Listen again to part of the lecture．Then answer the question. What does the professor mean when he says this:

Several hundred million years ago, a large body of water covered Manitoba．This ancient ocean was home to brachiopods, trilobites, and corals．As these organisms died, they settled in layers on the ocean floor, where they became compressed and cemented together, and over millions of years turned into limestone.
Wherever you find limestone, you also find crevices, tunnels and caves．A network of caves is formed when acidic rainwater seeps into the layers of limestone through cracks at the surface and dissolves some of the limestone．The limestone weakens and collapses near the surface and further below, creating sinkholes, or large pits in the limestone.
The limestone pits in Manitoba’s Interlake region are like nothing else in the world．What makes these particular pits so unique? Well, for one thing, huge numbers of snakes migrate to these holes in the ground．If you go anywhere near the pits in the spring or fall, you’ll find the region alive with thousands of red-sided garter snakes.
This is the world’s largest concentration of snakes．Tens of thousands of these snakes congregate at the surface of their winter dens each year．They emerge in the spring after spending the winter hibernating in what are known as the "snake pits." In the fall, the snakes return to the pits to prepare for winter.
And why are snakes drawn to the limestone pits? Well, the availability of winter dens in the limestone bedrock makes this sort of an ideal home．Like all reptiles, snakes are ectotherms．This means they warm their bodies mainly by absorbing heat from their environment．The pits are the perfect location for the hibernating snakes because they shelter the snakes from winter temperatures that sometimes dip as low as minus 40 degrees Celsius.
Animals interact with their environment in a number of ways-for example, by responding behaviorally or physiologically, or by adapting over evolutionary time．The migration of garter snakes to the limestone pits is an example of a behavioral mechanism for dealing with seasonal variations in temperature．Moving to a new location is an animal’s quickest response to an unfavorable change in the environment．By gathering in a mass below the frost line, the snakes are able to survive the harsh prairie winter.
Garter snakes have an optimal temperature for physiological function．They can, however, actually survive freezing temperatures for a short time-a few hours only- and at temperatures like minus-1 or minus-2 Celsius．They obviously can’t stay frozen all winter long, but to survive in cold climates, they’ve adapted to situations like being exposed to freezing temperatures outside their dens on cold nights in the spring or fall．This ability to endure short freezes will get them through till morning.
The beginning of freezing in their bodies is a physiological response that serves as a signal to the snakes．Freezing tells them to alter their behavior, to move and find a warmer place．This physiological-behavioral response might also be helpful in their winter dens because, if the den temperature falls too low and some snakes begin to freeze, it could be a signal for all the snakes to move to deeper, warmer parts of the den.
These response mechanisms are only marginal abilities that allow the snakes to cope with short-term freezes．About ten years ago, thousands of snakes died when they were trapped in shallower dens when the frost line fell during an extended period of freezing temperatures．They were apparently not able to get to lower depths in time.