CHS Blog

Night on Earth: An Extraordinary Nesting Aggregation of Eastern Hognose Snakes

March 24, 2020
Leslie Anthony

The 2017 CHS conference in Brandon, Manitoba, found us in Western Hog-nosed Snake territory, a fitting venue in which to present an interesting past experience with Eastern Hog-nosed Snakes. It was purely observational, but intriguing and question-begging—one of those field encounters that makes you go Hmmm

Because of the Narcisse snake dens, no Canadian knows better than a Manitoban that snakes occasionally aggregate. In nature, animal aggregations occur because of a limitation or efficiency around an important life-history stage or resource. Thus, aggregations of snakes occur in relation to one or more of: hibernation; mating (often associated with the previous); feeding (uncommon but observed); parturition (small numbers of females); nesting.

With respect to the last, communal nesting is common in snakes, can involve multiple species, and is tied to site quality and/or availability. This in turn can become philopatric/genealogical, with snakes returning across generations to the same site to nest. As a result, specific areas may see more usage. But while individual snakes may lay eggs at the same site, are there cases of snakes actually nesting synchronously en masse?

In 2009 I spent a week with a Masters student conducting a radio-telemetry study of Eastern Hog-nosed Snakes in Ontario. The study aimed to understand home-range usage and quantify road crossings. On an evening run I came across a large hog-nosed snake nesting at a sandy site near where we’d been tracking snakes. Though not particularly surprising given that it was good nesting habitat, the encounter was nevertheless fortuitous, so I stopped to watch.

A large snake laboriously moves sand one bend at a time as it digs. Click the picture for a video.

As I photographed the nesting snake, I noticed another a short distance away… and then another… and yet another… all heading in my direction. I realized I was surrounded by female hog-nosed snakes. Were they all intending to nest in this same small area? Apparently so, as I now saw similar scenes in several discreet areas of the tract. I returned the next night—under identical weather conditions—to the same astounding scene. In this small window of time, over the course of two nights, I observed ~50 adult Eastern Hog-nosed Snakes nesting in an area of approximately 50 m x 20 m. Was this a mass nesting event?

While one snake digs into a nest, another waits its turn a few metres away

If so, the first question would be Why the synchronicity? Considerations include: phenology and whether the timing was non-random; weather—hot, with recent rain and high humidity—as a potential trigger; philopatry, as in a returning population with a genealogical component; site characteristics and whether low availability of high-quality nesting sites in the area forced “aggregation”; accrual of a fitness benefit from communal nesting/egg-clustering.

Left: Sandface. You lookin’ at me? Right: A large female says “Enough of the photographs already!”

Whatever was going on was of phenomenological interest. It was clear, for instance, there was high site-specificity, as ascertained by old eggshells excavated by nesting snakes. Not only were snakes nesting where previous nests existed, but coveting the same specific sites; some of these were in such heavy rotation as to bear excavation tracks from up to four different snakes, with others awaiting their “turn” on the periphery, as no two snakes were ever seen conjointly digging at shared nest sites.

A second snake digs its way into a shared nesting site

Digging was tough work that took many hours. A snake would begin excavating with its nose and then, once her head was embedded, pull sand out with a body bend; as the animal got deeper it would move soil with multiple body bends in an accordion-like manner. I first observed nesting snakes at 7:30 p.m. and many had been there a while. A few were still at it the next morning, and seemed unconcerned about being approached, even when finished. This is odd in terms of vulnerability to predation.

Left: When a snake gets to this point in a dig, it is usually about to replace the front end with the back end and start laying. Right: A nesting site awaits the next user

As there were quite a few nests, another researcher set out to study them. Digging into one to place iButtons or remove eggs for incubation usually revealed spent eggshells, upping the ante on site-specificity. It was clear that these were key nesting micro-sites chosen for specific reason, returned to—and bloody crowded.

Eggs tend to be deposited in a linear fashion within an excavated nest

In a two-year study,researchers compared habitat selection and temperature profiles of 21 nests and 21 adjacent random sites. They found that snakes selected open, grassy sites that had lower amounts of vegetation than random sites, and were significantly warmer. Of 215 eggs from eight sites incubated in split-clutch design at mean nest (24°C) and random (22°C) site temperatures, eggs incubated at 24°C hatched earlier and had fewer anomalies; hatchlings were also larger and swam faster. The obvious conclusion was that snakes selected warmer nesting sites likely to improve hatchling fitness. This spoke to site selection but not synchronicity of egg-laying—or did it? In freshwater turtles, egg-clustering appears to increase fitness, albeit those eggs are laid at variable times. The question here is whether there is advantage to simultaneous laying.

A hatchling Eastern Hog-nosed Snake

Satellite photos reveal why this is a preferred site—an open sand barren in the middle of a heavily forested area. Yet it’s not a completely natural sand barren; at the time it was in use as an ATV and dirt bike track (signage stating the area was closed to motorized vehicles was regularly breached). Use of human-subsidized reproductive habitat (e.g., road and rail banks) is common in reptiles, and all areas surrounding this site that possessed similar qualities were heavily cultivated farmland. Could it also be that because the site occasionally hosted people on loud machines, predators spent less time there? On the other hand, vehicles utilizing the area during nesting season posed a clear danger to snakes and eggs; after nesting, a danger to eggs; and after that, to hatchlings.

ATV and dirt bike tracks near a nesting snake, suggesting the vulnerability of such sites

Another question was whether this was simply a localized phenomenon or more common to hog-nosed snakes in general. And a final wild card: in the second year of the study, a few males showed up during the nesting window to intercept females.

Although this putative mass-nesting event is biologically intriguing, these observations also highlight three key conservation concerns: potential impacts to nesting snakes; impacts to nests and hatchlings; and overall population impacts. If synchronized mass nesting is occurring, then its localization in time and space suggests clear choices in management strategies, such as permanent versus seasonal closure, and serious enforcement versus occasional monitoring. Regardless of whether the aggregation is real or coincident, it’s a fascinating aspect of the snake’s natural history that makes the site of clear importance to this threatened species in Ontario.

Nesting is a messy but satisfying business