Look up obligate in Wiktionary, the free dictionary.

As an adjective, obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:^[1]

• 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, an animal that relies solely on walking or running on its two hind limbs for locomotion
• Obligate carnivore, an organism dependent for survival on a diet of animal flesh.
• Obligate chimerism, a kind of organism with two distinct sets of DNA, always
• Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.
• Obligately intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell
• Obligate necrophage, an animal that uses carrion as its sole or main food source and depends on carrion for survival or reproduction
• 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 scavenger, an animal that uses decaying biomass (e.g. carrion, dead plant material) as its sole or main food source and depends on this for survival or reproduction
• Obligate symbionts, organisms that can only live together in a symbiosis

• Opportunism (biological)