Big Daddies (originally named Protectors, and occasionally referred to as Mister Bubbles, or Mister B, by Little Sisters) are genetically enhanced human beings who have had their skin and organs grafted into an enormous diving suit, with Alpha Series being an apparent exception to the grafting process, though still heavily spliced. Their primary purpose is to protect Little Sisters while they escort them around Rapture.
Aside from the Alpha Series, Big Daddies communicate by uttering haunting sounds similar to that of whale calls. Alpha Series communicate with low, inhuman grunts and roars. For easier interpretation (as stated in the Deco Devolution art book), production model Big Daddies have a bio-luminescent chemical substance which filled the helmet, displaying its state of emotion or mind. Big Daddy helmets have three light colors: green indicates the Big Daddy is hypnotized and friendly, yellow indicates awareness but indifference to his environment, and red indicates rage towards assailants.
The yellow mode is standard; in this state, a Big Daddy will react to people near it by roughly shoving them away from himself and the Little Sister he guards. If the first few attempts to drive away potential threats prove futile, Big Daddies will become enraged. The surviving Alpha Series, from BioShock 2, are in a permanent state of wrath due to prolonged separation from their Little Sister (this excludes subjects Delta and Sigma owing to their unique circumstances.)