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 See also Opportunism (biological) Biology terminology