This is specifically not thermal (blackbody) radiation, which is negligible at the visible frequency range for mice at these temperatures. The researchers find a difference in visible wavelength emission between living and dead mice at the same temperature
You should post this as a top-level comment because it's extremely informative and most of everybody is just assuming this is just talking about thermal radiation.
This point is addressed on page 2 of the paper.
Paper is accessible on bioarxiv: https://www.biorxiv.org/content/10.1101/2024.11.08.622743v1