Snakes & Frogs

In the glorious weather this past long weekend in the Ottawa area, during an outdoors excursion I stopped to rest in the cool shade of a particular tree. The exact location is unimportant, but it is recognizable and a comfortable place to rest. It reminded me of a curious if common event that happened in that exact spot a couple of years ago.

The tree was large and alongside an overgrown ditch and a bare patch of packed dirt and stone. As I was sitting there I noticed a brown frog sitting in the dirt a meter or so from my feet. It blended well into the browns and greens of its surroundings. Probably the only reason I noticed it is that it hopped a short distance. Looking down at it I also saw a garter snake slowly slithering towards it.

At first I didn't see what was happening. The frog was very clumsily moving away from the snake and the snake was ineffectively trying to sneak up on the frog. The frog only hopped about 10 cm each time, then it would sit there quite placidly. When the snake approached again it would do the same thing, always hopping directly away from the snake's general direction of movement. The frog was doing the absolute minimum to keep the snake at a distance. Conversely, the snake was getting nowhere in its apparent quest to catch the frog.

Looking at it from an evolutionary perspective, I should not have been surprised at the mutual tactics of predator and prey. If frogs were too successful at eluding snakes, we might expect fewer garter snakes in the world; if snakes were too successful at catching frogs, it is frogs that would see their numbers diminished. However it comes about there is an unsteady equilibrium between the two at the point where they are very nearly equal in their opposing strategies. That is a very neat conclusion if oversimplified since there are other species involved that would fill the void in the predator or prey landscape if the equilibrium between frogs and snakes is upset.

To continue the story, by means of a series of small hops, the frog landed a short distance in front of my feet. In its attempt to come around the frog, the snake discovered me, in particular my legs and shoes. Changing direction to make use of this welcome obstacle, the snake came around the heels of my shoes. While it formed itself to the curves of my shoes I kept as still as I could. Very slowly it used the screening afforded to gradually get closer to the frog than it yet had managed. The frog, ignorant of the looming threat, sat quite still.

When the snake's head reached the farthest reach of its blind, the tip of my right shoe, it was about 10 cm from the frog. That's when it coiled up and lunged. Its tiny fangs got hold of one of the frog's hind legs and held on tight while the frog tried to escape. The snake held on quite easily. Its quest for lunch was won.

I continued to sit very still since I was very curious to see how it would make its meal. I saw two problems. One was that the frog was about twice as thick as the snake's mouth and body. The other was that the snake's only tool was its teeth. To get more of the frog into its maw it would have to do it in steps, needing to release its grip at every step.

I discovered much later that garter snakes are mildly venomous, just enough to keep the frog from having the reflex speed to escape in the brief time the snake needs to use its backward-curving teeth to move up the frog's body. Probably that's why it waits after the initial grab before continuing its feeding.

There's no point in describing the deadly process further other than to note that the snake was perfectly capable of gradually drawing the entire frog into itself, taking about 15 minutes in all. With its now distended body the snake slowly slithered back into the shaded ditch to digest its meal, while I continued on my way.

Despite its macabre nature it was fascinating to watch something that in our urban lifestyle is often only understood intellectually rather than observed. Nature can be cruel, but that is the story of life on Earth. This never-ending game between predator and prey is one of the fundamental drivers of process of evolution, weeding out of the relatively weak while leaving the strong to reproduce, that over eons, quite accidentally, produced us.