Two Posts from Badger Run Wildlife Rehab

Ever wonder why birds can land on freezing metal in winter and their feet don’t get stuck to it?

Our fingers will get stuck on cold metal ice cube trays when pulling them out of the freezer. Our tongues freeze to cold metal ala A Christmas Story. That’s because the moisture on our skin freezes in contact with the icy metal.

Birds’ feet are covered with dry scales so there is no moisture to freeze to frigid metal. Birds have no sweat glands and essentially no secretory glands (not zero) so the skin does not secrete moisture through the skin on their feet.

The photo below of an Osprey’s foot shows these scales in the extreme.

Frigid temps can be hard on wildlife. How do they keep those bare feet from freezing? Countercurrent heat exchange.

Basically, the arteries carrying warm blood down to the feet are very close to, if not intertwined with, the veins carrying cooled blood back up to the body and the heart. So, the warm blood in the artery essentially rewarms the blood coming back up the leg’s vein so it does not cool the body’s core temperature.

And birds aren’t the only ones that use countercurrent heat exchange in their extremities to conserve body temperature. Other animals like arctic foxes and wolves use it, too. Deer species, as well. Also, beaver, muskrat, otters, and sea mammals.

  • Eiri@lemmy.ca
    4·
    3 days ago

    Great explanation. Thank you so much for all the effort. But the part you explained in the most depth was the part I did understand, I’m so sorry.

    But I did understand the part where I was confused anyway, to some degree

    What I wondered was why giving humans that ability would make the cold extremities feel worse. The answer seems to be that by reheating the returning blood, the outgoing blood gets pre-cooled, and starts at a lower temperature. So the feet are even colder.

    Now the question I still have is: so the birds’ get are still super cold, probably even colder than humans’. They probably even reach freezing temperatures, considering birds don’t disappear even at -20 °C. Why don’t they get frostbite, lose toes to necrosis, and all that stuff that a human going out barefoot in the winter would be sure to get?

    • anon6789@lemmy.worldOP
      3·
      3 days ago

      Ah, so the other reply was your question! I just replied to that comment.

      It would feel worse to us, because our fingers and toes are meaty and full of nerves. Bird toes aren’t, they’re largely bone and tendon and not many nerve endings, so isolating the really chilled blood to just that area isn’t all that uncomfortable to them. The parts of the lower leg that would feel that cold are getting the higher temp of the heat exchanged blood.

      The heat exchange takes heat away from the toes, yes, but it is still just high enough overall to prevent damage to what is there. That keeps them from getting frostbite, as there is little to actually freeze due, and the higher heat returning more quickly to the core fends off hypothermia, which would kill most things before frostbite could. So each end of the leg is tuned to each challenge.

      That’s about the extent of what I’ve been able to learn, so hopefully that has helped.

      There was a Simon Whistler video that interestingly said there was a study on ducks that showed a potential shortcoming of the system. If the ducks suddenly went from a more stable warm climate to being in icy water, that somehow tricked the system into not functioning properly or possibly quickly enough, it didn’t say in the video, that the feet actually could get frostbitten.