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.
