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Herring
Gulls in ice water
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Looking at seabirds in winter raises comfort questions that
we can scarcely relate to as thermally challenged humans.
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Mallards
on ice
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How are these Mallards
able to stand comfortably on ice? Why don't their feet freeze? Why don't they
lose an unsupportable amount of body heat through their unfeathered legs? As a
partial solution many birds at rest tuck one foot up against the warmth of
their bodies, under the insulating feathers. They balancing on one leg,
alternating the exposed and the protected one.
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Robin on ice
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The problem challenges any bird overwintering in
this climate, whether standing or swimming.
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Bufflehead
|
How do those legs
keep functioning if they're super-chilled? This Bufflehead's foot, besides
propelling the duck in powerful dives, dexterously satisfies an urgent itch.
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Red-necked
Grebe
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Bird’s
legs and feet are relatively free of soft tissue. The muscles that operate the
foot are mostly located higher up in the leg and connected to the bones of the
feet with long tendons. Because there isn’t much soft tissue in the lower legs
and feet, there is less need for warm blood. Many birds also have valves in
their leg arteries that control the blood flow, and there is evidence that some
birds can pulse the blood to the foot. Every once in a while, warm blood flows
to the foot to make sure it doesn’t suffer from frostbite.1
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Counter-current
heat exchange 1
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Ducks,
as well as many other birds, have a counter-current heat exchange system
between the arteries and veins in their legs. Warm arterial blood flowing to
the feet passes close to cold venous blood returning from the feet. The
arterial blood warms up the venous blood, dropping in temperature as it does
so. This means that the blood that flows through the feet is relatively cool.
This keeps the feet supplied with just enough blood to provide tissues with
food and oxygen, and just warm enough to avoid frostbite. Scientists who
measured it calculated that Mallards lost only about 5% of their body heat
through their feet at 0o C (32o F) . To put this in perspective, the rest of the duck is covered
with feathers and in contact only with air, not ice, but because the body is
relatively hot, 95% of the heat loss is from the head and body. 1
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Comfort
is all in how you feel
|
A creature doesn't
have to experience pain just because we do. If it's life systems are not in
danger because of its adaptations, it is not uncomfortable.
Warm blood from the
body going in the arteries to the duck’s feet is used to warm the blood coming back
from the feet and into the body, in the veins. This has two advantages. First,
it ensures blood going back into the body is warm. Second, it minimizes heat
loss. The bird's toes do indeed get cold, several degrees colder than the body.
But thanks to the counter-current exchange system, the body stays nice and
warm. 2
Online
sources:
1. T. C.
Pelletier, AskaNaturalist.com, April 22, 2010.
2. Chalisa Nestell, Phinizy Center for Water Sciences, 2014.
This is so weird. During the last birdwalk in February (the 16th) at Halibut, we talked about the countercurrent system in ducks. I remember studying this in anatomy/physiology in college and was fascinated. One important thing to also mention is that because of the remarkable function of the countercurrent system, ducks don't freeze to the ice even when their feet are wet, like human tongues do when they touch cold metal. Because the bottom of their feet are close to freezing, they don't stick to the ice. Very cool! This countercurrent system exists in many animals; some of them in warm climates. Camels have this system in their legs and large noses. Jackrabbits exhibit it in their ears. In these cases, it is to radiate heat OUT of their bodies and cool down the blood returning to their hearts. In addition, human beings exhibit certain systems that allow them to stay comfortable in their native environments. Inuit people have shorter limbs that retain heat. They also have arterial systems that allow them to put one hand in cold water while the other hand stays warm; if you or I do this, both hands cool down. Conversely, people from warmer climates have longer limbs that tend to dissipate heat, keeping the body cooler. Natural processes are fascinating, aren't they? Thank you Martin!
ReplyDeletei read this blog and it is so useful for me thank you
ReplyDeletewhy are my feet always cold