obligate

Etymology

Borrowed from Latin obligātus, past participle of obligō. Doublet of oblige, taken through French.

verb

1. (transitive, Canada, US, Scotland) To bind, compel, constrain, or oblige by a social, legal, or moral tie.
2. (transitive, Canada, US, Scotland) To cause to be grateful or indebted; to oblige.
3. (transitive, Canada, US, Scotland) To commit (money, for example) in order to fulfill an obligation.

adj

1. (biology) Requiring a (specified) way of life, habitat, etc..
   [A]nalysis of the chemical composition of their bones reveals that they were obligate carnivores. 2018, Tim Flannery, Europe: The First 100 Million Years, Penguin, published 2019, page 171
2. Indispensable; essential; necessary; obligatory; mandatory; unavoidably invoked.
   In addition to being the obligate food source for monarch caterpillars, milkweeds also provide abundant nectar for the adult butterflies.
   In some languages such signaling is optional, whereas in others it is obligate.
   Aquatic sites constitute obligate habitat for some species, and are critical breeding habitat for species with complex life cycles involving aquatic egg or larval development. 2009, C. Kenneth Dodd Jr., Amphibian Ecology and Conservation: A Handbook of Techniques, page 304
   Unlike for phagotrophic flagellates, bacteria serve as a facultative rather than an obligate food source for crustacean zooplankton. 2012, Ulrich Sommer, Plankton Ecology: Succession in Plankton Communities, page 351
   Light is the obligate energy source for the phototrophic microbes constructing these benthic mats 2013, K.C. Marshall, editor, Advances in Microbial Ecology, volume 11, page 472