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This recent question got me thinking about a creature concept I've tentatively named after everyone's favourite Moomins character, the Groke. In particular, the creature concept is based on her below-freezing body temperature.

So, this creature, which may or may not look like a large purple bed-sheet ghost, has a metabolism that has a highly endothermic component, which leads my grokes to hunt living beings for their body heat as much as the meat. They do not necessarily maintain this body heat when they aren't freezing a creature to death.

What I'm stumped on is, just what sort of a metabolic pathway is going on here? Is this even feasible?

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    $\begingroup$ Something is wrong here: if you can keep your body at below freezing temperature, why hunting when you can use environmental temperature already? $\endgroup$ – L.Dutch May 31 at 15:55
  • $\begingroup$ @L.Dutch maybe they live in an environment where the temperature is below body heat, or just that a living creature contains more thermal energy than is easy to obtain from air (I'd have to crunch the numbers on that one) $\endgroup$ – HAEM May 31 at 15:58
  • $\begingroup$ @HAEM The Groke is not my favourite Moomins character. The only Moomins story I ever read was "The Invisible Child" according to this question and answer: scifi.stackexchange.com/questions/210683/… and I remember very little about it. Many people on this board and others throughout the world have no knowledge of the Moomins, so saying that the Groke is "everyone's" favorite is clearly inaccurate. $\endgroup$ – M. A. Golding May 31 at 16:10
  • $\begingroup$ @HAEM then it would probably move south, or to somewhere volcanic. Murdersnacking is risky and a lot of effort, and starts to seem a bit silly when you could go somewhere which has a free all-you-can-eat buffet every day (and for somewhere with the right geology, every night, too). $\endgroup$ – Starfish Prime May 31 at 18:12
  • $\begingroup$ @StarfishPrime Thing is, polar bears also subsist on murdersnacking in a climate where moving south would be the smart solution. Evolution doesn't do what's smart, it does what's sufficient. $\endgroup$ – HAEM May 31 at 18:53
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They don’t need heat to gain energy from it. They need heat to survive the massively endothermic process of laying down their energy stores.

The Grokes evolved in a high temperature environment where food occasionally showed up in large quantities and was scarce the rest of the time. As a response they evolved a highly sophisticated, highly efficient, highly energy dense ‘fat’ analogue that uses a series of catalyst enabled endothermic reactions to ‘lock in’ as much of that energy as possible to save for later.

In their natural habitat these reactions aren’t an issue. The Groke would simply be quite cool just after feeding.

Here, however, it’s too cool. A freshly fed Groke needs to find a good heat source to keep itself warm while it’s body lays down it’s ‘fat’ stores. This means Grokes hunt not only for food, but for food and heat.

Now a sensible Groke will regulate its food intake, eating little and often and simply presenting as a cool bed sheet ghost rather than a freezing funereal shroud of death. Even more sensible Grokes have handy heat sources nearby and do their best not to kill them, as only a fool kills a still warm moomin.

Of course, once a Groke has fed it can go for a long, long time (at a more reasonable temperature) without needing extra food, or burn up those energy stores incredibly quickly (and hotly) by reversing the process. They can also survive up to quite extreme heats (which remind them of home).

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