Http://blogs.discovermagazine.com/seriouslyscience/2015/01/29/pandas-ancestors-ditch-meat-bamboo/
According to this study, it may have had to do with the deactivation (technically known as “pseudogenization”) of an umami taste receptor gene. Umami is the taste that makes things like meat, soy sauce, and mushrooms extra yummy. Apparently, at some point in panda evolution, the umami receptor became non-functional. Based on how much the gene has changed, the authors calculate that this happened around the same time that pandas started eating bamboo. Whether it’s cause or effect is unclear, although the authors think the switch to bamboo may have happened before the gene was lost. Regardless, the loss of the gene reinforced the panda’s vegetarian diet because it made meat less delicious to the bears.
Sorry it's a lot but hope it's useful
True every living thing needs energy
Thinking it would go with The peripheral nervous system will slow down digestion to conserve energy
