Why did endothermy evolve?
February 26th 2009 10:41
Many animals are endothermic. That is, they generate internal heat rather than relying upon the energy acquired directly from the ambient environment. This isn't an absolute delineation among animals, since even many animals that are nominally "cold blooded" - like bees and sharks - can raise their internal temperature when they really need to (though they do so in ways that are different to the way we do it, and some of them heat only certain organs) and some nominally "warm blooded" animals can in fact change their internal temperature at times. But ambiguities aside, what benefit is there to constantly maintaining a warm body? Is it actually an adaptation in and of itself, or is it a side-consequence of something else? Maintaining constant warmth expends a lot of energy. While reptiles, for example, need to eat relatively infrequently, relying for their heat by exposing themselves to the sun (and moving out of it to avoid overheating), we wouldn't survive for very long if we stopped eating.
Continuous endothermy had been thought to evolve with small, carnivorous animals as adaptations to allow them to maintain active lifestyles. Also, high aerobic capacity has been linked to high body temperature, but the link seems to be more correlative than causal, and there are many exceptions to the "rule". There are animals with high aerobic capacities that do not have endothermy. Theropod dinosaurs, closely related to today's birds (which are warm-blooded), were likely not endothermic, at least according to the fossil evidence (velociraptors, for example, lack the ). So the idea that selection for stamina selects for endothermy doesn't necessarily hold (this isn't to discount the idea. It's just that there could be a more parsimonious account that could explain a more diverse range of cases).
Recently, a new hypothesis was proposed about the origins of endothermy, and it puts herbivores rather than carnivores at centre stage. This wonderful article by Nick Lane in New Scientist spells out a new idea about the origins of this physiological state. The idea follows from the fact that herbivores, like carnivores, need to acquire enough nitrogen to meet their needs. Eating plant matter gives you plenty of carbon but not very much nitrogen. Endothermy could have arisen in birds and mammals as a way to burn this excess carbon. There's a prediction that flows from this: if we find a lineage that has switched from carnivory to herbivory, it should exhibit endothermy. So theropod dinosaurs that evolved into herbivores - like the Therizinosauroidea - may have had structures like turbinates (found in modern birds and lacking in other theropods) to facilitate endothermy. Finding such structures would lend credence to the carbon-burning hypothesis. I should mention that there are living herbivorous reptiles and they are ectothermic but they supplement their diet with meat (though I wonder how they get rid of excess carbon. Maybe they bask more in the sun?).
A problem may be that, with a higher metabolic rate, there is a greater need for the very proteins that allow this metabolism to maintained in the first place. But, according to Marcel Klaassen and Bart Nolet of the Netherlands Institute of Ecology, the scientists who came up with the carbon-burning hypothesis, endothermic herbivores can subsist on food sources that have far less nitrogen per mouthful, so this could make up for having to eat more. One should remember that animals always face trade-offs and compromises, and this could be an example. As long as a behaviour or physiological feature is beneficial <i>on average</i>, it can be favoured by natural selection. Paying a cost is inescapable, but it needn't be prohibitive.
I'm wondering, though, why endothermic carnivores like lions and cheetahs haven't reverted to ectothermy. This is more a question, however, of why endothermy in this lineages is maintained, rather than why it evolved in the first place. Perhaps there is a constraint that prevents this reversion? This possibility may be mitigated by the fact that endothermy looks easy to evolve, at least in terms of some of the molecular changes required, so why should it be difficult to get rid of? And there are mammals that can lower their body temperatures to an extent, like those that hibernate. Perhaps, then, endothermy was just a good strategy for mammals that went on to carnivorous lifestyles, for whatever reason. Someone might know more about this than me. Is there any evidence for a negative correlation between, say, the proportion of a mammal's diet that consists of meat and their average metabolic rate? Does something like this also hold for insects (though since there are so many differences between arthropods and vertebrates, there may be little similarity between these taxonomic groups in this regard anyway)?
The article in New Scientist can be read here.
Thermal signatures of a monkey. Image from Linda Hermans-Killam / outreach@ipac.caltech.edu
Continuous endothermy had been thought to evolve with small, carnivorous animals as adaptations to allow them to maintain active lifestyles. Also, high aerobic capacity has been linked to high body temperature, but the link seems to be more correlative than causal, and there are many exceptions to the "rule". There are animals with high aerobic capacities that do not have endothermy. Theropod dinosaurs, closely related to today's birds (which are warm-blooded), were likely not endothermic, at least according to the fossil evidence (velociraptors, for example, lack the ). So the idea that selection for stamina selects for endothermy doesn't necessarily hold (this isn't to discount the idea. It's just that there could be a more parsimonious account that could explain a more diverse range of cases).
Recently, a new hypothesis was proposed about the origins of endothermy, and it puts herbivores rather than carnivores at centre stage. This wonderful article by Nick Lane in New Scientist spells out a new idea about the origins of this physiological state. The idea follows from the fact that herbivores, like carnivores, need to acquire enough nitrogen to meet their needs. Eating plant matter gives you plenty of carbon but not very much nitrogen. Endothermy could have arisen in birds and mammals as a way to burn this excess carbon. There's a prediction that flows from this: if we find a lineage that has switched from carnivory to herbivory, it should exhibit endothermy. So theropod dinosaurs that evolved into herbivores - like the Therizinosauroidea - may have had structures like turbinates (found in modern birds and lacking in other theropods) to facilitate endothermy. Finding such structures would lend credence to the carbon-burning hypothesis. I should mention that there are living herbivorous reptiles and they are ectothermic but they supplement their diet with meat (though I wonder how they get rid of excess carbon. Maybe they bask more in the sun?).
A problem may be that, with a higher metabolic rate, there is a greater need for the very proteins that allow this metabolism to maintained in the first place. But, according to Marcel Klaassen and Bart Nolet of the Netherlands Institute of Ecology, the scientists who came up with the carbon-burning hypothesis, endothermic herbivores can subsist on food sources that have far less nitrogen per mouthful, so this could make up for having to eat more. One should remember that animals always face trade-offs and compromises, and this could be an example. As long as a behaviour or physiological feature is beneficial <i>on average</i>, it can be favoured by natural selection. Paying a cost is inescapable, but it needn't be prohibitive.
I'm wondering, though, why endothermic carnivores like lions and cheetahs haven't reverted to ectothermy. This is more a question, however, of why endothermy in this lineages is maintained, rather than why it evolved in the first place. Perhaps there is a constraint that prevents this reversion? This possibility may be mitigated by the fact that endothermy looks easy to evolve, at least in terms of some of the molecular changes required, so why should it be difficult to get rid of? And there are mammals that can lower their body temperatures to an extent, like those that hibernate. Perhaps, then, endothermy was just a good strategy for mammals that went on to carnivorous lifestyles, for whatever reason. Someone might know more about this than me. Is there any evidence for a negative correlation between, say, the proportion of a mammal's diet that consists of meat and their average metabolic rate? Does something like this also hold for insects (though since there are so many differences between arthropods and vertebrates, there may be little similarity between these taxonomic groups in this regard anyway)?
The article in New Scientist can be read here.
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