Bigger brains go with shorter guts, say Scientists

Brain is expensive tissue to maintain and hogs oxygen.  This is paid for with a trade-off  in smarter food harvesting which in turn means you can get by with less gut to digest the high value food you eat.   Science Daily News gives the example of some large brain Mormyrid fish (3% of body mass) compared to other Mormyrid fish with smaller brains.  (For comparison, human brains run 2.0 to 2.5% body mass.)  The same connection was found in a study of 30 species of frog and toad.

Illustration from the linked article