Snakes

SNAKES' WINTER HIDEOUT

When warm-blooded animals hibernate they have an internal safety device to wake them should the temperature drop below a certain limit. Cold-blooded animals do not have this, so for their winter sleep they must go deeper, beyond the limit of frost. This is one reason why snakes cannot live in polar regions, where the ground is permanently frozen to a great depth. Apart from the shortness of the summer, in such regions there is no safe refuge in which the snakes can hibernate.

Hibernation in cold-blooded animals cannot truly be called sleep. It is a state of torpor resulting directly from a lowering of the temperature. Even during spring and summer when snakes, for example, are out of hibernation they will be less active (i.e. more torpid) on cold days, and more active on warm days. As autumn approaches temperatures generally tend to drop, in temperate latitudes, and when the temperature drops to a certain level snakes begin to search for suitable quarters in which to lie up for the winter.

In the case of the European adder this search is initiated when the shade temperature has dropped to about 49°F, and the snakes come out of winter quarters when the temperature has risen to about 46°F. Even then a cold spell may send them back again. The length of the hibernation will therefore vary with geography. In northern Europe it may last for anything up to 275 days. In southern Europe it may be as little as 105 days. In Britain it is around 135 days, as compared with 150 days in Denmark, which lies at about the same latitude but has colder winters.

Some snakes are by nature burrowers and can therefore make their own winter quarters or can enlarge an existing burrow. Adders do not burrow, and they must find suitable holes in the ground or crevices among rocks.

One of the unsolved mysteries is how snakes, and especially adders, know in advance where to go. Normally they are solitary animals, but for the winter they come together and as many as 40 have been found in one hibernaculum, together with a number of toads and a large number of lizards. This massing together, whether of snakes only or snakes with other cold-blooded animals, is almost certainly a matter of conserving heat. Although we speak of cold-blooded animals this does not mean their bodies lack any heat at all, but that their body-temperature cannot be maintained much above the temperature of the surrounding air.

Temperature also plays a part in determining how deep the snakes will go. In Britain adders hibernate at ten inches to a foot below the surface. In Denmark, with its more severe winters they may be as little as ten inches, where the ground is covered with heather, or as much as four feet below the surface.

The mystery is how a number of adders, that have been living singly and spread well out from each other, all find their way to a traditional hibernaculum. We know that, if it is undisturbed, the same crevice or burrow will be used year after year. Marking the snakes in hibernacula shows that not all the same individuals come together each year. The only conclusion seems to be that one or more individuals, searching about as the autumn temperature drops, find the hibernaculum by the odour left behind in it from a previous winter's occupation. It seems also that other adders tend to follow the trails left by the first adders to reach the hibernaculum, picking up the scent of those that have preceded them.

STRANGE BEDFELLOWS

In North America rattlesnakes habitually use the burrows of prairie dogs for hibernation, and these may go down ten to twelve feet below the surface. According to one report the average number of rattlesnakes massed together in one hibernaculum is 250, and in some instances several times this number may be found together. These snakes usually choose burrows on a southern slope, possibly because these are the places where they live during summer because they are warmer. Certainly, at such depths below the surface the temperatures in winter or summer are not likely to be very different whether the openings to them are on the northern or the southern slopes.

In the United States the winter quarters of snakes have been found to contain as many as nine different species. And in these hibernacula, also, there may be other animals such as toads and frogs, lizards and tortoises, bees, mice, rabbits, ground squirrels, raccoons, skunks, the burrowing owl, and also the prairie dogs that made the holes. You do not find all these in any one place but the list shows the kind of associations that can occur. It used to be said that this coming together of such diverse animals conferred a mutual benefit. Thus, it was supposed that the coldblooded animals benefited from the warmth from the warm-blooded animals, and that the latter gained protection from enemies through the presence of the snakes. The first part might be true but it is hard to believe that torpid snakes would be a menace to anybody. On the whole, therefore, it seems more likely that such mixed associations are wholly the result of chance.