Wood frogs lay eggs in vernal pools

The hollow between the oaks fills with water each spring, and by early summer it holds something else entirely. Wood frog egg masses float in loose clusters near the surface, each one a translucent globe the size of a softball. The water warms in the long June light, and inside each jelly sphere, dark embryos twist and turn.

Wood frogs time their breeding to these temporary pools with precision that seems impossible. The adults emerge from winter refuges in the leaf litter when snow still patches the forest floor, drawn by the first warm nights to water that will vanish by midsummer. The males arrive first, their duck-like quacking filling the darkness as they call from shallow edges. Females follow, heavy with eggs, and the breeding happens quickly. Within days, the adults disperse back to the forest, leaving their offspring to develop in pools that exist on borrowed time.

Each egg mass contains between 600 and 1,500 eggs, bound together in a protective gel that swells when it contacts water. The jelly serves multiple purposes. It insulates the developing embryos, keeping them warm during cool spring nights. It contains antimicrobial compounds that ward off fungal infections. And it creates just enough buoyancy to keep the mass floating near the surface, where sunlight can penetrate and algae can grow. These algae form a partnership with the developing tadpoles, producing oxygen that the embryos need while consuming carbon dioxide they release. The relationship begins before the tadpoles even hatch.

Vernal pools exist because they dry up. This temporary nature makes them unsuitable for fish, which would otherwise devour wood frog eggs and tadpoles by the thousands. The pools fill with snowmelt and spring rains, warm quickly in the shallow basins, and support explosive growth of zooplankton and aquatic insects. Wood frog tadpoles feed on this abundance, growing rapidly through their larval stage. They must complete metamorphosis and leave the water before the pools dry, usually by late July. Those that develop too slowly die when their world disappears. The race against time has shaped everything about their life cycle, from the timing of breeding to the speed of development to the ability of newly metamorphosed frogs to survive immediately on land.

Spring peepers call from the same pools, their high whistles weaving through the wood frogs' lower chorus. But the wood frogs depend on these particular pools in a way the peepers do not. Peepers can breed in permanent ponds and streams if vernal pools are unavailable. Wood frogs cannot. They need the fish-free environment that only temporary water provides. The pools scattered through New Hampshire's forests represent critical habitat, each one a nursery that appears and vanishes with the seasons.

The water moves slightly in the evening breeze, and the egg masses bob at the surface like small planets in their own clear atmospheres. If you are walking near water that pools in forest hollows, the sound you hear might be spring peepers still calling in the gathering dusk, their voices thin and bright against the deeper quiet of early summer woods.