Is it possible that an endothermic physiology can evolve into an ectothermic (poikilothermic) physiology? I have a non-scientific gut feeling that it is irreversible. Whales and dolphins are evolved from land mammals, and they've remained warm blooded. While it doesn't give them any obvious advantage in the water, apparently they've compensated with massive amounts of body fat instead of devolving. If the ocean offers opportunity for mammals, then why haven't fish evolved to be warm blooded directly?
What do you mean by cold-blooded?
There's an issue with what you mean when saying "cold blooded". The correct words you may want to use are homeotherm, poikilothermic, ectotherm, and endotherm. In short…
Source of heat
- endo = inside
- exo = outside
Variance in warmth
- Poikilo = varies
- homeo = does not vary
Any combination of these two axes exist. For example: If the temperature in the environment never varies you can be homeotherm without needing to be endotherm. A fun example also are the large dinosaurs that are thought to be homeotherms because their metabolism produce some heat and they are so large that they remain warm thanks to this heat source. However, they were probably not able to regulate actively their temperature. Therefore, I would tend to qualify dinosaurs as homeo-exo-therm but I wouldn't be surprised if someone prefers to call large dinosaur home-endo-therm individuals. Hope that makes sense to you.
Note that the terminology might be a bit more complicated in reality as the diversity of mechanisms of body temperature regulation is important.
What did you mean in your question when talking about "cold blooded" species?
Transition from endothermy to ectotherm
Thanks to @tchrist. naked mole rat is ectotherm while its ancestors where endotherm.