On cold, rainy March nights, salamanders may be found crossing roads toward ponds and lakes. Spotted salamanders, Ambystoma maculata, other salamanders, and several frog species often move out of the woods on those nights with their destinations being the temporary pools that form in depressions as spring rains accumulate. These pools are called vernal (spring) pools because they are filled in spring and dry up by summer. Water usually accumulates in them over the winter, but the pools often freeze solid if this happens.
The rains of March or early April trigger the salamanders to move en masse – often to the very same pools they hatched in – to lay their eggs. There may be more snow storms and freezing weather after this movement, but the ponds do not usually freeze over again and cause the eggs or larvae to suffocate. Sometimes hundreds of salamanders and frogs cross roadways on the same rainy night, heading for these pools, to deposit the eggs that will ensure survival of their species.
Why do Salamanders Move When the Temperature is Cold?
Snakes and birds are the most common predators of adult salamanders moving over the ground. By moving on cold, rainy nights, most snakes are still hibernating and most birds are sleeping under shelter, and the adult salamanders have the best chance of avoiding predation on themselves. Unfortunately, roadways in wooded areas are often located between the woodlands and the vernal pools that these amphibians breed in. Sometimes, hundreds of salamanders and frogs are crushed by vehicles whose drivers may be totally unaware their woodland neighbors are hurrying to breeding ponds.
Problems With Dessication
There is sometimes so little rain in New England that many vernal pools dry up early. In 2007, egg masses of wood frogs, Rana sylvatica, and spotted salamanders were exposed on the dried, leaf covered depressions that were all that remained of many vernal pools. Although many embryos inside these egg masses dehydrated and died, the period of dessication in 2007 was short enough that some embryos survived to hatch and even metamorphose when the rains returned. Many dry spring seasons do not provide this respite and a dry spring season might prevent production of any offspring.
Why do Salamanders Lay Eggs in Pools That Might Dry Up Prematurely?
Fish cannot live in a pool that dries up, and fish are the primary consumers of amphibian larvae. If the frogs and salamanders lay eggs in pools with fish, they run the risk of all the larvae being eaten every year. If a pool dries up one or even a few years in a row, the adult salamanders may still live long enough to produce young in another year.
Other Problems for Salamanders
In the fall, some species of salamander, related to the spotted salamander, lay their eggs under the leaves in the depressions where the ponds will form in the spring. These eggs hatch as soon as there is water in the pools. The larvae then eat insects and algae that grow in the water - even if it is winter. When the spring breeding salamanders and frogs lay their eggs in the pool, the larvae of the fall breeders turn carnivorous and eat their cousins. If some of the spotted salamander larvae can grow to a large enough size before their cousins become carnivorous, they can escape this mode of predation.
The absence of fish in vernal pools makes these bodies of water essential for many amphibians to reproduce. Even without fish, salamanders still have enemies in the pools. Bullfrogs (Rana catesbianca), grass frogs (Rana pipiens), and garter snakes (Thamnophis sirtalis and other species) feast on salamander larvae and juveniles when they are available. Birds, such as herons and kingfishers prey on aquatic animals of all kinds. In addition to vehicular dangers, we humans cause many other problems for amphibians – but there is not enough space to discuss these here.
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