Summer World_ A Season of Bounty - Bernd Heinrich [15]
It may seem odd that although the frogs don’t care much about size, and perhaps other physical attributes of a pool or pond in which they spawn, they nevertheless are very particular about where in the pool they spawn. The females make an effort to deposit their eggs at the same place where others have already done so. The males are also nearby, gathered for their chorus. But you need a large pond to notice this. Three miles down the road from our house is a pond about 660 feet long and 165 feet wide. It has ample space for frogs to spread out, yet every spring the wood frog chorus is restricted to an area of several square yards on one end, and almost all the many females drop their eggs there, in one big pile. Why?
Although an individual egg clump is only walnut-size when deposited, it swells to the size of a baseball or softball within hours because the gelatin surrounding each egg absorbs water. When hundreds of individual egg clumps are deposited next to each other, there is a solid expanse of jelly densely dotted with the black eggs (they are white on the bottom, and they right themselves if turned). For female frogs it presumably feels right to swim to where others have spawned or will spawn. We might say that they respond to a “social releaser.” And for a male, the releaser to release his sperm is probably the female’s releases of her eggs. The frogs do not know the connection between their acts and the ultimate or evolutionary sense or consequences of these acts. Nor do they need to.
It seemed to me that the wood frogs’ egg aggregation might have something to do with elevating egg temperature to speed up hatching rate. Using my electronic thermometer, which I had then been using mainly to measure the body temperature of bees, I immediately got busy, waded into the icy waters of many pools, and measured the temperatures around the edges. I found no evidence that egg clumps are located in warmer parts of the pools relative to other parts, so the frogs did not search for or find any hot spots in their pools. But temperature could still be important in egg placement.
The upper black surfaces of wood frog eggs must absorb heat in sunlight, but the heat would normally be quickly dissipated by convection to the cooler water around them. However, a large mass of jelly embedding the eggs would reduce water movement and could aid heat retention. The larger the effective mass, the less heat loss from the center. So, to find out if there is a measurable effect, I took dozens of egg clump temperatures versus surrounding water temperature, and compared single and clumped egg clusters. The results: Egg clusters in shade are nearly the same temperature as that of the water surrounding them. However, in sunshine, the single egg masses were heated on average 3.5°F above the surrounding water temperature, and clusters of ten or more egg masses were heated 9 to 13°F above water temperature.
Temperature affects the developmental rate of the eggs. To find out how much, I brought egg clumps into the lab to determine the time until they hatched. In the woodland pools I had measured egg mass temperatures of 43°F to 79°F, and in the lab none of the eggs hatched that were held either below 41°F or above 86°F. In between these extremes, the developmental rate of embryos was directly related to temperature. For example, eggs at 46°F required thirteen days to hatch, and eggs at 68°F hatched in six days. Thus, within the physiologically suitable temperature range, every increase of 3°F in egg temperature speeds up the hatching date by a day. That could be a huge potential advantage, since the snow-melt pools that these frogs use are highly ephemeral in the summer.
After finishing my temperature measurements I was both pleasantly surprised and also a little disappointed to learn that I had indeed been on the right track. Egg clumping for heating was an idea that had already been proposed. Bruce Waldman from Cornell University had published