Cold Feet Warm Heart -- By Countercurrent Exchange

March 22, 2018

Bufflehead in frozen cove 3 (1)

March 2018 Viewer's ChoicePhotographer: Rob Sheridan 
Summary Authors: Rob Sheridan 

This tough little male bufflehead duck (Bucephala albeola) is wintering in a partly frozen cove off the Massachusetts coast, having migrated from summer breeding grounds in northern Canada. Buffleheads are diminutive diving ducks that congregate in small groups in open, shallow waters amongst sea ice. They feed on a wide range of invertebrates that they find on the seafloor. The bow wave and wake created by the little bird’s paddling feet can be seen in the photo, as can nearby sea ice.

Being warm-blooded, the bufflehead must preserve its core heat, while still providing nutrition and oxygen to these feet in the extremely cold salt water, which can have a temperature below the freezing point of fresh water because the dissolved salt depresses the liquid-to-solid phase transition temperature. An ingenious countercurrent heat exchange system evolved in the proximal legs of such birds. So warm arterial blood leaving the heart for the legs transmits much of its heat to returning venous blood, preserving the heat for the animal’s core while still providing nutrition and waste removal for the distal legs and feet via the cooled arterial blood. If temperatures become critically cold, threatening freezing of the feet, pulses of warm arterial blood will bypass this system to keep them just above freezing temperature.

Also important is the evolved paucity of muscle tissue in the feet, with muscles moving the feet located proximal, near the warm core and connected to the feet by long tendons that are relatively impervious to cold. Remarkably, a similar countercurrent exchange system and proximal muscles with long tendons evolved separately in cold water marine mammals to preserve core temperature while providing nutritional blood flow to moving extremities. Given time, evolution is an incredibly clever engineer. Photo taken on January 7, 2018.

Photo Details: Camera: NIKON COOLPIX P900; Exposure Time: 0.0008s (1/1250); Aperture: ƒ/6.5; ISO equivalent: 220; Focal Length (35mm): 2000; Software: Windows Photo Editor 10.