Monday, December 28, 2015

Gender bender

Click pictures to embiggen

A lot of natural history comes from patience, from watching for events that only happen rarely, and then are over, in the blink of an eye. There are, of course, some very spectacular natural events that aren't exactly sneaking up on you. More like, they walk up to you and grab your attention by the collar. Collective behaviour, has definitely been one of these: a large number of organisms doing things all at once, and somehow, together. Crowds, swarms, flocks, schools, all sorts of collectives are being studied. Advances in theory, measurement and notions of emergent behaviour have all led up to this.

The emergence of red sided garter snakes (Thamnophis sirtalis parietalis)  in Narcisse, in Manitoba is most certainly a collective event. And the sheer number and coordination is impressive. How do thousands of these animals time their collective exit so exactly? How do they know that now is the time to crawl to the surface for their annual spring orgy? A few thousand snakes out at the same time is definitely the thing that brings so many people out to Narcisse, in the middle of the otherwise featureless flat Manitoba plains. But it isn't necessarily the most interesting thing going on here.

Well, what is going on here?

Before there were the flat lands, there was a first a huge ice sheet and then a huge lake in Manitoba, lake Agassiz. Agassiz was formed from the melt waters of that glacial ice sheet from the last ice-age. The ice-sheet was so large and heavy that Manitoba may still be undergoing isostatic rebound (i.e. the land-mass is still lifting back up after a weight was removed from it!). It is the reason the land is so flat and featureless, the ice sheet scraped everything down to nothing.

So where did all the water go?

Some of it remains in the remnant lakes, lake Manitoba and lake Winnipeg, that are nearly large enough to rival the great lakes. Some of it went underground and carved very deep caves into the bedrock. Some of these caves are so far below under the ground, that they are below the frost line. When winter comes to Manitoba and surface temperatures drop below -30 degrees Celsius, in these deep caves, its still above freezing. So at some point in the geological and evolutionary history of Manitoba, red-sided garter snakes started using these caves to hibernate. In the fall they go underground and they sleep off the winter there. Come spring, when the temperature is a bit more conducive to a happy life for  a cold-blooded animal, they all wake up nearly at once and out they come!

And when you see the numbers, its seems like it is all at once, a few thousand at once. This is already quite the feat: for a few thousand snakes to time their metabolism so they can all emerge within a day or so of each other. But it's not even the best of it, it gets even more interesting. It isn't all at once. Males tend to come out first, when it still isn't quite warm enough outside. And once they are out, they hang around, soaking up what little watery sun they can get; here they wait. They're waiting for the females.

When the females do wake up and make their way out of the cave, a whole bunch of males are waiting for them right at the entrance. The males will then proceed to chase every single female as she comes out of the cave. Several males chase every single female that comes within their ambit. They try their best to align their bodies up against the females, to wrap around her and to mate with her, all the while signalling to her with pheromones and by rub her with their chins. If enough of them try to do this at the same time, it leads to balls of snakes wound tightly around each other, often known as mating balls.

A mating ball
The balls form and dissolve constantly and not always because the deed is done. Mating balls do not always have a female core with a male outer layer.

Sometimes a mating ball is all male.

Some males will pretend to be female in order to attract courtship from other males. The snakes seem to recognize each others sex only through pheromones which they detect on each others skin. Snakes don't have external genitals and male hemipenises are held inside the body and everted from the cloaca only during mating. So all the males need is a chemical disguise. So gender-bending males produce female pheromones and this is enough to fool the other males into courting it and making mating balls around it.

Why on earth though? Well, for the one thing that most cold-blooded animals are short of: warmth. The core of a mating ball is warm and is constantly being warmed up by the bodies that pile up around it. Typically these 'transvestite 'males come out later than the other males, they might be in worse physical condition, and so cold and unable to court other females by themselves. By attracting a mating ball around themselves, they can raise their body temperature a few degrees. For a cold-blooded animal, that's a pretty big deal. When this is done, the gender bending males seem to turn the female pheromone off magically. With the other males no longer pursuing them, they get down to finding some mates of their own. In fact, the current theory is that the female pheromone has just the right degree of volatility: when the male's body temperature rises to where it needs it to be, the pheromone just evaporates off on it's own!

There can be a cost to this trickery, a cost some females pay and some males probably do as well. The mating ball is a pretty frantic affair. These congregations last only a few days, so the urge to get it on is very very strong. And sometimes the mating balls end up suffocating the female they are formed around. With the males there is the added fear of injury from forced copulations. So this strategy isn't without risk, but what a cool adaptation!?

In almost all systems, there's a lot of amazing detail beyond the obviously impressive spectacle and I hope we can keep looking beyond the obvious and finding it.

Have a great Christmas break, guys!

PS: A lot of the research done here was done in the labs of Richard Shine and Robert T Mason in case you want to read some of the primary literature.