It should come
as no surprise that European explorers were astonished by their
first encounters with freeze-tolerant frogs--for although the
globe suffers no shortage of cold places, freeze tolerant frogs
live only in North America.
current range of the wood frog (in red) and extent
of glacial ice 18,000 years ago (light blue.)
interprets the narrow geographic distribution of freeze-tolerant
frogs to mean that they evolved only very recently, as the last
Ice Age drew to a close some 15,000 to 20,000 years ago. As glaciers
retreated from their greatest extent (as far south as present-day
Indiana), frogs moved north to occupy the newly exposed land.
frogs could have evolved the ability to survive freezing in so
short a time may sound far-fetched, but Storey believes that frogs
are ideally suited for freeze tolerance. He sees freeze tolerance
as a natural progression from the ability to withstand severe
dehydration, which most frogs already possess.
does one get from dehydration to freezing solid? Dr.
Storey explains.Since frogs spend much of their time in water,
their skin is water-permeable; as a result, they are especially
prone to dehydration in times of drought. Many frogs survive losing
over 50% of their water.
Dehydration tolerance may be an ace in the hole for an enterprising
frog looking to become freeze tolerant, but it doesn't count for
everything. The spadefoot toad (despite its name, really a frog)
can lose 60% of its body water, yet freezing kills it. So what
else must a frog accomplish to evolve
Despite their great
croaking, these frogs garnered little attention until a 1982 paper
published by University of Minnesota biologist William Schmid.
Shortly afterward, Storey began to investigate the wood frog's
strategy for freeze survival. By 1990, Rubinsky, who was already
studying more traditional techniques for organ preservation, took
an interest in the wood frog as an alternative route to success.
By the time that Rubinsky began studying the wood frog, Storey
already understood the importance of slow cooling to its survival.
Slow cooling allowed the frog several hours to respond by producing
and saturating its body with prodigious amounts of glucose, which
functioned as a cryoprotectant. Glucose concentrations within
the frog's central organs typically soared to 100 times their
original values. Cryoprotectants like glucose are nothing new
to the science of organ preservation. They are known to accumulate
within cells, where they bind to water molecules, therefore preventing
the cells from undergoing dehydration when freezing occurs in
the extracellular spaces.
Yet, many attempts
to freeze and revive organs using cryoprotectants like glucose
have previously failed. Even for the wood frog glucose isn't always
the ticket. A wood frog frozen to -5°C survives without a
hitch. But lower the temperature another ten degrees and the wood
frog fares no better than any other animal given the same harsh
treatment would--inevitably, it dies. Some unknown factor limits
just how far the wood frog can be cooled and still survive.
Rubinsky and Storey
set out to identify this factor by observing the freezing process
on a microscopic scale. They watched through a microscope as sections
of liver tissue from the wood frog were cooled to either -7°C
(a survivable freeze) or -20°C (an unsurvivable freeze).