NewScientist - What's the point of being warm-blooded?
This relatively detailed article discusses the differences between warm-blooded creatures (endotherms) and cold-blooded ones (ectotherms), as well as the two competing theories as to why endotherms have evolved at all. Ectotherms are very efficient-- they don't have to be constantly eating-- they can effectively shut down some or all of their bodies to conserve energy that endotherms waste on a constant basis. (This distinction isn't hard and fast.) The first theory that was developed about 30 years ago observed that being endothermic enables higher stamina-- the ability to sustain longer physical exertion in a hunt or fight. Because of the selective advantage granted by this ability, endotherms evolved. The problem with this is that the endothermic heat-generating organs are largely those with more mitochondria (the brain, liver, and other visceral organs that are always functioning) not the cardiovascular system and muscles that can supply stamina. The alternate model being developed currently is that herbivores need many nutrients to build proteins and DNA (for more cells) but among them they need both carbon and nitrogen. The difficulty for them is that the amounts of nitrogen in plants is a fraction of the amount of carbon. Thus to get enough nitrogen you need to eat a lot more carbon than your body can reasonably use efficiently (if you're ectothermic). The evolutionary solution? Increase mitochondrial production (by switching on perhaps just one gene) and burn the carbon off with heat. This new theory posits that the Cretaceous period (140m years ago), which began to see flowing plants with a higher nitrogen content, created an opportunity to evolve this trait.
This relatively detailed article discusses the differences between warm-blooded creatures (endotherms) and cold-blooded ones (ectotherms), as well as the two competing theories as to why endotherms have evolved at all. Ectotherms are very efficient-- they don't have to be constantly eating-- they can effectively shut down some or all of their bodies to conserve energy that endotherms waste on a constant basis. (This distinction isn't hard and fast.) The first theory that was developed about 30 years ago observed that being endothermic enables higher stamina-- the ability to sustain longer physical exertion in a hunt or fight. Because of the selective advantage granted by this ability, endotherms evolved. The problem with this is that the endothermic heat-generating organs are largely those with more mitochondria (the brain, liver, and other visceral organs that are always functioning) not the cardiovascular system and muscles that can supply stamina. The alternate model being developed currently is that herbivores need many nutrients to build proteins and DNA (for more cells) but among them they need both carbon and nitrogen. The difficulty for them is that the amounts of nitrogen in plants is a fraction of the amount of carbon. Thus to get enough nitrogen you need to eat a lot more carbon than your body can reasonably use efficiently (if you're ectothermic). The evolutionary solution? Increase mitochondrial production (by switching on perhaps just one gene) and burn the carbon off with heat. This new theory posits that the Cretaceous period (140m years ago), which began to see flowing plants with a higher nitrogen content, created an opportunity to evolve this trait.
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