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A pit full of crawling snakes is not where most people like to spend their time. But when Canadian zoologist Michael Aleksiuk came across thousands of garter snakes squirming around in a limestone pit one spring, he just had to go back for some answers. What where all those snakes doing? Why were some neatly mutilated when Aleksiuk returned one morning? Where did they all disappear to at the end of May and how did they survive the cold Manitoba winters? In this 1975 National Geographic article, Aleksiuk explores the strange behavior and life cycle of the red-sided garter snake.
By Michael Aleksiuk, Ph.D.
I stared in disbelief as thousands of snakes milled around me. They were everywhere—the surface of the ground seemed literally in motion! The countless elongate bodies slithering through the dry, brittle grass of spring made an eerie rustling. Although I was used to snakes, I couldn't help feeling a little uneasy.
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I was standing near the bottom of a limestone sink—a pit perhaps fifteen feet across and ten feet deep. As I watched, it struck me that in spite of their unceasing movements the snakes seemed to be moving aimlessly. They crawled among the rocks, up small shrubs, even over my feet. Tongues constantly flicking in and out, they investigated everything, but didn't seem to find anything.
'What the devil are you doing down there?'
Startled, I whirled around and looked up. A large man stood at the edge of the pit, hands on hips, glowering down at me.
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Reluctantly, I stepped through the writhing mass of snakes and climbed out of the hole. I suppressed my annoyance, recalling what had brought me to this bizarre place.
Wild stories about snake pits had reached me ever since I had arrived in Manitoba a year earlier. At first I was skeptical. Finally I had located and journeyed to this snake pit in the aspen parklands 60 miles north of my Winnipeg home. I had verified the fantastic numbers of snakes and recognized their potential for scientific research.
'I lease the rights to this pit,' the man gruffly informed me, 'and I need 20,000 snakes for my customers this year. You can't take any of these snakes!'
I explained that I was a zoologist from the University of Manitoba, and the snakes were of major research interest to me and my colleagues. 'We're trying to learn how these snakes survive and reproduce,' I added.
The man's face brightened. 'I'm sorry,' he said, 'I thought maybe you were one of my competitors.'
Stan Spakowski introduced himself and explained that he was a professional snake and frog collector, selling to biological supply houses. Naturally he had a proprietary interest in his sources of supply.
'Please feel free to study them as much as you wish,' he added apologetically.
I left the snake pit aware that I was dealing with the red-sided garter snake, Thamnophis sirtalis parietalis, a subspecies of the common garter snake. However, I didn't know much about its habits.
Back on campus I found the university library confirmed what I suspected: Such large aggregations of land snakes appeared to be unknown to the scientific world. What I had observed probably occurred on this scale nowhere else. Here was a scientific gold mine! Elated. I immediately prepared for another trip to the snake pit.
Next morning a macabre scene greeted me at the pit. The masses of snakes were nowhere in sight—only several bloody individuals lying here and there in the grass and among the rocks. I picked one up and examined it. There was a gaping hole in its belly, and its liver was missing. Others in the grass revealed the same condition. Still alive, the snakes crawled away weakly when I put them down.
I climbed down into the pit and touched one of the few snakes visible among the rocks. Most of its head was missing. Though mortally wounded, it struck out at me blindly, its survival reflexes still working.
What a contrast to the swarming activity of the previous day! Because snakes are cold-blooded and depend on the sun for heat, in colder regions they are almost always inactive at night, and whenever it is cool and cloudy. This explained the inactivity; it was still only 8 a.m. Most of the snakes were apparently hidden beneath the broken rocks in the pit. But in spite of my best efforts I could find no clue to what had caused the mutilations.
I chose a shaded part of the pit and sat stock-still. Within an hour snakes began to emerge from the rubble and mill around. A pattern seemed to develop. Although male and female snakes are difficult to tell apart, males seemed to be chasing females. The snakes were mating.
Activity increased as the sun warmed the torpid snakes. Soon countless males were courting females. The competition was fierce. Sometimes as many as a hundred males attempted to mate with one female at the same time. This produced huge 'mating balls,' in which the female often couldn't even be seen.
After mating, most females tried to leave the pit. But in many cases they were caught by a tangle of males and the whole mass of bodies tumbled back to the bottom. Usually, the females managed to escape the pit within an hour or two after mating.
The males did not leave after the first mating attempts, but waited for more females to emerge. Although it was difficult to make a reliable estimate. I believe that a male probably remains in the pit area for about a week after first emerging from underground.
Watching the activity. I found the answers to the mutilation mystery. At one point, a partially beheaded snake slowly crept from a crack. Fresh blood oozed from the wound. Cautiously, I felt the victim: It was still ice-cold from hibernation. Earlier, I had seen a mouse scurrying from crevice to crevice in that area. I concluded that mice and shrews nibble at the heads as the snakes slowly work their way up through the rubble.
Later, the flapping of wings and a raucous cawing heralded the arrival of a crow. The crow hopped up to a newly emerged, torpid snake and grabbed it with one foot. It pecked swiftly at the snake four or five times, ate something, and moved to another victim. I jumped up with a yell, and the crow flew off, complaining hoarsely.
I picked up the snake: The skin had been broken only in the area of the liver, and that organ had been neatly excised. Nothing else had been touched. How the crow performed the surgery with such precision is a mystery.
Back in Winnipeg that afternoon, I asked a colleague, Dr. Kenneth W. Stewart, about the probable origin of the limestone sinks.
