
respectively, after successive seasons of growth, are as follows:
yearlings, 27-3/4 and 29; two-year-olds, 34-1/4 and 37-1/4;
three-year-olds, 37-3/4 and 41-3/4; four-year-olds, 40 and 44-3/4;
five-year-olds, 41-1/2 and 46-1/4; six-year-olds, 42-1/4 and 48-1/4;
seven-year-olds, 43-3/4 and 50; eight-year-olds, 44 and 51-1/4.
Judging from trends in the small samples available, sex ratio in
hatchlings is approximately 1:1. In the summer trapping of blue
racers, males are caught in larger numbers than females, but seemingly
this is because of their greater activity. In fall along the limestone
outcrops where the racers hibernate females were caught in slightly
greater numbers than males among the young adults, and made up a
little more than 60 per cent of the old adults. Presumably the males
are eliminated more rapidly, because of their greater activity, their
smaller size, or a combination of both factors. The composition by age
groups of the adult racers captured was as follows: two-year-olds,
41.5 per cent; three-year-olds, 17.8 per cent; four-year-olds, 12.6
per cent; five-year-olds, 9.5 per cent; six-year-olds, 6.1 per cent;
seven-year-olds, 4.3 per cent; eight-year-olds, 2.7 per cent;
nine-year-olds, 2.4 per cent; ten-year-olds, 1.2 per cent; more than
ten years old, 1.9 per cent.
Attempts to census blue racers on the study plots, from the
capture-recapture ratios, yielded highly variable figures for
different areas and even for the same area at different times.
Variability is believed to result mostly from the small sizes of the
samples, none of which is large enough to yield a census figure that
is statistically reliable. Combined samples yielded figures indicating
populations of one to three adults per acre in early summer in areas
of favorable habitat. To represent the entire population in late
summer--the time of its annual maximum--these figures would need to be
approximately tripled.
The blue racer depends mostly on speed to escape its enemies. In
escaping it often takes advantage of a downhill slope to increase its
speed. A racer startled at close range often sets out with a violent
thrashing that attracts attention to a given spot, then glides away so
silently and rapidly that its course is not detected. The snake may
circle back and approach the scene of disturbance slowly and
stealthily, from the direction opposite to that taken in departing.
Often a racer seeks concealment by climbing into a bush or tree, and
occasionally escape is effected by swimming. A racer that is caught or
cornered usually makes a spirited defense by striking and biting. An
alternative reaction, seen most typically when the snake is
handicapped by injury or by low temperature, is to coil with the head
concealed, and, with writhing movements, to smear the surface of the
body with musk discharged from glands in the base of the tail.
Many natural enemies prey upon the racer, but in the present study the
majority of records pertained to the red-tailed hawk. The broad-winged
hawk, marsh hawk, red-shouldered hawk, sparrow hawk, and barn owl also
are among the raptors that feed on this kind of snake. Among mammals
the striped skunk is the only species definitely recorded to prey on
the racer, feeding on the eggs as well as upon the young and adults.
The common king snake, prairie king snake, timber rattlesnake,
copperhead, and even the slender glass lizard have been recorded to
prey on the blue racer, but probably all are of minor importance as
natural enemies. Shrews (_Blarina_, _Cryptotis_) and mice
(_Peromyscus_, _Reithrodontomys_) have sometimes killed and eaten
racers confined in traps, and under natural conditions they possibly
prey upon snakes that are immobilized when torpid from cold, in their
hibernacula and temporary shelters. Chiggers are the most common
ectoparasites of the racers. Four species of the chigger genus
_Trombicula_ have been recorded on racers from the Reservation. The
racer is an important host of the common pest chigger, _Trombicula
alfreddugèsi_, which often attacks humans. Most adult racers on the
Reservation and nearby areas carry the fluke, _Neorenifer
lateriporus_. In spring these flukes are conspicuous in the mouths of
the racers, but in late summer they are not in evidence.