Restricted to a single or particular mode of behavior or environmental condition; such as, an obligate aerobe that is dependent on the presence of molecular oxygen in order to breathe.

1. To bind, to compel, or to constrain by a social, legal, or moral tie.
2. To cause to be grateful or indebted; to oblige.
3. To commit; for example, money, in order to fulfill an obligation.
4. Etymology: from Latin "to bind by oath", from Old French obligier, from Latin obligare; from ob-, "to" + ligare, "to bind".

An organism that requires air or free oxygen for life: Obligate aerobes are dependent on molecular oxygen to breathe and to metabolize elements, like fats or sugars, in order to acquire energy.

An organism that can live only in the absence of oxygen: In comparison to an aerobe, obligate anaerobes are life forms, like some bacteria, that can only thrive in environments lacking free oxygen or air.

1. An animal that requires meat, or other animal products, in its diet in order to obtain the nutrients that are found in sufficient quantities.
2. An obligate, or true carnivore, is an animal that subsists on a diet consisting almost exclusively of meat.

They may consume other products presented to them, especially animal products like cheese and bone marrow or sweet sugary substances like honey and syrup; but, as these items are not essential they do not consume them on a regular basis.

True carnivores lack the physiology required for efficient digestion of vegetable matter; in fact, some carnivorous mammals eat vegetation matter specifically just as an emetic (for vomiting).

3. An animal that by its genetic makeup must eat the tissue of other animals in order to thrive.

Obligate carnivores may eat other foods; such as, vegetables, grains, or fruit, but they must eat meat as the main source for their nutrients.

An organism that can exist only as a parasite.

A predator that is narrowly restricted to a specific kind of prey.

An organism which can only live off dead organic matter. These fungi are very important as decomposers of organic material.

An organism that cannot function unless it is combined with another organism.