Summer World_ A Season of Bounty - Bernd Heinrich [13]
Fig. 6. Male tree frog calling and advertising himself.
IF ANIMALS’ MAIN SUMMER PREOCCUPATION IS A RACE of reproduction, then the chorus of wood frogs on a night in early April is the starting gun. The frogs burst out from under the decaying leaves on the ground, overnight meet at a pool that has just melted, and start their convocation, which is rowdy, loud, and brief. One might assume that the males call to attract females specifically to themselves, but now, after getting to know them a little better, I think the story of what they do is more interesting. As we shall see, it can involve cannibalism, and more.
For about eight months the wood frogs crouch, with their heads down and their limbs tucked tightly to their sides, under the leaves that settle on the ground in the fall, and they and the leaves are then covered with snow. The frogs often freeze solid, and in that condition they don’t have a heartbeat, breathing, digestion, or activity of the brain cells. A reputable human pathologist, applying the same clinical standards to them as he would to one of us, would conclude that they are dead.
The wood frogs’ cue to revive and arise as from the dead, like that of the alder, hazel, and poplar flower buds, usually comes on the first warm (40°F) rainy day in April. By the millions freshly thawed frogs crawl out from under the cool damp leaves, and each of them starts hopping in a beeline to a little pool somewhere in the woods. They arrive at it from all directions. The whole population in any one area will travel mostly at night, and most of the frogs arrive on only one particular night. But adjacent pools are not necessarily on exactly the same schedule. Traffic to a pool on these nights can be intense; up to 4,000 frogs have been counted coming to a single pool in three hours (Bevan 1981).
All fall, winter, and spring the frogs had fasted and waited for their cues to arise and become active. During thaws in January or February, the aboveground temperatures on occasion rose to nearly 60°F, but the frogs still did not budge. Even after they do become active, they still do not feed for some time. First things first. For wood frogs that means sex and egg laying, which they accomplish simultaneously.
Fig. 7. A male wood frog in calling position on a pool.
I wrote in my diary entry for 14 April 1995 that I had arrived at about ten o’clock the night before at camp in Maine, driving in a drizzle and being impressed by the “traffic.” The main traffic on my trip from Vermont that night happened to be crossing the road, and it was mostly wood frogs. While coming through New Hampshire I saw them as pebble-like lumps in my headlights against the black, wet tarmac. At one point I was induced to stop my pickup truck, and I caught twenty of them, both males and females. Every one of them had been facing or hopping toward where I could hear a male chorus at full throttle. The road was also littered with the flattened dead—those who had previously attempted to join the gathering throng. From a distance a wood frog congregation sounds like a gaggle of peripatetic ducks. Presumably it is irresistible to female frogs, and I suspect it is to the males as well.
Fig. 8. Part of an aggregation of male wood frogs on a pool.
Although the wood frog choruses in different pools are often on their own schedules several days apart, group chorusing is not entirely due to similar arrival times of the participants in any one pool. Getting there on the same night makes it possible to sing together, but that alone does not ensure a chorus. Only the males call, and not at random with respect to one another. Already timed to arrive at the pool within about a day or two, the individuals further synchronize their calling with each other.
The wood frogs’ chorusing is, like that of most frogs, an energetically extreme exertion. In their case it’s done on a stomach that has been empty since fall. But that exertion is only a prelude for the wrestling