Each day spring is a little closer. This is the time of the year for anticipation of the annual amphibian migration to local vernal pools and wetlands. Predictable, and yet unpredictable, this migration will occur every year without fail. The unpredictable part is the weather. Relatively warm rainy nights are what drives amphibian migration. The predictable part is the timing of events that occur once the weather is suitable. This annual migration is one of the amazing natural history events in this part of the world.
Woodland vernal pool
Amphibian migration can begin early in the calendar year even when there is still some ice on the vernal pools. The first raining night of the year initiates some of the first salamanders to begin their march. The initial movement is typically male Jefferson and Small-mouth salamanders, as these woodland species, often referred to as mole salamanders; live as adults in the soil of the forest floor. Early arrivals begin to set up their territory in the pools and will eventually start to deposit spermatophores on the bottom of the pools. Spermatophores are little sperm packages left on the bottom of the pool by the males later be picked up by females for internal fertilization. These early amphibians can be caught by cold weather and may become trapped beneath the ice of the frozen pool. Typically this is not a problem if the cold snap is not a prolonged event these animals will absorb enough oxygen through their skin to survive until the thaw.
The next wave of migration consists of female Jefferson salamanders and the initial movement of male wood frogs. The Jefferson salamander males begin dancing for females to attract them to their area of the pool where they have deposited their spermatophore. Once the spermatophores are picked up, a series of jelly egg masses will be laid on twigs in the vernal pools. The first arriving wood frogs claim a territory and begins calling in anticipation for the later arriving females. After spending the winter hibernating and mostly frozen solid, these frogs only have one thing on their mind, and it is not food!
The waves continue when the weather events are again suitable for migration and this is when activity really picks up. The early salamanders have deposited their eggs and have already begun to disperse back into the forest. The number of male wood frogs increase and their chorus peaks in great anticipation of the arriving females. The first male spotted salamanders arrive, another mole salamander, and will follow the same behavior pattern as the earlier Jefferson and small-mouth salamanders, setting up territories and depositing spermatophores. Now, leopard frogs, pickerel frogs and spring peepers are on the move. Again the males arrive first and now the chorus can be so loud that you need to shout to a friend standing next to you.
The next wave will bring the peak of activity to wetlands and vernal pools. Female wood frogs are now arriving and are greeted by enthusiastic males that will outnumber females by about 4 to 1. Often males will amplex (grab) any moving objects that come by in hopes that it will be a receptive female. This can be other male wood frogs, salamanders or even a hand if placed in the center a group of calling males. Each female will lay one egg mass containing approximately 400–600 eggs, and often the entire population will complete this task in 24 hours. The first female leopard frogs, pickerel frogs and spring peeper begin to arrive as well as the first male American toads. This brings a new song to the wetlands and their long melodic trill fills the damp air. Female spotted salamanders arrive and as they maneuver around overzealous wood frogs. They will find a suitable male and utilize his spermatophore to fertilize her eggs. She will then deposit round egg masses again on small twigs and substrate in the vernal pool. Now, the female leopard frogs, pickerel frogs and spring peepers have all arrived. As a singing male attracts a suitable female, amplexus occurs. When the female releases her eggs the male will release sperm onto the eggs for external fertilization to take place. At this time most of the Jefferson and small-mouth salamanders have dispersed back into the forest.
The peak of the explosive breeding season is now nearing an end. The salamanders, wood frogs, spring peepers, leopard frogs and pickerel frogs are now dispersing back into their summer habitats and the attention is turned to the toads. The males’ chorus peaks and females begin to arrive. These are champion egg layers as long strings of up to 2,000 eggs can be deposited.
The mad rush is now complete and the transition to summer breeding amphibians in now occurring. Green frogs, American bull frogs and gray tree frogs songs fill the steamy summer nights with their songs as their breeding season continues throughout the summer.
Back in the vernal pools the eggs have began to hatch. These larvae grow very rapidly and ready themselves for their transition to a terrestrial life before the heat of the summer dries their pools. The frogs graze algae and the salamanders are veracious predators eating any aquatic animals that can fit into their mouth including insects, crustaceans and other amphibian larvae. If all goes well the cycle will be complete as they disperse into the surrounding habitat to hopefully one day return to these wetlands to again play out this cycle of life.
Watershed Stewardship Center