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Garter Snakes: Urban Survivors



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Scientists believe as many as fourteen species of snakes occur in DuPage County, but the snake the average resident will most likely encounter is the Garter Snake. Despite the superstitions that cause many people to kill any snake making an appearance, and despite the destruction of marshes and the blacktopping of open spaces, Garter Snakes manage to survive in DuPage County because of their ability to adapt to an urban environment.


Actually, there are two species inhabiting DuPage's forests, fields, and marshes-the Eastern and the Plains Garter Snake. The most widespread is the Eastern Garter Snake, Thamnophis sirtalis, which ranges from the Atlantic coast north and west to the Yukon and the Pacific coast. Northeastern Illinois has its own variety, appropriately called the Chicago Garter Snake, one of the eleven subspecies of this wide-ranging serpent.


The Chicago Garter Snake is comparatively slender and moderately sized. Adults range from 20 to 24 inches in length. A dark yellow stripe runs down the spine of this subspecies, and a somewhat lighter stripe runs along the second and third row of scales on each side. These side stripes are interrupted by brown cross-bars in the front quarter, and this feature distinguishes the Chicago Garter from other subspecies of the Eastern Garter Snake. Most of the Garters found in our area have a brown or olive "checkerboard" between the stripes and a bluish stomach.


The Plains Garter Snake, Thamnophis radix, which inhabits the Central Plains states, is the second Garter in DuPage County. Although the average Plains Garter Snake is slightly shorter than the Chicago, I have seen individuals from our region as long as 26 inches. These serpents are stouter in appearance than the other species, and have a proportionately shorter head. The stripe along the spine of this species is orange rather than yellow; this feature is the best field mark for those who don't want to pick up a snake to examine it closely. Those willing to handle the serpents will find other identifying features.


The Plains Garter Snake has black borders on the edge of each lip scale, or labial. Also, the side stripes are on the third and fourth scale rows. Finally, there is a behavior difference: while the Plains Garter will usually just thrash about and then curl when caught, Chicago Garters will attempt to bite. Of course, there are exceptions! But neither serpent is poisonous and inflicts only a small wound if provoked to bite.


When two closely related animals occupy the same area, some feature in their ecological requirements keeps them from competing with each other. In the case of DuPage's Garter Snakes, their natural habitats separate the two species. Although both are more common in the vicinity of water, the Chicago Garter occurs in open woodlands, while the Plains Garter resides in marshes and wet fields.


But snakes don't read textbooks, which may explain why I have found individuals of each species only a few feet from each other in some locations. The Chicago Garter Snake s more wide ranging and abundant, but the Plains was present in half the sites where I found the Chicago Garter. Why would the two species occur together? Perhaps as suitable habitat for both snakes is reduced within DuPage County, especially the wetlands preferred by the Plains Garter Snake, the two populations are forced to retreat into the same areas.


Another possibility is that they were released into the habitat by collectors who couldn't house them (or by small boys whose mothers wouldn't allow the pets they brought home from camp inside the house). But wherever their ecological requirements are met, Garter Snakes survive, and I have seen them in almost every Forest Preserve I have visited within the county.


The best time to observe Garter Snakes is in the spring shortly after they emerge from hibernation and are seeking mates. Perhaps one reason why Garter Snakes survive in DuPage County is that they have large numbers of young. In Illinois, the Plains Garter averages 14 young a season, but as many as 92 young have been reported from other parts of its range. The Chicago Garter Snake also produces a large number of off-spring, and broods from 12 to 42 been reported. One of my Chicago Garters had a brood of 20 young in early September, while another had 41 young in late August. All Garter Snakes are viviparous; that is, the females retain the eggs inside them until they are nearly ready to hatch. The young are "born" in a thin membrane, from which they emerge within a few minutes prepared to survive on their own.


Young Garter Snakes of both species begin feeding a few days after birth, and they choose from quite a varied menu. Garter Snakes eat many small creatures which are common inurban habitats. Earthworms form the bulk of their diet in our area. While one large Chicago Garter Snake I kept ate nearly a quarter-pound of worms during its weekly feeding, smaller or younger species seem to require lesser, more frequent meals. When in captivity, both species eat fish, although I doubt if either is able to catch a healthy fish in the wild. I had one Chicago Garter Snake that atesardines and tuna, two fish it would never encounter in the wild, as well as discarded parts of frozen trout.


Both Garters also prey upon amphibians. One of the principal foods of the Plains Garter Snake in the western part of its range is frogs. In the DuPage habitats where I have seen Garter Snakes, frogs are uncommon or absent. However, the American Toad is often present in Iarge numbers and enters the snakes' diets, either as tadpoles or as adults.


