Spring Amphibians

The rain started after midnight on March 11th.  The first night of warm spring rain.  Each year, this night marks an event of epic magnitude, the timing of which has been perfected by millions of years of evolution.  This night belongs to the amphibians.  Out in the woods, millions of frogs and salamanders descend on seasonally filled pools to fulfill a singular mission: to breed.

Unlike the long and drawn out migration season of birds, the movement of amphibian species that use vernal pools is more like an invasion.  In this area, this invasion consists of three main players, the Spring Peeper (Pseudacris crucifer), Wood Frog (Rana sylvatica), and Spotted Salamander (Ambystoma maculatum).  The first to sing in early spring are the Spring Peepers (one of these thumbnail-sized entrepreneurs was singing at 9:15 on a relatively warm night on my birthday, January 31st).  The Peepers are followed by perhaps a handful of early Wood Frogs, and finally, on the next warm, rainy night, by the much anticipated invasion of all three species to the woodland pools.

A gorgeous female Wood Frog  

The day after the rain I was out at the largest of the four vernal pools in the woods at my home on Honey Branch Farm.  At this time of year, I can hear the amphibian chorus from my house, more than a quarter-mile away.  As young children, my siblings and I cleverly named this pool the “Woods Pond,” and were well aware of its springtime inhabitants.  I’m happy to say that, fifteen years later, I again find myself thigh-deep in the same pond, albeit carrying an expensive camera instead of a cheap dip net.  The rain that had started the night before had just slowed to a drizzle.  After they grew bored with my presence, the Wood Frogs’ and Spring Peepers’ singing quickly rose back to a deafening chorus.  Spring at last!

A spectating Wood Frog

Wood Frog egg masses

In the two hours I spent wading in the pond and taking pictures, I failed to see any evidence of Spotted Salamanders.  So at sunset, I hopped in the car and headed about ten minutes north to the Sourland Mountains, where dozens of vernal pools decorate the forest floor among the diabase boulders.  Early Spring is one of the few times these largely sub-terrestrial salamanders can be seen above ground, as thousands of individuals converge on these pools from the surrounding woodlands, crossing whatever barriers that stand in the way of their natal pools.

A male Spotted Salamander crossing a road in the Sourlands.  

Roads are the cause of a large number of amphibian deaths 

during their spring migrations to vernal pools.

At the pools, the salamanders were already hard at work.  Dozens of salamanders were in the water, lazily walking across the pool bottom and looking quite prehistoric.  These giants of eastern amphibians have converged here from the surrounding woodlands, the males arriving well before the females, gathering in so-called congresses near the center of the pool.  Their spermatophores, packets of sperm that resemble white bird droppings, had been deposited in clusters on top of the submerged leaf litter.  When the larger females arrive, they are courted by the males, whose displays involve nudging and swimming in circles around their prospective mates.

A male Spotted Salamander resting on the bottom of the pool

The salamanders, Spring Peepers, and Wood Frogs put on a terrific show – one amorous male Wood Frog even attempted to hold a male Spotted Salamander in amplexus, much to the salamander’s annoyance.  The night was spectacular.  I crawled into bed at 2:00am. 

A much larger female Spotted Salamander on her way to the pool

The Vernal Pool Strategy:

When you’re an amphibian in the vernal pool business, arriving early counts.  Early arrivals can claim the best real estate, call for mates longer, and breed and lay eggs sooner, all of which improves the chances that they will reproduce and give their offspring adequate time to develop.  However, for exothermic amphibians that are at the mercy of ambient temperatures, early activity also means risking exposure to sub-freezing temperatures.  Without freeze-safe adaptations, punctuality would be suicide.  For Spotted Salamanders (Ambystome maculatum), that simply means staying underground below the frost line until the time is right.  Wood Frogs (Rana sylvatica) and Spring Peepers (Pseudacris crucifer), however, have an adaptation that allows them to be more opportunistic: Just before freezing, fast-acting enzymes convert the glycogen that had been stored in the frog’s liver into glucose in the bloodstream.  The glucose spreads throughout the frog’s body and, like the antifreeze in a car, lowers the freezing temperature and protects the individual cells from rupturing.  It makes good sense, then, that in some spring-breeding amphibians, liver glycogen accumulates in the fall in anticipation of sub-zero winter temperatures, and decreases again after the breeding season in spring.  This handy adaptation allows these frogs more leeway for trial and error.

The reliance of many amphibians on vernal pools is probably largely a result of one important characteristic: aquatic habitats that remain filled for only a portion of the year lack some of the potential predators of amphibians and their eggs and developing larvae.  While predators still exist in the vernal pool environment, the risk of predation is presumably greatly reduced.