Wrong! I give you ROBOTS POWERED BY DEAD BODIES:
From Fox News:
A Maryland company under contract to the Pentagon is working on a steam-powered robot that would fuel itself by gobbling up whatever organic material it can find — grass, wood, old furniture, even dead bodies.
Robotic Technology Inc.'s Energetically Autonomous Tactical Robot — that's right, "EATR" — "can find, ingest, and extract energy from biomass in the environment (and other organically-based energy sources), as well as use conventional and alternative fuels (such as gasoline, heavy fuel, kerosene, diesel, propane, coal, cooking oil, and solar) when suitable," reads the company's Web site.
That "biomass" and "other organically-based energy sources" wouldn't necessarily be limited to plant material — animal and human corpses contain plenty of energy, and they'd be plentiful in a war zone. [bold added for scary emphasis]
(Robotic Technology Inc.'s nice touch is that the robot is steam-powered. Hurray. Not just robots -- STEAM PUNK robots.)