Friends

 

I was returning home from work the other day when I heard something moving in a small pile of rocks near the side of my building. Looking over, I saw two eastern garter snakes (Thamnophis sirtalis sirtalis) retreating into a hole between the rocks, along with two others sunning themselves on top of the pile. When I moved to get a closer look, these two snakes also retreated between the rocks.

That these snakes would be sunning themselves so close together, and then retreating into the same hiding place when disturbed, may seem a bit odd. After all, we don't usually think of snakes as very sociable creatures and there has been very little research into the social dynamics of snakes due to this very prejudice. All that has started to change over the past few decades, however, with some truly interesting results. 

Garter snakes have long been known to gather together in large numbers during at least two times of the year. First, during the winter, garter snakes living in colder regions will often gather together in communal hibernation* spots that may also host members of other species, such as rattlesnakes and copperheads. Hibernating together like this, snakes are able to better retain heat by reducing their total surface area-to-volume ratio and large dens will often support hundreds or even thousands of animals!

Upon surfacing in the early spring, sexually mature male garter snakes begin to search for a mate by following the pheromone trails of female snakes. Sometimes, if multiple males converge on a female at the same time, they will form a "breeding ball" - a riving mass of up to a dozen or more male snakes all competing to mate with the female in the middle. Larger males or males that were able to gather more resources the previous year are able to engage in this competition for a longer amount of time and so are generally able to achieve more copulations. The female, for her part, has pockets of folded tissue in her cloaca that allow her to store viable sperm in her body and decide when to let it fertilize her eggs. She may even be able to control specifically which of several males she mates with will be the father of her offspring. 

In addition to these two, well understood periods of social interaction among garter snakes, I found some sources claiming that individuals may gather together at night in order to retain body heat in a similar fashion to what they would do while hibernating over the winter. I also saw references to pregnant female snakes gathering in groups during the summer, but one paper used as a source for this claim was later retracted for reasons I don't know, and I wasn't able to gain access to a book that also apparently makes this claim. 

Nonetheless, more recent studies have at least demonstrated that garter snakes can be quite sociable in captive environments outside of hibernation and mating contexts. In one study, Skinner and Miller (2020) placed 40 eastern garter snakes together in enclosures containing ten snakes and four shelters each. They observed the snakes' social interactions for eight days and found that juveniles in particular seemed to prefer spending time in shelters with more snakes in them and that males were less likely to explore or wonder around the enclosure when they were in groups. The study also suggested that when they were gathered together in groups, individual snakes had "friends" who they preferred to be in proximity to. This was demonstrated by shuffling the snakes around twice a day and observing that they would consistently return to their preferred groupings over the next several hours. 

Why do garter snakes seem to be so sociable? The authors of the study draw a distinction between two kinds of social aggregation in animals - that which occurs as a result of different individuals being attracted to the same resource, and that which occurs when different individuals are attracted to each other. Since juvenile garter snakes appear to prefer shelters with more snakes in them and have preferences about who they share those shelters with, it would seem that the latter situation is occurring here. One possible explanation for this behavior is that juvenile snakes are more susceptible to predation and so seek out groups as a source of protection. Another possibility is that garter snakes have developed a general tendency to seek out company in response to its advantages in specific situations where temperatures drop and retaining extra heat is necessary. The snakes that I observed appeared to be adult females, so this latter explanation, or a similar one related to the claims about gatherings of pregnant females I described above, are probably most relevant to their particular aggregate. 

Whatever the reason for snake sociality, it probably has to be balanced with competition for resources that must occur in a species that cannot share food. Another, earlier study looking at sociality in Butler's garter snakes (Thamnophis butleri) found that they preferred to spend time around other snakes that were on different diets, suggesting a sensitivity to both whatever advantages are connected to aggregation and to threats of competition (Lyman-Henley and Burghardt 1994).

If snake social life is indeed more complicated than we once thought, then it could have some important implications for conservation efforts. Scientists seeking to restore threatened snake species to their former habitats report difficulties with snakes becoming disoriented and wondering far from their release site into potentially unsafe areas (Butler et al 2005; Lee and Park 2011; Nash and Griffiths 2018). The authors of the 2020 paper suggest that if other species have similar social patterns to the garter snakes observed in their study, then it is possible that releasing them in groups could help reduce wondering and associated mortality. Only time and more research will tell, but in the meantime, I will be enjoying the idea of four snake friends rooming together in their basement apartment just outside my building.

*I use the word "hibernation" here, but technically speaking, snakes don't do that because they are awake and will even leave their chosen den site on warmer days. Hibernating animals, on the other hand, are in a deep sleep the whole winter. The sluggish state that snakes do enter is called brumation.

Sources:

Butler, H., Malone, B., & Clemann, N. (2005). The effects of translocation on the spatial ecology of tiger snakes (Notechis scutatus) in a suburban landscape. Wildlife Research, 32(2), 165-171.

Lee, J. H., & Park, D. (2011). Spatial ecology of translocated and resident Amur ratsnakes (Elaphe schrenckii) in two mountain valleys of South Korea. Asian Herpetol. Res, 2(4), 223-229.

Lyman-Henley, L. P., & Burghardt, G. M. (1994). Opposites attract: Effects of social and dietary experience on snake aggregation behaviour. Animal behaviour.

Nash, D. J., & Griffiths, R. A. (2018). Ranging behaviour of adders (Vipera berus) translocated from a development site. Herpetological Journal, 28(4), 155-159.

Skinner, M., & Miller, N. (2020). Aggregation and social interaction in garter snakes (Thamnophis sirtalis sirtalis). Behavioral Ecology and Sociobiology, 74, 1-13.