Wood Frog
It’s
not every day that a freeze-resistant frog is discovered in your
own back yard.
Often referred to as the frog with a robber’s mask, the wood
frog, Rana sylvatica, is a remarkable species, only recently
discovered in Oklahoma. A member of the "true frog" family, this
small frog ranges from one-and-a-half to two-and-three-quarters
inches long and comes in a range of colors. Usually brown or
rust colored, they can be green or gray, but all have the
distinctive dark patch that runs from the eye to the circular
hearing organ, called the tympanum. In addition, the wood frog
has two ridges running parallel down the back, a feature common
to most frogs in this family.
While the wood frog is typically found in the northeast part of
the country, it can be found as far south as northern Alabama,
and a western subspecies is even found in the Rocky Mountains.
In Oklahoma, the wood frog has a limited range. The first
specimen was discovered in 2005 in the southern portion of Adair
County in a wildlife refuge adjacent to the Ozark Plateau
Wildlife Management Area. In the following years, additional
frogs were found in the same wildlife refuge, but the breeding
area was never located.
Every spring, males travel from their hibernation sites back to
favored breeding spots and begin calling to attract females.
Although the chuckling, duck-like voice may sound like another
northern species — the Northern leopard frog — it is shorter in
length, and not as low pitched; generally one of the first to
call in early spring. Breeding can begin as early as March,
sometimes before the ice has completely melted, and lasts until
the beginning of May. Females lay large egg masses in the
deepest part of the pool, with 1,000 to 3,000 eggs laid by a
single individual. After breeding, the adults leave the area and
return to drier ground. Development of the egg is dependant upon
water temperature. If laid in cold water, it can take as long as
a month for the tadpoles to appear, but if laid in warmer water,
development can take only nine to 10 days. Complete
metamorphosis from tadpole to a frog generally takes two months,
and the froglets reach sexual maturity two years later.
Unlike many frogs, the wood frog hibernates in upland sites
where they are more vulnerable to harsh winter conditions. Even
with obvious drawbacks, this unusual behavior of trading a more
secure hibernation site for leaf litter and stumps has
advantages. Because they are closer to the surface, wood frogs
are better able to detect subtle temperature changes that
announce spring sooner than frogs that hibernate underground or
underwater. This early emergence is a way to take advantage of a
longer growing season for the offspring, and to some extent,
escape from predators that are still in hibernation. Even still,
the cold winter weather requires a unique adaptation – wood
frogs are freeze tolerant. Amazingly enough, these frogs are
able to survive even when up to 50 percent of their total body
water is turned into extracellular ice. Forming underneath the
layer of absorbent skin, these ice crystals are scattered among
the skeletal muscles and only the presence of specialized
proteins and glucose keep the cells from bursting.
Like many other amphibians, the wood frog primarily breeds in
seasonally wet depressions called vernal pools. Intermittently
filled from either raising water tables or from precipitation or
water runoff, these pools are especially important to breeding
frogs and salamanders because they can’t support fish—a major
predator. One of the leading threats to these frogs and many
other amphibians is the loss or damage of these sensitive
breeding areas. Even slight changes in the landscape from small
construction and ground moving projects to increased litter can
damage this new-found Oklahoma species.