As an adjective, obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:

  • Obligate aerobe, an organism that cannot survive without oxygen
  • Obligate anaerobe, an organism that cannot survive in the presence of oxygen
  • Obligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphere
  • Obligate biped, Bipedalism designed to walk on two legs
  • Obligate carnivore, an organism dependent for survival on a diet of animal flesh.
  • Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.
  • Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell
  • Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host
  • Obligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions
  • Obligate symbionts, organisms that can only live together in a symbiosis

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