Narcisse Snake dens, from Youtube.

As an ectotherm living in the chilly wilds of Canada, this snake hibernates through the winter, kept warm by hundreds of other slithering bodies. Thousands of snakes will pile together in one hibernaculum (a den) to spend an eight month long winter (Parker and Mason 2009). Some young snakes are actually crushed under the weight of this cuddle puddle (Shine and LeMaster 2001). Much of the research around hibernation has involved how it prepares the snakes for their subsequent two to four weeks of mating.

What happens during hibernation?

For the garter snakes, hibernation offers a chance for hormones to run rampant. For females, this means the creation of hormones to increase attractiveness upon waking. Parker and Mason (2009) demonstrated that females only begin synthesizing one of their most powerful pheromones after a cold snap. Successful mating for the next spring, then, requires a spell of low temperature. For both males and females, hibernation seems to be the time for peak sex hormone activity. Researchers once touted these garter snakes as a prime example of dissociated mating, whereby sex hormones such as testosterone (males) and estradiol (females) seemed absent or have little effect during mating season. Lutterschmidt (2012) argues that this may have been a myopic line of research. Looking at sex steroid during winter reveals enticing correlations between hormone levels and mating activity.

What wakes them up? A look at hormonal cycles and brain surgeries.

Every spring, the snakes rise from their dens in a fairly coordinated manner, the males leave the den first and the females follow shortly after. What initiates this is, however, still unknown. One group hypothesized that rising levels of water might prompt the snakes to awaken. Eight out of their fifteen snakes continued hibernating through rising levels of water, even after becoming fully submerged—suggesting water levels have little to do in their waking cycles (Todd 2009). Temperature has been hypothysized to play a role (Todd 2009); the warming of the gound in the spring-time could simply warm the bodies of the snakes, iniating wakefulness. While an elegant hypothesis, Lutterschmidt 2006 suggests otherwise. She looked at the actual ground and body temperature of the hibernating and waking snakes, but was not able to find a correlation. Rather, her research argue that it is actually cold snaps during dormancy that reset biological clocks, preparing the snakes for imminent awakening.

As the pineal gland is known to process information about both temperature, light, and melatonin (a hormone associated with maintenance of 24-hour sleep-wake cycles), it seems well suited to play a mediating role in the hibernation patterns of the garter snake (Mendonça 1996). Initial attempts to support this hypothesis failed, pinealectomy of the male garter snakes upon hibernaculum emergence left nearly half of the males unaffected (though Spring pinealectomy was able to induce mating in previously non-courting males). However, pinealectomy in males during the Fall prior to hibernation produced continually high levels of melatonin, and an apparent disinterest in courting. So while the pineal gland is known to play some role in the waking and subsequent mating habits of at least the male snakes, it probably acts through a complicated signalling network during dormancy (Lutterschmidt and Mason 2009).

Geographic Variation

Cold temperatures and complex hormonal cycles--probably involving melatonin and the pineal gland--are thought to play some role in the hibernation cycles of these garter snakes. Really, not much is understood. In an attempt to rectify this, Lutterschmidt and Mason (2009) opted for a more phylogenetic approach, looking at populations of garter snakes from three different environmental regions, Oregon, Florida, and Manitoba, Canada. Indeed, their results suggest that these hibernation and mating cycles of the different populations are regimentally different between the populations of snakes. Further research will be needed before causal genetic variations or environmental signals during key portions of development can be teased apart. This research will have significance not only in questions about how annual cycles are maintained, but also in how organisms will be able to respond to perturbations within the environment (Lutterschmidt and Mason 2009). How much plasticity is there within the regulation of these cycles, and what will be the consequences should they prove rigid?

Homeward bound

During the summer and fall feeding seasons, Thamnnphis sirtalis travels far away from its hibernaculum, yet they return to the same den at the onset of winter (Southwood 2010). For conservationists looking to rear, repopulate, and relocate populations of these garters, this has proven a critical question (Burke 1991; Germano and Bishop 2009; Lisa 2002; MacMillan 1995). What are the factors necessary to get a snake born in a lab to attempt to winter in an unknown hibernaculum? Young snakes have been noted to cluster around their mother and follow her to a den (Lawson 1994). Lawson notes that these females may be able to find their way home as gravid females tended to lay pheromone trails away from their den. The young can follow these mothers to the den, but Lawson holds concerns about the significance of his results. The non-gravid females, and all of the males, tend towards two way of finding home. For general orientation, they tend to orient themselves with the trails led by the leading mothers. From there, it has been suggested that they pay attention to solar cues, specifically polarized light (Lawson and Secoy 1991; Lawson 1994 LeMaster, Moore, and Mason 2001). In the closely related, non-migratory, species Thamnophis ordinoids, none of this has shown to be true (Lawson 1994).