[geor9e]

Animals don't give off much waste heat. They evolved not to. And it's very hard to extract energy from small temperature gradients. The thermoelectric chargers that convert body heat to energy are only ~1% efficient. Compare to sunlight, which gives ~20x the watts per square inch, and can be converted at 20% efficiency. So a body heat charger needs to be roughly 400 times as big as a solar panel does.

[giantg2]

This doesn't seem right. Anyone who has been in a cattle barn knows the kind of heat that cows give off. The energy demand for the sensors should be very small. 20% efficiency for the solar cells might be true on paper, but not in practice - diet, orientation, even needing 4x the solar cells because you cant guarantee the cells will always face up (collar rotates).