Summer World_ A Season of Bounty - Bernd Heinrich [17]
It may seem that there was little left to learn about the mating habits of wood frogs, given the solid empirical results. However, I still wondered: If females don’t choose, then why do males call at all? What could they call for?
To answer this, or any other relevant biological question, it helps to look first at the context in the field, in the animals’ natural environment. Wood frogs are unlike any of the other species of local frogs in that wood frog males are not spread out. By contrast, male tree frogs are separated from each other by trees; spring peepers, green frogs, bullfrogs, and other pond breeders are usually scattered along a shoreline or over an expanse of marsh where they can be hidden in little niches under leaves and grass that allow them to control space around themselves. Calling wood frog males are easily visible, massed on open water near the center of their little pool.
Several days after I had seen the wood frogs crossing the road at night while I was driving to Maine, I sat down near the edge of a pool near my camp. This pool, which is no larger than the floor space of an average room, contained at least fifty highly visible male frogs. They were spread out, as is typical, about a foot apart all over the surface of the pool, with only the tops of their heads out of the water while their hind legs trailed behind. They floated in place and occasionally paddled with alternate strokes of their hind legs. They approached any other frog they came near. I saw only one female jump into the pool. At least I assumed it was a female, because only this one was pounced on almost instantly and not released. In seconds, three males were on top of her, and one of them got a tight neck lock. It was, as always in wood frogs, a classic competitive scramble with the males in an intense contest for the females, who are literally up for grabs.
In one ball of ten squirming males that I untangled I found a dead female at the center. I threw her back into the pool, and she was again mobbed and embraced in the same way. I suspect that there were so many males on this dead female because she could not escape. But males’ preference could also be involved, because males “should” prefer more rotund females: such females would make the males’ sometimes prolonged wrestling efforts more worthwhile, since they would get more eggs with one shot (so to speak). My dead female happened to be rotund indeed—she was bloated with gas, though, instead of eggs. In any case, she could not have chosen any of these males; they all chose her. Whatever the calling behavior of one or the other of these amorous males, it had made no difference to this particular long-dead female. However, an anthropomorphism readily suggests itself to describe what might be going on. Is the frogs’ chorus a collective effort of the males to get females to come into their pool, like guys at a Saturday night college fraternity party playing loud music to attract the most babes to go to their house as opposed to a neighboring one?
At first I watched the frogs from a distance of about twenty feet, so as not to disturb their activities. Their calling was, as usual, in concert; some of the time the whole crowd seemed to be sounding off, and then there were periods of silence, as though the band played all together for most effect, then took an occasional break before resuming with renewed vigor. After a while, one or two of the frogs started up again, the rest then joined in, and their voices blended in. (By contrast, with many other frogs and toads one can easily pick out individuals by differences in pitch.) I had brought along a tape player to record the chorus. I reversed the tape and played back their calls during a silent period after I had disturbed them and they had dived to the bottom. Almost instantly after I turned on the sound,