Why Don't Duck Feet Freeze?

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This mallard hen is standing on ice and snow as if it wasn’t cold at all. An amazing design keeps her feet from freezing.

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Watching a mallard duck waddle across a frozen pond completely unfazed by the cold march, I wondered at how I would feel doing the same thing in bare feet. I suspect that pain and misery aside, it wouldn’t take long before my feet began to freeze into blocks of ice. Why then don’t duck feet freeze?

In a bit of brilliant natural bio-engineering, the duck uses a system of blood vessels, known as a countercurrent system, or more specifically, rete mirabile, to keep feet from freezing. This is a dense network of blood vessels formed by dividing up larger vessels into many small branches, like a net, and then re-uniting them into one trunk. Along the way, they intertwine with vessels containing blood flowing in the opposite direction and a heat exchange occurs.

So, in a nutshell, in the venous system, cold blood is rising toward the body from the feet. In the arterial system, blood at body temperature is headed toward the feet. As the venous and arterial veins split into many tiny vessels and intertwine, heat is transferred from the arterial side to the colder venous blood, warming it as it enters the body and thus maintaining the core temperature where it should be.

From the standpoint of the feet, the blood entering the feet is substantially cooler, lowering the differential between the feet and the environment. The blood flow to the feet is relatively constant. The goal is to keep the feet just above freezing, not warm and toasty. In this way, the duck’s feet don’t lose much heat to the environment and don’t freeze.

This incredible system of rete mirabile (plural retia mirabilia), which means marvelous network, isn’t just reserved for birds, or even heat exchange, though.

Some fish use a rete mirabile system to help fill their swim bladders with oxygen to improve buoyancy. A change in pH causes a pressure shift in the oxygen molecules still in the venous system. The oxygen seeps out of the veins of the rete mirabile and back into the arterial side. When pressure in the arterial side exceeds the oxygen pressure in the swim bladder, the oxygen transfers to the swim bladder.

The giraffe has a problem that I get a hint of every time I tie my shoes and blood rushes to my head. With the giraffe though, the problem is greatly magnified, not in tying shoes but in getting a drink of water and lowering its head from 16 feet to the ground. Enter rete mirabile in the neck which equalizes blood pressure when its head gets below level.

Retia mirabilia are also found in the limbs of a number of mammals. As with the duck’s feet, this countercurrent system helps to reduce the temperature gradient and thus heat loss to a cold environment. However, cooling of the limbs also serves to reduce the metabolic oxygen demand by the muscles. A highly developed rete mirabile is part of how animals such as sloths and lorises can hang on to a branch long past what would exhaust another animal.

   And, at last, back to birds. How do seabirds, which may spend their entire lives in salt water handle the massive amounts of salt they ingest? They use a countercurrent exchange into a “salt gland” via a rete mirabile. This gland collects the salt and then the bird “sneezes” out the brine.

That this incredible design crosses so many species and serves such different purposes is amazing to me. I truly do find it “mirabile”.  

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Help Idaho Wildlife

When we traveled across the state in October 2017, most of the vehicles we saw using the wildlife management areas did not have wildlife plates. Buying wildlife plates is a great way for non-hunters and hunters alike to support wildlife-based recreation like birding.

C'mon folks, let's help Idaho's wildlife by proudly buying and displaying a wildlife license plate on each of our vehicles! 

See below for information on Idaho plates. Most states have wildlife plates so if you live outside Idaho, check with your state's wildlife department or vehicle licensing division for availability of state wildlife plates where you live. 

And tell them that you heard about it from Nature-track.com!

Wildlife License Plates

Great news! as of 2024, there are three NEW designs for license plates. They still are bluebird, cutthroat trout and elk, but they are beautiful.

Idaho Wildlife license plates provide essential funding that benefits the great diversity of native plants and wildlife that are not hunted, fished or trapped—over 10,000 species or 98% of Idaho’s species diversity. Game species that share the same habitats (such as elk, deer, antelope, sage-grouse, salmon, trout) also benefit from these specialty plates.

No state tax dollars are provided for wildlife diversity, conservation education and recreation programs. Neither are any revenues from the sale of hunting or fishing licenses spent on nongame species. Instead, these species depend on direct donations, federal grants, fundraising initiatives—and the Idaho Wildlife license plates.

Both my vehicles have Bluebird Plates. I prefer the bluebird because the nongame program gets 70 percent of the money from bluebird plates, but only 60 percent of the money from elk and trout plates - 10 percent of the money from elk plates supports wildlife disease monitoring and testing programs (to benefit the livestock industry) and 10 percent from cutthroat plates supports non-motorized boat access.

Incidentally, in 2014, the Idaho Legislature denied the Department of Fish and Game the ability to add new plates or even to change the name of the elk and cutthroat plates (very specific) to wildlife and fish plates, a move that would have allowed for changing images occasionally and generating more revenue. It would seem that they believe that we Idahoans don't want a well funded wildlife program.

I think it is time we let the Legislature know that Idahoan support wildlife funding and that we would like to see these generic plates come to fruition.