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MOUNTAINLIFEONTARIO
WINTER2016
largely in temporary ponds and ditches formed
by meltwater, their choruses are heard over a
month before aquatic species like green frogs
or leopard frogs begin their own breeding. But
hibernation on land also has its cost. Frogs can’t
bury themselves underground like toads do and
thus, must hibernate in damp leaf litter to avoid
drying out. Their soft, water-permeable skin is
no barrier to ice and so, when frost penetrates
to their winter home, they simply freeze.
Within only a few hours of the animal being
exposed to freezing temperatures, ice fills fluid
compartments like the abdominal cavity. Large,
flat ice crystals run between the layers of skin
andmuscle, and the eyes turn white because the
lens freezes. With some 65 percent of the frog’s
total body water converted to ice, blood stops
flowing and breathing, heartbeat, andmuscle
movements all stop. The “frog-sicle” exists in a
state of suspended animation until it thaws.
Freeze tolerance has three main components.
The first is control of the freezing process itself.
Lowering temperatures trigger special nucleators
to actively seed ice formation in the frog’s body
(imagine a starter grain on which an ice crystal
can grow), allowing animals to begin freezing
just below 0˚C. The second is production of
cryoprotectants
—high concentrations of sugars
and sugar alcohols that prevent excessive
reduction of cell volume and increase the
content of bound water. Like the glycols added
to our car’s radiator for the same purpose,
cryoprotectants keep the insides of cells from
freezing. Thus, ice forms
around
the internal
organs rather than through them, drawing water
from them but leaving a thick, syrupy solution
inside unfrozen cells. Finally, because the blood
is frozen, organs require an elevated ability to
survive without regular delivery of oxygen or
nutrients; during thawing, antioxidants resist
the cellular stress produced when oxygen is
suddenly reintroduced, and repair mechanisms
go on high alert—for instance, the level of
clotting proteins rises to staunch any bleeding
from ice-damage to organs.
Behaviour also plays a role. As frogs freeze, they
assume the “water-holding” position typically
employed by animals to avoid desiccation: head
down, limbs drawn in. This position reduces
evaporative water loss over what can be weeks
of continuous freezing. What hasn’t been well-
studied are how signals for breathing, heartbeat,
and brain activity are reactivated during thawing,
when all organ functions miraculously re-boot
within two hours
.
Though it, too, is somewhat freeze tolerant,
the red-sided gartersnake (
Thamnophis sirtalis
parietalis
)—with notoriety as denizen of
Manitoba’s famed snake dens, the world’s largest
concentration of reptiles—is no wood frog.
Rather, the species is tolerant by degree and
with regional variation—northern populations
can handle the freezing of skin and skeletal
musculature, but not for periods stretching
beyond hours or days. Winters where sub-
zero temperatures reach deeply into dens for
extended periods still cause massive die-offs.
Nevertheless, certain subspecies of
T. sirtalis
have the longest activity season of any North
American snake, often roaming the surface
with body temperatures lethal to other species:
I’ve seen gartersnakes moving through spring
snowfalls inAlgonquin Park, and a friend
documented them emerging from a den on
the Alberta/Northwest Territories border with
internal temperatures as low as 0.5˚C, active and
crawling across patches of snow.
When it comes to freeze tolerance, Ontario’s
frogs and snakes give you something to think
about next time you’re feeling cold and want to
say: “I’m freezing.”
Withsome65percent
of the frog’s total body
water converted to ice,
bloodstops flowingand
breathing, heartbeat, and
musclemovementsall stop.
The“frog-sicle”exists
inastateof suspended
animationuntil it thaws.
Like a frog, the red-sidedgartersnake is freeze-tolerant, thoughnot to the same degree. LESLIEANTHONYPHOTO.
