Monday, October 10, 2011
The Quintessential North American Reptile
Article and photos by Wayne Ferrier
I had that unmistakable feeling of being watched. It was a sunny autumn afternoon, and I was helping my father dig up an old drainage ditch at their Central Pennsylvania home. I was pretty far down in the ditch, pitching gravel over my shoulder onto the bank above me. I paused and looked around.
It didn’t take long to find out who was spying on me. A common garter snake, Thamnophis sirtalis, lay curled up on the bank, watching me with an intensity that I would have to say bordered on fascination.
A curious thing about the encounter was that the snake was half buried in gravel. She was too enchanted watching me work to worry much about being buried in stones.
No doubt I was excavating a favorite hunting ground. Digging up and replacing the old drainage system, I was uncovering a lot of salamanders (Eurycea bislineata), most certainly a staple in this particular garter snake’s diet.
I do not know how long she had been there, inches from my head. For a moment we remained motionless, eyeing one another, but eventually she lost her nerve and darted off towards the stone wall. Slick yellow and brown lateral stripes proved to be excellent camouflage gliding through a background of burnt grass and autumn leaves, and she quickly disappeared from view.
I put down my shovel and took a coffee break. This encounter sure brought back memories. There have always been garter snakes in my parent’s yard, and as preschoolers we used to play here at this very same wall. My friends and I would often get that eerie feeling that someone was watching us. Often it turned out to be one particularly curious garter, and in our ignorance we would chase him back to the stone wall screaming, wailing, and hurling rocks. But within an hour he would be back to continue his espionage, we’d get that feeling and in panic try to unload more punishment on the poor creature.
Actually he was too fast for us and we never did catch him. What makes garters act like that? Most snakes go out of their way to avoid people, so one that actually chooses to live amongst us, hangout during daylight hours, and even engage in people watching seems to be rather oddball behavior for a snake.
A partial explanation is that garter snakes rely heavily on their vision when they hunt. This does not mean, however, that their vision is that great—a garter snake cannot see very well unless what it is looking at is moving. If the prey stays perfectly still, the snake may not detect it. The slightest movement, however, can give the prey away. Garter snakes are hypersensitive to the very slightest twitch. So things that move fascinate them.
In addition, it seems that garter snakes like to eat all the time, at least compared to many snakes that spend long periods between meals, and the thought of food sometimes overpowers their flight mechanism. I once caught and temporarily kept a fully mature garter at my country home in upstate New York and had her feeding out of my hand within twenty minutes. I used to play a game with her, where I’d wiggle my finger as if it were a worm and she’d get all excited, flick her tongue and start pursuit; that is until she figured out that my finger was attached to me and that ended the game; and she wouldn’t fall for that trick anymore.
She had no interest in leaving captivity until the autumn leaves started falling, and she knew she had to go hibernate. That’s when it was her time to put one over on me. In order to feed her I had to open the lid at the top of her terrarium. If she wanted fed, which was like several times a day, she’d rise, balancing her belly on the glass and propping up on the tip of her tail. I’d hoover an earthworm just above her head. She’d check it out for a few minutes, and then quickly snatch it from my hand. One October day she took a particularly long time making the strike and I was getting bored, eventually my attention started to drift; she took the opportunity and darted past me and out of the terrarium! Was that planned? Sure seemed like it, but I’ll leave that to evolutionary psychology to debate. I will say this though, she tried the trick several more times, but when I wouldn’t fall for it again, the ruse ended.
Garter snakes seem to be smart. Most snakes detect prey primarily by olfaction using their Jacobson’s organs. Pit vipers, such as rattlesnakes and copperheads, also have heat sensors. Compared to these snakes, garters are more visual. If movement is sensed overhead (e.g., a hawk), it is to be avoided; but if the movement is perceived at or below eye level (e.g., a frog), it may be pursued, analyzed, and perhaps eaten—unless it’s my finger. My pet snake once accidently bit my finger during a sloppy strike aimed at a worm. She knew immediately she had missed and hit the wrong target. I may be over anthropomorphizing a bit, but she actually appeared to be embarrassed, genuinely sorry, and ran and hid. When she saw I wasn’t the least perturbed by the mishap, she came back and finished the worm. She never missed after that and I was never bitten again.
Back to the story of digging the ditch and the garter at the stone wall: I’m sure watching me work, that it didn’t take her long to figure out that I was way too big to eat. Down in the ditch below her, I wasn’t particularly threatening, but when I noticed her and stood straight up, it was a different matter, and that was when she decided that the show was over, and maybe the best thing to do was skedaddle.
Hands down the garter snake is the dominant reptile in North America. It has the widest range and is the most common reptile found on the continent. The genus Thamnophis (garter and ribbon snakes) can be found anywhere from southern Alaska to the Maritime Provinces. The common garter, Thamnophis sirtalis, has the most northerly range of all North American reptiles, going as far as the border between Alberta and the Northwest Territories. Every one of the lower 48 states has at least one species of Thamnophis, and a few species live as far south as Costa Rica. Mountains, plains, deserts, swamps, even cities—it doesn’t matter—as long as there is suitable food around, garter snakes can usually be found.
