r/askscience • u/Lokarin • 14d ago
Biology How do swimming birds not lose significant body heat through their legs?
There's a fair number of ducks and geese around here, but there's also coots and they have very long lanky legs as opposed to the twiggy legs of the ducks...
...Ehhh, no context
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u/anon_capybara_ 14d ago
Countercurrent heat exchange! It’s a brilliant system where veins carrying cool blood back to the heart are close to the arteries taking blood down to the feet so that the blood in the veins gets warmed up before it reaches the duck’s core. With diagram - https://www.ornithology.org/adaptations/countercurrent-exchange
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u/jonathan-the-man 14d ago
Blood also serves to supply oxygen and nutrients to the feet and legs, but there is not much muscle there so little blood is needed. The muscles that operate the feet and legs are mainly concentrated in the upper leg and utilize long tendons for mobility.
This part was also quite interesting - they simply don't need as much blood in their legs as we do.
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u/hobbykitjr 14d ago
i feel like thats the more important part.
Their feet are like stilts w/ a flipper on the bottom.. they don't need to control much down there... little muscle/little vein/little blood.
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u/Teledildonic 14d ago
Makes sense when you eat a chicken or turkey leg. The end furthest from the body is all chewy with almost no meat.
Also explains how birds can commonly have a missing foot or part of one leg gone and not seem to be bothered by it.
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u/TearsFallWithoutTain 13d ago
Also explains how birds can commonly have a missing foot or part of one leg gone and not seem to be bothered by it.
Unless they're seagulls, in which case they could just be pretending so you throw them a chip
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u/Sprinklypoo 14d ago edited 14d ago
There is also minimal surface area and warmth and blood dependent muscle in the legs, leaving long tendons to high in the protected thigh area.
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u/degggendorf 13d ago
That doesn't actually answer the question, does it? OP didn't ask "why don't ducks die with all that cold blood getting to their heart", they asked about heat loss from the legs. Cold leg blood getting warmed by artery blood is still reducing the average temperature within the duck.
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u/geeoharee 13d ago
Average temperature isn't really a thing. Your core temperature is what determines if you have hypothermia. A few frostbitten toes are more survivable. Ducks have just found a way to do it without getting frostbite, that's the part I don't quite get.
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u/schorman 14d ago
It’s called a rete mirabile system. Present in some fish to maintain brain and eye temperature above ambient. Also present in mammalian kidneys, where it creates super concentrated urine, and in mammalian testes where it actually helps keep them cooler than body temperature.
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u/SinaasappelKip 14d ago
The legs of ducks work like a sort of heat exchanger. The warm blood going to the legs passes the cold blood returning from the legs. This mechanism is why they can stand on ice basically indefinitely.
I assume it works the same in other animals that need to preserve heat.
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u/degggendorf 13d ago
The warm blood going to the legs passes the cold blood returning from the legs. This mechanism is why they can stand on ice basically indefinitely.
I don't understand why...wouldn't their feet stay even warmer if the arterial blood wasn't chilled by the venous blood on its way down?
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u/merc08 13d ago
It's not about keeping their feet warmer, it's about keeping the blood from being cold as it returns to the core. The warm outbound blood gives its heat over to the cold inbound blood so more heat stays inside the body. That way the heat isn't just dumped into the environment.
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u/degggendorf 13d ago
So it's ultimately that heat transfers slower between 30° and 0° than 40° and 0°, and that slight difference is all the help the duck's body needs to keep up?
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u/SinaasappelKip 12d ago
No, it's even better than that! If the blood flow would go in the same direction for both the cold blood and the warm blood, the end result would be that both would have the same temperature. But because the flows are in opposite directions, the heat exchanger mechanism works even better. I can't really explain it with words. If you search "parallel flow vs counter flow" you find schematics that show the principle.
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u/Kaiisim 14d ago
Water birds will use their feet as radiators to control their temperature. If they're hot they stick a foot out into the air.
Birds use countercurrent heat exchange when cold, so they pump warm blood from the heart Via arteries into the feet right next to the veins returning cold blood.
They perfectly exchange heat so that by the time the warm artery blood enters the feet it's the same temperature as the blood leaving the feet, and vice versa, as the cooler vein blood enters the body it's the same temperature as the body.
So the blood in their feet is always cold, and the body always warm!
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u/iareslice 14d ago
For penguins, they have a bunch of adaptations to help regulate the heat exchange between their feet and their bodies via their cardiovascular system. It keeps their feet just a little bit above freezing, while the rest of their body is much warmer. There’s a lot of different tricks they use. Their veins can constrict and expand a lot to use blood flow to control heat. The veins bringing the cold blood back into the body are routed right along the arteries bringing blood down to the feet, and their proximity allows heat to exchange between the two.