Every year, I eagerly look forward to the first rains of early March. For me, with these cold rains comes the arrival of spring. When the air temperature rises to about 40° F and an extended rain begins in the afternoon and continues into the evening, its time to put on my raingear and visit the vernal pools in search of amphibians.
Vernal pools are characterized by being small bodies of water that generally go dry by late summer. This eliminates predaceous fish from the pools. The pools are fueled by the decaying leaf vegetation that support insect larvae, fairy shrimp, and other invertebrate life that feed the amphibian larvae. Vernal pool amphibians are migratory breeders that move into pools to reproduce but spend most of their lives elsewhere. They include most members of the mole salamander family (Ambystomatidae) and many anurans (frogs and toads).
The first species I will look for is the spotted salamander. In the course of one evening, a well-timed rain event can result in hundreds of these large yellow-spotted mole salamanders traveling from the upland woods to these ephemeral water-filled basins for a few nights of frenzied mating. For reasons unknown to me, biologists have termed this mating mayhem a “breeding congress”. Nuptial dances precede the deposition of numerous spermatophores by each male. Females pick up these sperm packets, and lay up to 250 eggs in the water over the course of several days. The underwater egg masses quickly swell with water into softball-sized masses. They are often white-cloudy (not always), firm in the hand, with the peripheral individual eggs somewhat flattened.
The spotted salamander is one of approximately 30 extant species in a single genus which make up the Ambystomidae family. This family of large salamanders is widespread in North America, but not found on any other continent. Ambystomatids are called mole salamanders for their tendency to live under litter or in burrows, emerging and returning to water only to breed. While the spotted salamander is an early season breeder, there are other mole salamanders known to breed earlier. The tiger salamander can cross snow and ice to breed during January and February thaws, but, depending on how you look at it, perhaps the earliest breeder is the marbled salamander, who gets a head start by breeding in the dry vernal pool sites in the fall. The female will wrap herself around the eggs until the fall rains fill the pond, at which point, she will leave them to their own care. The big advantage of this timing is that when the spring-breeding salamander eggs hatch, the large marbled salamander larvae, being carnivorous (like all salamander larvae) have an unlimited and convenient food source.
Most of the mole salamanders will breed in these vernal pools for a week or less and then retreat to their subterranean habits for the rest of the year. Species that adapt such a breeding technique are known as explosive breeders. The deposited eggs then begin a race against time. The abandoned eggs require about three months to hatch and metamorphose into juveniles (depending on species) and must be able to leave the aquatic environment before the pool dries up.
The March rains bring more than just large salamanders to these vernal pools. There is perhaps nothing more exciting than to hear the first wood frogs of the season. Their duck-like calls will fill the air for several days as these explosive breeders emerge and congregate in the hundreds. The wood frog egg mass contains 500 or more eggs, is more often clear and more gelatinous, with the peripheral eggs retaining their spherical shape. Egg masses are laid in communal clusters, with often more than a hundred females producing egg masses in the same location of the pool.
Wood frogs are truly remarkable creatures. They are the most widely distributed amphibian in North America, and are the only ones found north of the Arctic circle. One of their unique capabilities is their freeze-tolerance. They can withstand temps of 21 degrees and freezing of 60-70% of their internal body water. This they do by flooding the cells with glucose, a natural sugar, which acts as an anti-freeze, protecting the cell walls from erupting. Additionally, much of the cell water is osmotically removed from the cells and organs to the intracellular space, where it can freeze and not affect the integrity of the cell walls or internal organs. When this frozen condition persists for sixteen hours, the heart goes into cardiac arrest, with no further heartbeat until the frog is thawed out. Gray tree frogs, chorus frogs and spring peepers also have a freeze-tolerance, although research has demonstrated a freeze tolerance of up to only 50% of the internal body fluids. Surprisingly, toads do not have this ability. They survive the winter simply by burrowing below the freeze line.
Three species of tree frogs breed in vernal pools; the chorus frog, the gray tree frog and the spring peeper. Of these, only the summer-breeding gray tree frog will not be found in the March pools. Chorus frogs can be identified by their song, which resembles running your thumb along the teeth of a plastic comb. Females deposits several small masses of 20 to 100 eggs attached to grasses or other material in shallow water. Another explosive breeder, their calling period is very ephemeral, lasting about a week, depending on the weather.
Beginning with, or shortly after the wood frogs and chorus frogs begin their calling, the spring peeper will begin its serenade. Everyone recognizes the high-pitched “preep” of the spring peeper. It’s believed that spring peepers exert more energy in an evening of calling than any other animal in the world with respect to their size. Up to a thousand eggs are attached singularly to underwater grass stems. Unlike the explosive breeders, the spring peeper is an extended breeder, with its mating call being heard over the next six or more weeks.
If you were to visit a breeding pond after the eggs are laid, you can guess the amphibians by the nature of the egg masses. Frog eggs are found in a surface mass, toad in a long double string of eggs, and mole salamander eggs in firm masses in deeper water.
Vernal pools are critical habitats for these highly seasonal amphibians and their predators and prey. The dynamic interactions of the pool residents create a living classroom that provides an ideal ecological teaching environment. And this is the time to go out and discover their beauty.