Though it surely ranks among the world’s oddest-themed picnicking sites, the reaction of many a visitor to the park’s raison d’être makes the notion of eating first and seeing the sights later odder still. For beyond these convivial tables, a post-prandial stroll along a three-kilometre loop through grassy alvars and stands of dwarf aspen delivers diners to a sight that — fascinating though it may be — is for some disconcerting: the Narcisse Snake Dens, where a fluctuating population of around 75,000-150,000 red-sided gartersnakes overwinters in limestone sinkholes and, after emerging in spring, spends several weeks vigorously mating under a Prairie sun. The planet’s largest aggregation of snakes, it is observed by some 35,000 people each year, close enough to have the amorous animals slither over their shoes or be picked up by the more intrepid.

Depending on who you are, this de facto Wonder of the Natural World is a scientist’s dream, bucket-list Instagram brag or, for those addled by the thought of even a single serpent, serious trigger-fodder. Yet regardless of where one fits on the spectrum, all leave Narcisse with two important gifts: a first-hand, David Attenborough-free recalibration of nature’s “wow” factor, and newfound respect for these mostly misunderstood creatures.

The red-sided gartersnake is one of a dozen recognized subspecies of the common gartersnake, a continental superspecies ranging from the Atlantic to the Pacific, Mexico to the sub-Arctic. Red-sided gartersnake males average 40-60 centimetres and females, 60-80 centimetres (though they can reach 110 centimetres). The den is surrounded by snake-friendly chain-link fencing to keep overly curious humans from disturbing the proceedings — or falling in. A wooden viewing platform stretches out over the pit, where we contemplate the fortuitous reasons for these annual gatherings.

That such a profusion of snakes occurs this far north may seem strange at first blush, but makes sense in the details. While summers here are warm enough to accommodate a number of reptile and amphibian species, winters are long and harsh, with surface temperatures far south of -30 C for extensive periods and frost moving deep into the ground. For ectotherms like snakes, whose body temperature remains close to air temperature, decent hibernaculae are at a premium — they must be below the frostline but above the water table, yet humid enough that snakes won’t dry out over the seven to eight months they spend in them. The Narcisse area features classic karst topography, where surface water and rain have dissolved calcium carbonate along weaknesses in the limestone bedrock, creating deep fissures and underground chambers. These chambers often collapse in on themselves as sinkholes, allowing snakes access to an extensive labyrinth several metres below the surface.

Mason — whose name is now synonymous with this giant, self-contained outdoor laboratory and who is fond of mentioning he met his wife “in a snake pit” when she joined his 1997 Narcisse fieldwork as part of a citizen-science ecotourism group — continues his voluminous research with a stable of graduate students to this day. Among their many startling discoveries: not only can sex be differentiated by pheromones, but so can breeding versus non-breeding body condition; males prefer larger females, whose pheromones are more attractive; males can differentiate female pheromones from different dens and prefer those from their own; some males produce female pheromones to fool other males and gain warmth and protection within mating balls (in experiments, female-performing males warmed to 28 C would cease impersonating females, while males whose body temperature remained at 10 C kept up the pheromone drag show). All this sensory showmanship comes courtesy of snakes’ ability to detect sexual, prey and same-species scents at mere parts per million — the reason for the constant flicking of their tongue, which delivers airborne molecular cues to the sensitive vomeronasal organ in the roof of their mouth that detects scents. 

But while scientists inform much of what we know about the biology and behavioural ecology of these snakes, there’s plenty of nature on display for visitors to get their own sense of how the snakes fit into the larger ecosystem. “This trail is usually littered with de-livered snakes,” remarks Mooi as we stroll toward the second den, referencing the favoured snack crows extract from easily caught serpents. “But then, we’re early.” 

Recalling the fastidious re-creation of a snake den in the Manitoba Museum that Mooi showed me yesterday, replete with a hungry crow lurking in an aspen, begs a question: what is the value of an actual visit? “The dens provide huge opportunity for learning about all sorts of aspects of ecology,” he says. “In a day and age where it’s harder and harder to find natural experiences because we’ve surrounded ourselves with concrete, something relatively close that you can bring a school group to opens up whole new worlds. Kids can also learn how this is a professional opportunity — that they could grow up to study these kinds of things for a living.” 

Dealing with the dens was supposed to be 10 per cent of Roberts’ time but most years took up a lot more. “For better or worse, I also became the regional snake guy,” he says. “If anyone had one in their basement I’d get the call. Then came the tunnels and fences.” Around the new millennium, the dens’ ecological importance was becoming better-known and people were taking more interest in the environment and conservation. It all fed into a desire to deal with a perennially upsetting problem: the mortality of snakes crossing Highway 17—particularly as they returned to dens in the fall when, according to a report Roberts wrote in the late ’90s, some 30,000 were killed each year. In a joint project between Manitoba Hydro, Manitoba Conservation, Manitoba Highways and local organizations, tunnels bored beneath the highway roughly every 100 metres over a kilometre-and-a-half stretch were fitted with pipe passages and low drift-fences to encourage snakes into them. Since snakes don’t like cold, dark tunnels, it could have been an expensive but ineffective gesture if it weren’t for Bob Mason. In an applied-science conservation collaboration, Mason put his Narcisse research to work by painting the tunnels with synthetic red-sided gartersnake pheromone to entice the animals to travel through. The snakes responded; fortuitously, they also left more pheromone along the way, compounding an effectiveness that saw the road toll drop to 1,500 per year. 

When the system was first installed, some 25,000-30,000 snakes used the tunnels during fall migration. “I’d put a box trap at a tunnel in the morning and come back in the afternoon and there’d be 300-400 snakes in [it], so we were having significant success in reducing mortality,” says Roberts.

Protecting the snakes remains an uphill battle. Snowmobiles and snowplows damage the fence each winter, requiring extensive annual repair. Other initiatives have waned. At one time, “snake-crossing” highway warnings were erected and taken down annually. “Not many people slowed,” recalls Roberts. “And most locals didn’t care, so the highway department flat-out refused to lower the 100 kilometre-per-hour speed limit. That was frustrating.”

The embattled fence remains an issue — every hole and downed section a potential death for hundreds of snakes. As any good road ecologist would advise, Roberts would like to see a more permanent tunnel-and-fence system. “Before I retired, I provided a highways biologist with a drawing of what they could do based on what I’d seen elsewhere in North America. If they ever redo that highway — not a priority because few people live out there — maybe that could happen, but it’s unlikely.”

Roberts believes the Narcisse snake dens should have been an ecological reserve in which no one was allowed, but a decision was made early on that it would be a place for public interpretation. “That’s been successful, but the downside is the population has been impacted by the cumulative effect of everything going on there. We’ve never really done enough.”