Summer World_ A Season of Bounty - Bernd Heinrich [18]
Like most summer activities, the frogs’ vocal signaling requires an impressive expenditure of energy (Taigen and Wells 1985) and therefore presumably has an advantage. However, it is not immediately obvious why the calling of an individual wood frog in a chorus could aid him snag a female that has jumped into the pool. If not, then why call at all, as long as other nearby males are doing all the work and bringing females in by their calling? Instead of having a mating advantage, calling would seem to be disadvantageous, because the noncallers, who save their energy, should have an advantage in the inevitable tussling contests that ensue among the males seconds after a female jumps into their crowd. In the extensive literature on the mating game there are indeed innumerable examples of “satellite” males (those that wait to intercept females coming to the displaying males they are attracted to), who adopt the energetically more economical mating strategy. So, why don’t they all stay silent? And after one frog called, then all the satellites should especially be silent. Instead, all the neighbors joined in. It didn’t seem to make sense from the perspective of satellite males. But I knew it makes sense—somehow.
Although the frogs’ synchronous chorusing in crowds was puzzling to me, someone thoroughly steeped in frog literature would not have been confused. One reviewer commented about an article I had submitted that was summarily rejected: “Why of course they would chime in. Frogs do that. When your competitor is calling you had better immediately call as well to remain in the picture as far as a potential mate is concerned.” This idea makes intuitive sense, of course, but only in terms of females’ choice. I had regularly observed exactly that—in other frogs, those that are hidden and widely spaced, such as gray tree frogs, spring peepers, green frogs, and maybe bullfrogs (although those in our pond also aggregated to produce deafening pulses of sound where hundreds joined in, separated by moments of absolute silence). Why should male wood frogs chime in with the chorus of voices when there is no way for an attentive female to choose specific preferred individuals out of the chorus line? I didn’t know. But I didn’t think they were performing like the participants in a summer music festival. Or were they?
In a scenario of an intense, proximate, competitive male-male scramble for females, the idea that frogs could be “cooperating” for anything is not intuitive. But as an example of possible ultimate cooperation at the same time, suppose a frog’s voice reaches one mile. Then the area of a circle with a radius of 1 mile is pi (about 3.14) times 1 mile squared, or about 3.14 square miles. Suppose further that there is one female in this area, and that the calling male frog could on average attract only one female. Then if ten frogs start calling synchronously from that same spot, and their combined volume rather than the individuals’ volume as such counts, the combined vocal volume (pi × 10 squared) is now increased tenfold and would sweep about 314 square miles of area, reaching an audience of 100 females. That is, the ten frogs who call synchronously to amplify the attractant could expect to increase the number of females attracted up to a hundredfold, while their interindividual competition increased only tenfold. Per individual, then, each achieves a tenfold advantage by joining in the communal chorus.
How to Describe the Young Frogs’ Summer Race?
Regardless of ultimate cooperation by males in attracting mates, northern wood frogs breeding in woodland puddles live on the edge of survival and proximally compete for their lives. In their ephemeral pools, wood frogs have only about two months of summer to complete their larval development. Often they run out of time. In 1995, twenty-one