A new British robot called Ecobot III can survive for seven days at a time by consuming biomass and excreting waste. The article points out that it's only able to convert about 1% of the material it consumes into energy so I guess it's kind of like a panda eating nutrient-low bamboo. Whether you see the takeaway here as being a robot that eats or a robot that poops is up to you.
"Diarrhea-bot would be more appropriate," lab director Chris Melhuish told New Scientist. "It's not exactly knocking out rabbit pellets."