Toads often inflate their bodies when seized, and this prevents a small snake from eating them. But usually the snake manages to swallow the toad by dislocating some of the bones in its own jaw and slowly "walking" over the toad until it is consumed. In this dramatic way I watched a 34-inch Chicago Garter Snake swallow a toad that had inflated itself to a six-inch circumference, although it took the snake about an hour to finish. The toad was digested within four days, and in the meantime, the snake also ate a dozen large earthworms.


Snakes are noted for preferring live prey, but Garters will eat carrion. Dead fish and amphibians are taken by both species, and I kept a Chicago Garter Snake which ate skinned mice and sparrows that were intended for a Fox Snake, Elaphe vulpina, also living in its cage. A Minnesota scientist reported flnding the leg of a Pocket Gopher in the stomach of another subspecies of the Eastern Garter Snake. This confirms that the species feeds on carrion in its natural habitat, since gophers are too large an animal to be the prey of any Garter Snake.


The varied diet of both the Chicago and Plains Garter Snakes enables them to survive in areas where other snakes would have difficulty maintaining a large enough population for breeding. Nevertheless, I have observed that there are several groupS of animals which captive Garter Snakes will not accept, even though various writers have reported the opposite. I have seen a Chicago Garter Snake seize a large gray slug, but almost immediately release it and wipe its mouth on the floor of its cage. Nor have I seen any captive Garter Snakes feed on insects or other arthropods. Grasshoppers are reported to be a major item in the Plains Garter Snake's diet in the west, but none of the DuPage snakes I have kept has shown any interest in these insects when they were made available.


Our Garter Snake species need to be able to adapt to changing environments, reproduce themselves in large numbers, and accept a variety of food in order to survive their many enemies. Many carnivores eat Garter Snakes, as do a number of birds of prey. A Canadian scientist reported that mice attack and kill Garter Snakes when the reptiles emerge from hibernation and that crows flock around hibernating sites, kill the snakes and eat only their livers. Garter Snakes are even preyed upon by other snakes as well as by turtles and large frogs. Judging from the number of scarred, nearly tailless snakes I have seen, narrow escapes are common. But Garter Snakes have several methods of defense to thwart predators.


The preferred defense method is concealment. Garter Snakes, like other serpents, seek shelter under logs, planks, blocks of concrete, vegetation, tar paper and other objects. I have found snakes under all of these, as well as inside a rotting log. It is not unusual to find several snakes together, and large numbers have been collected from animal burrows and ant nests. Garter Snakes typically conceal themselves both before and after hibernation. They also seek shelter prior to moulting their skin.


In the open, Garter Snakes either freeze or flee. The Plains Garter is a master at remaining motionless, an effective technique in the fields and marshes it inhabits. There, the direct rays of the sun form strong shadows and intense streaks of light on the ground, and the snake's body pattern blends in well with the light and shadow. Also, the matted vegetation permits the snake to lie partially concealed, increasing the effectiveness of remaining motionless. One time, after crisscrossing a field for an hour without seeing any snakes, I knelt to tie my shoe. Just inches away from my foot was a large females Plains Garter Snake which I must have walked by several times.


The Chicago Garter Snake is likely to speed away silently when discovered, and speed away it does! Although experiments suggest that even the fastest snakes don't exceed speeds of more than three and a half miles an hour, natural conditions and their contoured bodies give snakes an advantage. One of the most frequent sights I have had is the glimpse of a tail as the reptile slips to safety. One scientist noted the fastest snakes are either solid colored or have the Garter's striped pattern; this pattern, besides blending in with natural surroundings, makes it hard to focus on the creature as it moves from side to side.


Garter Snakes use two other defense measures in dealing with their enemies. The Chicago Garter often strikes, and although such a bite is only a superficial scratch (easily treated with iodine), the effect is startling. Both Garter Snakes of DuPage County share a defense method used by others of their kind: while gyrating around, they spray the air with the offensive contents of their anal glands and disgorge any stomach contents. Crude but effective, the method usually gains the snake a few moments to slip away.


While the Chicago and Plains Garter Snakes have been able to survive in certain DuPage localities, other snakes have not managed to adapt and survive. Although I expect to locate a few more species, one snake I don't plan to find here is the Ribbon Snake, Thamnophis sauritus, another species of Garter Snake. This serpent had been recorded from adjacent areas in Cook County, but because of the destruction of its woodland stream habitat through pollution and urbanization, the species probably no longer exists in northeastern Illinois. It is one of three snakes whose populations have dwindled to the point that they are included on the Illinois endangered species list.


Even though its two relatives have been able to adapt to similar problems, continued preservation of natural areas within DuPage County and respect for snakes as important residents of their natural communities is necessary to spare even such tough urban survivors as the Chicago and Plains Garter Snakes from the Ribbon Snake's fate.