{"id":24,"date":"2007-03-30T00:03:00","date_gmt":"2007-03-30T04:03:00","guid":{"rendered":"http:\/\/web.colby.edu\/mainebirds\/2007\/03\/30\/24\/"},"modified":"2007-03-30T00:03:00","modified_gmt":"2007-03-30T04:03:00","slug":"24","status":"publish","type":"post","link":"https:\/\/web.colby.edu\/mainebirds\/2007\/03\/30\/24\/","title":{"rendered":""},"content":{"rendered":"

For the Birds – Cold and Unfeathered Legs<\/span><\/span><\/p>\n

In the last column, I described some of the ways that birds are able to survive the challenges of cold weather. I did not have enough space to describe how birds keep their unfeathered lower legs from freezing. Several readers inquired about this problem for birds in cold environments. I\u2019ll tackle that problem this week, including a foray into some chemistry.<\/p>\n

For most birds only a portion of the legs is unfeathered. The long bone you see without feathers is called the tarsometatarsus. This compound bone corresponds to the bones of your foot. The tibia (shin bones) and the femur are usually well feathered. So, it\u2019s the elongated foot bones and toes of birds that have no insulation.<\/p>\n

The unfeathered lower parts of birds\u2019 legs have a very high surface area, just the ticket for losing heat to the cold winter air. To keep the legs as warm as the rest of the body, birds would have to pump a huge amount of heat-carrying blood to those exposed areas. Instead, birds allow the legs to cool, pumping a modest amount of blood into the extremities. In Herring Gulls, the upper part of the exposed leg may have a temperature of only 40 \u00b0F and the bottom of the foot is just barely above freezing!<\/p>\n

Allowing the tissues of the lower leg to get so cold requires some fascinating modifications of the cells of those body tissues. Every living is covered with a membrane. This membrane is mostly made of molecules called phospholipids. Each phospholipid has two strings of carbon atoms, called fatty acids, pointing toward the center of the membrane. To function properly, the phospholipids of the membrane need to be able to move past each other. The membrane needs to be fluid enough to allow small molecules like oxygen to diffuse in and carbon dioxide to diffuse out. When a membrane is exposed to colder temperatures, the phospholipids pack in tightly. Oxygen cannot get into the cell; frostbite and often cell death occur.<\/p>\n

Each carbon atom in the fatty acids of a phospholipid has four binding sites. Usually, two of the binding sites are used to bond to the two adjacent carbon atoms. The other two binding sites usually bind to a hydrogen atom. However, it is possible for two adjacent carbon atoms to each lose one of their hydrogen atoms and establish two bonds with each other. When these double bonds occur, they introduce a kink in the fatty acid.<\/p>\n

You can see the effect of these types of bonds in your kitchen. Butter is made of fatty acids with no double bonds; it is a saturated fat. At room temperature, butter is a solid. Not the condition you would want for your membranes! Olive oil, liquid but viscous at room temperature, has one double bond in its fatty acids. In other words, it is a monounsaturated fat. Safflower oil is even thinner than olive oil because it has polyunsaturated fatty acids (two or more double bonds).<\/p>\n

By adding double bonds to the fatty acids of the cell membranes in the leg, birds ensure that their membranes remain fluid in cold temperatures. The kinkiness caused by the double bonds keeps the fatty acids from packing too close. The membranes remain fluid.<\/p>\n

Birds have a trick to keep their legs from losing too much heat. They arrange the arteries that bring blood to the leg and the veins that return blood to the heart as a counter-current system. You can see counter-current systems in many heating plants in large buildings. The furnace is situated at the center of the building. Cold air from outside is pumped into the furnace through a long pipe. The warmed exhaust air in the building is pumped out through a pipe parallel to the intake pipe. As the exhaust air moves toward the outside of the building, much of its heat is transferred to the colder intake air moving in the opposite direction. By the time the intake air reaches the furnace, the air has been warmed significantly by the exhaust air. Much of the heat of the exhaust air is recovered before the air is pumped outside. The counter-current arrangement of bird arteries and veins accomplishes the same energy savings.<\/p>\n

Why do most birds not have feathered legs? The answer lies in the fact that a bird\u2019s metabolism must be raised to provide the energy for flight. Excess heat is produced by this rise in metabolism and must be eliminated, even in the coldest weather, so a bird doesn\u2018t overheat. The unfeathered legs are the sites where that heat is lost.<\/p>\n

[Originally published on March 3, 2007]<\/p>\n\n

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For the Birds – Cold and Unfeathered Legs In the last column, I described some of the ways that birds are able to survive the challenges of cold weather. I did not have enough space to describe how birds keep their unfeathered lower legs from freezing. Several readers inquired about this problem for birds in […]<\/p>\n","protected":false},"author":146,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_s2mail":"","ngg_post_thumbnail":0,"footnotes":""},"categories":[1],"tags":[],"builder_content":"","_links":{"self":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/24"}],"collection":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/users\/146"}],"replies":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/comments?post=24"}],"version-history":[{"count":0,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/24\/revisions"}],"wp:attachment":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/media?parent=24"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/categories?post=24"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/tags?post=24"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}