# Biochemists: Cellular diff between endo- & exotherms

**URL:** <https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238>\
**Category:** Factual Questions\
**Created:** [September 5, 2004, 6:15pm UTC](https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238 "2004-09-05T18:15:13Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Vlad\_Igor](https://avatars.discourse-cdn.com/v4/letter/v/7feea3/32.png) [@Vlad\_Igor](https://boards.straightdope.com/u/Vlad_Igor)\
**Post date:** [September 5, 2004, 6:15pm UTC](https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238/1 "2004-09-05T18:15:13Z")

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What biochemistry differences are there between, say, reptiles and mammals? Or perhaps phrased another way, what evolutionary change occured to allow endothermic dinosaurs or other endotherms to become exothermic? I think it has to do with more than just the TCA cycle, but I’m not **that** up on biochemistry to know where the energy would come from.

Vlad/Igor

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**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\
**Post date:** [September 5, 2004, 6:27pm UTC](https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238/2 "2004-09-05T18:27:33Z")

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See [Poikilothermic](http://www.eurofreehost.com/po/Poikilothermic.html)

> [@](#):
>
> Poikilotherms often have more complex metabolisms than homeotherms. For an important chemical reaction, poikilotherms may have four to ten enzyme systems that operate at different temperatures. As a result, poikilotherms often have larger, more complex genomes than homeotherms in the same ecological niche. Frogs are a notable example of this effect.
> 
> Because their metabolism is so variable, poikilothermic animals do not easily support complex, high-energy organ systems such as brains or wings. Some of the most complex adaptations knowninvolve poikilotherms with such organ systems. One example is the swimming muscles of Tuna, which are warmed by a heat-exchanger.
> 
> In general, poikilothermic animals do not use their metabolisms to heat or cool themselves. For the same body weight poikilotherms need 1/3 to 1/10 of the energy of homeotherms. They therefore eat only 1/3 to 1/10 of the food needed by homeothermic animals.
> 
> It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms in the same ecological niche often have much shorter generations than homeotherms: weeks rather than years.

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**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\
**Post date:** [September 5, 2004, 6:43pm UTC](https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238/3 "2004-09-05T18:43:42Z")

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> [@Vlad/Igor](#):
>
> Or perhaps phrased another way, what evolutionary change occured to allow endothermic dinosaurs or other endotherms to become exothermic?

I think you’ve got the direction reversed. If dinosaurs were reptilian in their metabolic physiology, they would have been “ectotherms” (heat is obtained from external sources). Somewhere along the way to becoming birds, they would have become “endotherms” (heat generated internally). Endothermy in general is a more derived trait than ectothermy.

Of course, it is still unresolved as to whether dinosaurs as a group were truly endo- or ectothermic (with the more correct terms being, as noted by **astro** , homeothermy and poikilothermy, respectively).

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**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\
**Post date:** [September 5, 2004, 7:13pm UTC](https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238/4 "2004-09-05T19:13:30Z")

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For more information about the distinction between endotherms/ectotherms and homeotherms/poikilotherms, see the Staff Report by myself and Doug:

[What makes some animals cold-blooded and others warm-blooded?](http://69.20.125.245/mailbag/mwarmblood.html)

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**Author:** ![Vlad\_Igor](https://avatars.discourse-cdn.com/v4/letter/v/7feea3/32.png) [@Vlad\_Igor](https://boards.straightdope.com/u/Vlad_Igor)\
**Post date:** [September 6, 2004, 2:55am UTC](https://boards.straightdope.com/t/biochemists-cellular-diff-between-endo-exotherms/263238/5 "2004-09-06T02:55:19Z")

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Terminology and links noted. Thanks, guys.

Vlad/Igor