Why Do Garter Snakes Form Mating Balls? The Narcisse Dens

Why there are so many males at once

The biggest mating balls happen at large communal dens in the north, where the common garter snake spends winter underground. Several conditions come together there:

• Everyone is in one place. Thousands of snakes overwinter in the same den, so in spring the whole local population is packed into a small area.
• Males come out first. Males leave the den earlier and stay around the entrance for days or weeks, waiting for females.
• Females leave quickly. Each female emerges, mates, often only once, and soon departs for summer feeding areas.
• The season is short. In places like Manitoba the snakes are active for only about four months, so there is no time to wait.

The result is a hugely male-biased crowd at the den. At any moment there may be many males for every receptive female, so each new female becomes the center of intense competition.

The Narcisse snake dens of Manitoba

The most famous place to see mating balls is the Narcisse Wildlife Management Area in the Interlake region of Manitoba, Canada, north of Winnipeg. Here red-sided garter snakes (Thamnophis sirtalis parietalis) spend about eight months of the year in sinkholes and crevices in limestone bedrock, below the reach of the frost. Such winter shelters are called hibernacula, and how snakes survive inside them is explained in Do snakes hibernate?.

Narcisse holds the largest known gathering of snakes anywhere in the world. Counts vary from year to year, but tens of thousands of snakes use the dens, and in recent years the population has often been estimated at about 75,000 or more. In late April and May, as the ground warms, the snakes surface in waves, and the den floors and surrounding rocks become a moving carpet of snakes and mating balls.

The population has had setbacks. An early, severe cold snap in autumn 1999 is thought to have killed a large share of the snakes before they reached the dens, and thousands were also being killed each year on a nearby highway during their migrations. Snow fences now steer snakes into small tunnels under the road, road deaths have dropped sharply, and the dens have recovered.

How males compete inside a mating ball

A mating ball looks chaotic, but each male is doing the same thing: trying to get into the right position. Male garter snakes court by:

• Following scent. Males detect a female’s skin pheromones with their flicking tongues, see why snakes flick their tongues, and can tell her from a male by smell alone.
• Chin rubbing. A male presses his chin along the female’s back.
• Aligning bodies. He lines his body and tail up alongside hers.
• Rippling. He produces waves of muscle contractions along his body, which appear to stimulate the female.

Only one male mates with her at a time. Studies at the Manitoba dens found that larger, longer males and those that align quickly tend to be more successful, but there is a lot of luck involved in such a crowd.

Females are not all equally attractive. Larger females, which produce more young, release stronger attractive pheromones and draw bigger mating balls.

Males that smell like females

One of the strangest discoveries at the Manitoba dens is that some males briefly smell like females. In 1985, Robert Mason and David Crews reported in the journal Nature that a minority of males produced female-like skin pheromones and attracted courtship from other males. They were nicknamed “she-males.”

Biologists have proposed several possible advantages:

• Distracting rivals. A male that draws other males away from a real female may have a better chance with her.
• Stealing warmth. Newly emerged snakes are cold and sluggish. A male covered by courting males may warm up faster, a form of heat theft called kleptothermy.
• Protection. Being buried in a ball of other snakes may also offer some protection from predators such as crows while the male is still cold.

Later work suggested that female-like scent is common in newly emerged males for a short time and fades as they warm up and become active, so it may be partly a side effect of the transition out of winter dormancy.

What happens after mating

After a successful mating, the male leaves a gelatinous mating plug in the female’s cloaca. The plug, along with chemicals in it, makes her unattractive to other males for a while, so she can leave the den without being mobbed again.

The female then travels, sometimes several kilometers, to marshes and meadows where she feeds through the summer. Garter snakes give birth to live young rather than laying eggs, and her litter is born in late summer, typically 10–40 babies. Females can also store sperm, so some mating happens in autumn and the sperm is used the next spring.

Mating balls in other snakes

Mating balls are not unique to garter snakes. They are reported in other species where many males converge on one female, for example green anacondas in South America. However, nowhere else do they occur on the scale of the northern garter snake dens, which is why the red-sided garter snakes of Narcisse have become a classic subject of research on snake behavior and a popular wildlife spectacle each spring.