Everybody Into the Pool

Everybody Into the Pool

By Bernd Heinrich

Wood frogs mate and their eggs and tadpoles mature en masse.

In the Maine woods, winter lasts well into April, but toward the middle of the month I begin to watch some dozen roadside ditches and small pools in anticipation of a rite of spring. Snowstorms still come and go. On some nights, ice covers pools and ponds. But whenever I'm outdoors, I watch and listen. Then, from one day to the next, the whole surface of a newly ice-free pool will sprout wood frogs. Spaced almost evenly a foot or so from one another, dozens upon dozens of the three-inch-long frogs float with their legs extended, hind legs buoyed apart, and snouts above water. If I had failed to see the frogs, I would have heard them. Calling, they pump their cheeks in and out, Dizzy Gillespie--like. But on the pool there are no solos. When one frog calls, the others immediately join it in a concert of quacks and croaks.

As I creep toward the pool for a closer look, all the callers fall silent. Many remain motionless on the water's surface. One step closer, and they submerge with kicking strokes, dive the foot or two to the bottom, and hide under last year's fallen leaves. But I've found I can conjure them up again, simply by playing back their calls with a tape recorder. Thus summoned, they will cautiously resurface, approach the sound, and resume their concert.

The wood frog, Rana sylvatica, is well named, being adapted to life in a sylvan, or woodland, setting. In summer, one seldom sees these tan frogs with the black eye stripes, and I consider it a rare treat to encounter one in the woods near my cabin. For the six months of winter, they are totally under cover, having burrowed under fallen leaves or loose soil. In a physiological feat, wood frogs survive subfreezing temperatures by manufacturing a glucose that protects their cells (which shrink in winter) from being penetrated by ice, while the fluid outside the cells freezes. After their spring resurrection, they abound in temporary bodies of water left by the previous fall's rains and by melting snow.

By far the majority of frogs I see are males. They have lost the tan tones of summer and taken on a rich chocolate brown to almost black color, save for a delicate touch of yellow on the head and a chalk white belly. Occasionally, however, I see pinkish tan, slightly larger individuals. These are the females. I seldom see more than six or so females at any one pool, but that does not mean they are not there. They come to the water mostly under cover of darkness, when they can be seen crossing roads and heading directly toward song-filled pools of waiting males.

Once at a pool, a female is not to be found sprawling at the surface like the males. If she is at the surface, the female will be engulfed in a bobbing ball of up to twelve males, all jockeying for position and pushing in a frenzied contest to claim her as a mate. A male takes possession by perching on the female's back and clasping his powerful front legs around her neck. Once this headlock--the scientific term is amplexus--has been achieved, the two can be disentangled only by force. (A woman once brought a thus engaged wood frog couple to me, thinking it was a two-headed frog.) The female, with male attached, sometimes dives to the bottom of the pool. The male will hold on until she sheds her cluster of eggs. In captivity, I found, he will cling for several days. In the pool, he holds fast for a day or less; the female lays her eggs on the evening of their coupling. With the male appended, she swims to a spawning site near submerged vegetation and extrudes black, gelatin-coated eggs in a mass that is partially exposed at the water's surface. The male fertilizes the eggs, and the pair decouple. Each female deposits about a thousand eggs--I counted 1,025 in one egg mass. The gelatin encasing each egg is highly hygroscopic and thus quickly absorbs water. Within hours, the egg cluster swells from walnut size to baseball size.

Wood frogs are aptly known as "explosive" breeders. After just the first night of the mating orgy, hundreds of egg clusters float in a supercluster at one end (in my experience, usually the eastern end) of the pool. Why would all the females in a particular pool put their eggs into one big "basket"? It may lessen the risk of predation; alternatively or additionally, it could relate to the temporary nature of the breeding places, where timing is a matter of life and death to the young. I've measured egg-mass temperature and found it to be at least 2 [degrees] C higher in communal masses--where the output from many females is bunched up--than in a single mass. This temperature difference speeds up the hatching rate of the eggs by a day or more. And a clumping of thick, gelatinous masses reduces convective heat loss as the black eggs are warmed by the sun.

Speedy development of the eggs and tadpoles is crucial. As a survival strategy for their offspring, wood frogs generally avoid permanent bodies of water, which may contain fish, leeches, and other predators. Suitable temporary pools are colonized quickly; one that I dug by backhoe near my cabin attracted dozens of breeding wood frogs (determined by counting egg masses) the first spring and nearly a hundred the second. But while eggs and tadpoles thus escape becoming a meal for a predator, they must develop into froglets by the time the pools have dried up in late June or early July.

The tadpoles of some species, such as the bullfrog, take as long as two years to metamorphose into young frogs. Wood frogs take just forty-five days on average to go from egg to tadpole to froglet. This metamorphosis is fast even when compared with the development rate of other amphibians that breed in temporary pools. Also, unlike other tadpoles I know of here in the Northeast, wood frog tadpoles will, like fish, swim around in schools. I have seen them swimming in long lines and sometimes in giant circular formations. Slow moving, they stay close to the water surface. At frequent intervals one can see their silver bellies as they twist about at the surface to gulp air or graze on algae.

Many questions posed by the wood frog's unusual, seemingly social, behavior are not yet answered. Why do the males go precisely to those pools where the competition for females is greatest? Why do they prefer to congregate in specific pools when they could go to a pool with no other wood frogs and thus avoid competition? Why do they join the chorus to produce even louder, and presumably stronger, vocal lures that attract still more males to the arena to compete? I think one answer is that their behavior is driven by females. Given the ephemeral nature of the pools, females need to deposit eggs early and quickly. This means going to pools where the presence of males--and plenty of them--is guaranteed. The same aural cue that entices the females also lures more males: the loudest chorus will be the one that is closest and that also has drawn the most females. I am reminded of some species of tropical fireflies in which males also converge and "call" (flash) synchronously to enhance a communal mating display.

Wood frog reproduction is a boom-and-bust affair. Of an original clutch of a thousand eggs, almost all may develop into froglets and leave the pool, or almost all may perish. In late June, when the pools shrink, the bottoms sometimes turn soupy and black with congealed masses of dead tadpoles. As long as there is a little water, the dying individuals sustain the others, their bodies providing sustenance to their fellows. Just before metamorphosis, the tadpoles weigh only a fraction of an ounce. Within several days, they lose much of their larval body weight, absorb their one-inch tails, and sprout legs. The tiny, freshly metamorphosed wood froglets then leave the water for a life on land. They usually reach at least half their adult body weight by summer's end in the north woods.

Bernd Heinrich is a professor of biology at the University of Vermont in Burlington. His latest book is Mind of the Raven (Cliff Street Books, 1999).