Summer World_ A Season of Bounty - Bernd Heinrich [16]
Why Do the Wood Frogs Call?
If there is one tenet that I knew beforehand to be firmly established in the scientific literature on frogs and toads, it is that males compete in attracting mates by making conspicuous vocal displays, and that females choose. Like the singing by birds and many insects, most famously crickets, katydids, and cicadas, the calling by frogs is an advertisement in which a male draws attention to himself or to some resources he holds that females need for reproduction. I know of no published exception to this explanation for male frogs’ mating calls. Biologists agree that calling sets up competition among males, allowing females to sort them out and choose. If several calling males are near each other, they can presumably be all the more easily compared, so that females can exercise even better choice. Indeed, aggregations of males where females come to be mated are considered the equivalent of mate marts where the males strut their stuff (if they can, or if they have enough to be allowed by the competition, or both). The females generally choose a small number of individuals out of the participants. Do wood frogs? My hunch was that they do not. And this time I consulted the literature as well as the frogs.
Three research papers on wood frogs’ mating aggregations appeared between 1980 and 1985. The first one, by Richard D. Howard (1980), then at the University of Michigan, established that the males outnumber the females at the breeding pools by about six to one. The skewed sex ratio apparently results from different mortality; fewer females live long enough to reproduce because it takes them one year longer than the males to become sexually mature. Each female was found to pair with only one male, and vice versa—a condition that is, I think ironically, described as “monogamy.” There is an intense competitive scramble among the males, but Howard was unable to demonstrate any choice by females. A nearly simultaneous study by Keith A. Bervan (1981) also reported no evidence for female choice. Bervan also found that any female would be clasped long before she reached any calling male that she might choose. The males don’t have the luxury to choose, either. Bervan noted that they attempt to clasp with each other, with any females, and even with already firmly clasped pairs. That is, males cast a wide net, try to capture a mate first, and discriminate later; those males on the receiving end of a “trial embrace” give a call that identifies their gender, and they are then immediately released. However, they are not so easily deterred from females who already have a male attached to them. If the female doesn’t make a quick enough getaway after she has a male, she will quickly accumulate a surplus of males that restrict her mobility. The other males try to grasp her around the abdomen and then move upward in an attempt to pry the other male off her back. They rarely succeed, but they can do so if the competing male is small enough and the female is so large that her male can’t reach all the way around her neck to secure a solid lock with his thumbs.
If these two studies weren’t enough to dispel the notion or expectation of female choice in wood frogs, another one did. This next study, by Richard D. Howard and Arnold G. Kluge of the University of Michigan (1985), emphatically concurs with the previous studies. These authors write, confidently: “Our results were unambiguous: the slightest movement by females resulted in their immediate amplexus [locking on of male] by the nearest male.” And females did not dislodge those potentially unwanted males; only other males did that, in athletic wrestling contests where size and strength mattered. Thus the most exhaustive and broadest study of “mate choice” yet (involving reams of data on survivorship, growth rates, numerical estimates of zygotes produced by females and sired by