Summer World_ A Season of Bounty - Bernd Heinrich [19]
After getting the earliest possible start following the spring melt, the next step in the wood frog’s race with time is to get the tadpoles developed into frogs and hence out of the pool before it dries up. This step mainly involves the development of the larvae; they must grow fast, or be able to act like adults (hop on the ground; breathe air), or both. Two main ingredients make the difference in this race: food availability and temperature. Any increase in body temperature above the near-freezing water temperature that the tadpoles find themselves in permits an increase in growth rate, and they must simultaneously have access to sufficient amounts of the right kinds of food, particularly protein.
Wood frog tadpoles are primarily vegetarian. They feed on algae. But algae are not their only food. When I kept tadpoles indoors in an aquarium, adding nothing but a few decaying leaves picked out of woodland pools, they fed on these leaves and skeletonized them so that only a fine latticework of leaf veins remained. Larvae fed on dead leaves were still alive at the end of February, when I found them under the ice of a rain barrel where I had dumped eggs the previous April. Apparently an insufficient (low-protein) diet can extend the tadpoles’ life span from several weeks to at least ten months. Conversely, when I fed them some “fish flakes,” a high-protein commercial aquarium diet, they had a growth spurt.
Freshly metamorphosed froglets look and act like adults. However, they weigh as little as 0.007 ounce (0.2 gram), about one-hundredth of the adults’ mass. The specific size of the tadpole where evolution has set the developmental switch to produce the adult form is flexible, but in wood frogs it has presumably been strongly influenced by time. The smaller the tadpole at the set point, the shorter the time to get there. Still, in many years the larvae run out of time and don’t make it to the froglet stage. When I checked on one of my biggest study pools on 11 June 1999, I found the center a moist black goo of dead and dying wood frog tadpoles. It was surrounded by the tracks of raccoons and great blue herons. Carrion beetles and maggots were mopping up the edges. Similar scenes were repeated at other wood frog breeding pools, but as I found out later, this did not mean the wood frogs’ breeding at those pools was necessarily a failure that year.
I scooped up a few spoonfuls of the dead and dying wood frog tadpoles and dumped them into my aquarium with their living relatives. The larvae immediately consumed the dead and weakened of their own kind, and literally overnight they grew hind legs. The next day they added front legs as well, and the following day their tails shrank. They still swam like tadpoles when in the water, but they hopped like frogs the minute they were on land. In three days the change to a cannibalistic diet had caused them to become frogs, whereas the other larvae of the same batch who were maintained on decaying leaves were still tadpoles seven or eight months later. I suspect therefore that as the pools dry up in the summer, feeding frenzies on their own kind impel some froglets to life on land.
Initially a pool where no more than 100 females have deposited their clutches of 300 to 1,000 eggs each may contain at least 50,000 tadpoles. Collectively they represent the nutrients that have been concentrated from highly dispersed and often microscopic food particles by the collective grazing of the tadpoles over the span of a month or two. They become a nutrient store that could later give their brethren a boost to lift them out of the pool as the water disappears. If the same scenario is repeated regularly through evolutionary time, then cannibalism could be an