One of my hobbies is inventing fringe science theories. Here’s one of them.
There is a shrew called Eurasian least shrew (Sorex minutissimus). It weighs around 2 g (0.07 ounces). In Yakutia it lives in places where winter temperatures drop below -60C, there’s only a couple inches of snow on the ground, and immediately underneath the snow is a layer of permafrost half a mile thick. The shrew has short fur and naked snout and paws, yet remains active all winter, maintaining body temperature of 39C, which can be 100+ degrees above the ambient. This would be absolutely impossible to achieve by chemical metabolism (Hanski, 1984) even if food was abundant, which it isn’t. Sorex shrews actually decrease their metabolism and spend little time foraging in winter (Aitchison, 1987), and their metabolic rates at lower temperatures are inconsistent with chemical heat production (Schaeffer et al., 2020).
Obviously, the shrews utilize some unknown energy source. I strongly suspect cold fusion along some unknown pathway. We know that (1) nuclear fusion usually ends with iron production because iron is the first element that can’t be fused further without consuming energy rather than producing it, and (2) Sorex shrews deposit excessive iron in their teeth, hence their common name “brown-toothed shrews”. It doesn’t necessarily mean that the shrews actually run fusion all the way to iron: they could be fusing deuterium atoms on some kind of iron substrate, or maybe inside carbon-iron nanospheres.
This would explain why Sorex shrews living in coldest areas evolve to be the tiniest: the smaller you are, the easier it is for high-energy particles and gamma rays – dangerous byproducts of nuclear fusion – to escape your body without causing tissue damage. It also explains why Sorex shrews famously shrink their brains and other radiation-sensitive organs in winter (Dehnel, 1949) and switch from lipid to glucose metabolism (Thomas et al., 2023) apparently to be able to stop cell division to prevent radiation sickness. But the protection is not complete: Sorex shrews seem to have high mutation rates and some speces have numerous chromosomal races due to high chromosome instability (Zima et al. 1996).
This theory would be easy to test by catching a Sorex shrew in Alaska or Siberia in winter and checking it for isotopic anomalies and radioactivity, but this has never been done as far as I know. And if my theory is proven wrong, perhaps some studio would be interested in making a horror movie based on it?

