Obligate
# Obligate

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 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.
* 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