Ideal habitats can accommodate as many as 10 snakes per acre, and several species can coexist in the same area by hunting different prey and being active at different temperatures. Add to this their habits of frequent feeding and daytime activity, and it is not surprising that a garter snake is the first—and perhaps only—snake that many North Americans may ever encounter in the wild.
Most Thamnophis are opportunistic, varying prey according to what is available. Earthworms are their favorite food, amphibians are their second choice. Sometimes they eat smaller snakes, and may even resort to cannibalism. Insects are consumed when abundant in the fall. Occasionally Thamnophis kill and eat rodents (e.g., voles, mice, chipmunks) or nestling birds. Their versatile diet may also include fish and crustaceans, and even carrion.
Perhaps the most extreme example of the garter snake’s love for food is their taste for dangerous delicacies. In some high-end sushi restaurants you may order fugu, a dish prepared from the extremely poisonous pufferfish or blowfish. A skilled chef knows how to prepare the dish by removing the poison; and if you buy it you must have a lot of trust in the skill and integrity of the chef. If a garter snake were to be fed a bad batch of fugu, it might not notice. The snakes have evolved resistance to blowfish poison (tetrodotoxin), because they regularly eat rough-skinned newts (Taricha granulosa), which also secrete the toxin. The newts and snakes have been engaged in an evolutionary arms race, and the last time I checked, it seems that the newts are losing. They just can’t make themselves toxic enough to dissuade the garters from eating them.
Generally, Thamnophis capture prey with their mouths and swallow it alive to slowly suffocate in the snake’s digestive tract. It is thought that the saliva of some species of Thamnophis may be medley toxic. Some garters may resort to constriction when subduing rodents—western plains garters have been seen doing this.
When a garter snake first ventures out to hunt, like any other snake, it flicks its forked tongue trying to locate prey. A young snake analyzes the scent substances give off by potential prey before it will strike, but an experienced snake relies more on visual clues. This skill is especially useful when hunting frogs and toads. Normally diurnal, many species prowl at night during the anuran breeding season. At this time the smell of frogs may be ubiquitous and relying on scent alone would not be productive for the snake. So it lies in wait and when it sees a frog or toad move it strikes. A snake may be right on top of a frog, but as long as the frog remains motionless, the frog will go undetected. Now the frogs are well aware of this phenomena—they themselves can’t see their own prey very well unless it is moving—so when the snake is around they keep still. Western ribbon snakes (Thamnophis proximus) have been seen solving this problem by systematically striking the vegetation, obviously smelling the frogs but unable to see them. Striking the grass disturbs the frogs to the point that they lose their nerve and make a break for it. The snakes were actually flushing the frogs out.
Many Thamnophis species are as comfortable in the water as they are on land. Sometimes they maneuver through the shoreline brush or climb trees and overlook the water from that vantage point. Ribbon snakes are frequently found among the reeds and cattails in shallow water—an ideal ecological niche, where land, water, and sky come together. The reeds offer good cover and usually abound with insects, fish, frogs, snails, and leeches. But the reeds also offer death. Those who like to eat aquatic serpents also hang out in the reeds; wading birds, predatory fish, and ophiophagous snakes (cottonmouths for example), are among the ribbon snakes worst enemies. When a ribbon snake comes across the scent of a predatory snake, it leaves the area immediately. If pursued, ribbons will sometimes dive into the water and submerge.
Many Thamnophis are generalists making use of a variety of habitats and prey.
Generalists are more versatile and less susceptible to starvation if one food source is scarce and this is a primary reason for the success of this reptile. But they have numerous enemies—mostly other snakes, large birds, and mammals such as opossum, fox, mink, and skunk. Young snakes may also have to contend with large toads and frogs. When faced with danger, the snake either tries to flee or conceal itself. Which one may depend partly on the weather. On chilly days a garter snake cannot move very fast and may not even attempt to flee, knowing full well that it would probably lose the chase. On warmer days a garter may even become aggressive but may quickly switch to passive measures if touched. Thus aggressiveness may be only a bluff—but don’t take this for granted! Sometimes if handled they emit a strong musky odor, which makes you want to put them down.
In the autumn and early spring you often encounter a garter snake en route to and from its winter hibernacula. In some areas this may be as far as 15km from where you find them. They often find other snakes to hibernate with. I’ve learned a lot about snakes since my original experiences with garter snakes when I was a kid. My autumn encounter with the spy might have been a strange experience had it been any other kind of snake, but it was a Thamnophis—the quintessential North American reptile. It was not so strange, there are a lot of them here. Perhaps the old stone wall has been a Thamnophis hibernaculum since I was a kid. I finished my coffee and my reminiscing and went back to work in the ditch and awaited her return. But not this time, the last I saw of her had been her stripes blending into the leaves and grass, as she slipped into the recesses of the stonewall.
Posted by Wayne Ferrier at 12:05 AM | Permalink