Could ectotherms keep warm in winter?

Question

While ectotherms are generally able to operate at a wide array of temperatures, there are still limits

This would present a problem to any ectotherms wishing to visit (or even live in) colder regions. While endotherms like us can simply add more layers, an ectotherm wouldn't produce enough heat for it to be effectively kept in

The ectotherms for this question would be humanoid in their anatomy and size, and they have access to modern technology. They would need to keep their body above around 5°C, with ambient temperatures falling to just under 0°C at the coldest times

Is there an efficient/cost-effective way for these ectotherms to stay warm in winter?

Answer 1

More Winter Gear!

Insulation is the solution here. Coats, jackets, etc to prevent heat loss. Really, they would just wear more than a normal human because they do not generate much heat themselves. (Muscle movement produces heat no matter what you are.) Make sure to prevent air from pumping out of the insulating clothes. Put them on inside THEN go outside.

Couple this with modified hand warmers when going outside (a controlled oxidation reaction), and they can realistically go into cold climates. Maybe not indefinitely, but enough to get to another source of heat like a house, school, or office.

I realize this is all things we normal humans do in cold climates, but these creatures would just do it more.

Answer 2

Just Be An Icefish

(I guess this is kind of a frame challenge?)

Icefish (Channichthyidae) can live water down to -2C / 28F. (Note that seawater freezes at a lower temperature than freshwater!)

They have simply evolved proteins that are adapted to operate at those temperatures, rather than at the higher temperatures preferred by... well, almost everything else, along with natural antifreeze molecules that keep their own bodily fluids liquid at below freezing temperatures. That indicates that the limits are a good bit below your stated 5C goal, and no technological solutions or additional exotic biology would be required to survive your merely-zero-degree environment.

But, say you want them to have a 5C body temperature anyway. It's a less extreme adaptation, and when the ambient temperature only gets a smidge below 0C anyway, 5C isn't that much of a delta to maintain. As PipperChip suggested, just adding more clothes probably would be sufficient after all, as long as they move around to generate muscular waste heat. But, you can also aim to have clothing that accumulates heat passively (or, for a lower-tech solution, evolved skin that does so). All you need are pigments that are black in the visible and near IR range (or whatever the peak spectral range of your world's star is, presuming that these ectothermic humanoids do not live on Earth), while having extremely low emissivity / high reflectivity in the far IR. Just as materials with the opposite properties (low visible emissivity, high IR emissivity) can passively cool themselves to several degrees below ambient temperature (by effectively radiatively coupling to deep space), your special cold-weather clothing would be able to raise its temperature several degrees above ambient by maximizing absorption of radiation and minimizing radiative losses.

Your humanoids could also do what a lot of cold-weather birds do to minimize heat loss: employ counter-current blood flow to the extremities, allowing them to operate at colder temperatures while conserving heat in the core. That would further reduce the amount of thermal energy that needs to be collected by clothing or skin, as well as maximizing usage of any internally-produced metabolic heat.

Answer 3

Associate with endotherms.

Some of them are pretty warm. Several small ones are more easily arranged than one large one. Choose endotherms that are not very wiggly.