'Acidic groundwater percolating through fissures in the limestone slowly dissolves it, forming caves,' Ken replied. 'At one time the region between Lakes Winnipeg and Manitoba must have been underlain by an extensive network of caverns.
'The weight of ice during the last glaciations probably collapsed the roofs of many caverns, leaving rubble-filled sinks. The roofs of others have perhaps been weakened by centuries of freezing and thawing of water, finally to fall in, leaving more of these bowl-shaped depressions.'
The existence of caverns explained how so many snakes were able to disappear into the bottoms of the pits.
According to my friend Henry Yakielashek, whose home is near the pit, there were at least four large limestone sinks used by snakes in that area. 'But there aren't as many snakes around as there once were,' he added. 'Maybe the collectors are getting most of them.' The snakes first begin to emerge in late April, Henry told me, two to three weeks after the snow disappears.
By mid-May mating was at its peak. I watched almost daily, but the numbers of snakes in the pit began to dwindle rapidly until finally, at the end of May, few were left.
Back in my office at the university, I pondered the information I had obtained—and the questions that it posed. What made the snakes come out of the caverns when they did? What caused the vigorous mating in May? Where did the snakes go after leaving the pit? When did they come back? Why did the snakes congregate at the pits? How did they survive the region's extremely long, cold winter? These questions formed the basis for several years of intensive research.
Patrick Gregory, a graduate student, undertook a study of the snakes' seasonal movements. Working at several pits, he marked about 6,000 by removing scales in various patterns from the tail of each. Then he spent the summer combing ridges, fields, forests, and marshes, looking for the ones he had marked.
His study showed that most of the snakes spend the summer in marshy areas, where they feed on abundant frogs, as well as on toads, slugs, and earthworms.
Pat found some snakes as far as ten miles from their pits—a remarkably long journey for a snake only 18 inches long! It is while the snakes are dispersed in their summering areas that the females produce their offspring. Most births occur in mid-August, as many as 40 in a litter, though the average is 15. Born alive, the young are capable of rapid activity within hours. Completely independent of the mother, they must fend for themselves from the moment they are born.
All summer I kept a careful watch on the principal snake pit. It was lifeless, except for an occasional mouse. Then on August 20, during a routine check, a small movement caught my eye. A snake slithered over the rocks into the shelter of the crevices. The fall migration back to the pit had already begun.
By the middle of September, the pit looked much as it had in mid-May, with two major exceptions. Instead of moving outward, the snakes were now funneling down into the pit. And, once in the pit, the snakes lay motionless. In contrast to the frantic activity and rustling of spring, an uncanny silence now prevailed. Heads poised elegantly, the snakes seemed to be waiting for something. At one point I estimated there were some ten thousand pairs of unblinking eyes fixed on me. It was a spectacular sight.
Keeping close watch on the pits, Pat Gregory found that not only did many of his marked snakes return, but nearly all made their way to exactly the same pit where they had been marked. This meant that a snake probably uses the same pit year after year.
Traveling to a distant marsh in spring, and back again in fall, these small snakes somehow navigate across broad reaches of forest, pasture, and swamp, like migratory birds. How they do it with such accuracy is an important subject for future research.
Even young snakes born in summer eventually return to the sinks. It would be easy to speculate that they learn the route from their mothers, but the theory does not hold water. For one thing, the young spend little time with their mothers. And Pat's work showed that the young don't even spend their first winter at the pits. Where they do hibernate that first cold season isn't known. Somehow, on their own, the yearlings migrate to pits they have never seen, through miles of totally unknown territory.
In early October, fewer and fewer snakes could be seen in the pit. They were moving underground, beneath the rubble, where they would spend the winter. By late October the pits were once again apparently lifeless.
Because these garter snakes are inaccessible during the winter, we made preparations to conduct winter studies in the laboratory. Together with Anne Ismond, then a graduate student at the University of Manitoba, I had captured about five hundred snakes in September and placed them in a controlled environment room. To simulate winter, we kept most of them at 39° F. and in constant darkness.
From time to time we removed a few snakes from the dark holding room for study. In late January something totally unexpected happened: When I brought a number of snakes out into a warm, well-lighted laboratory, they mated vigorously I tried again, and again the change somehow triggered mating activity.
Al Hawley, another graduate student, took up a study of the environmental regulation of mating behavior. He dug snakes out of the pit in late April before they emerged naturally, transported them to the lab in an icebox, and held them in a cold, dark room. He could then transfer them from simulated hibernating conditions to various conditions of light and temperature.
It worked out beautifully. Snakes transferred from 35° F. and darkness to 86° F. and light always mated within one hour. Thus the snakes, within certain time limits, will 'mate upon command.'
Al also demonstrated that temperature, not light, is the important variable in mating. Furthermore, it seems the snakes must be exposed to winter cold for a minimum period of about five months before transfer to higher temperatures will induce mating. These results neatly demonstrate nature's way of timing mating behavior to the spring.
Although many questions remain, we have come to understand much about how this remarkable animal is able to cope with the rigorous climate of Manitoba. Nature has produced a specialized snake capable of overcoming problems not faced by subtropical and tropical reptiles.
The red-sided garter snake is clearly a prime subject for further research in environmental adaptation. But after years of study, I see it as more than that. To me it is a living work of art—intricate, yet simple in its design. Indeed, as the ever-evolving product of natural selection, it is the handiwork of the grandest Master of all.
Source: Aleksiuk, Michael. “Manitoba’s Fantastic Snake Pits.” National Geographic, November 1975.
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Garter Snake; Reptile; Snake (reptile)
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