Definition of facultative in English:



  • 1Occurring optionally in response to circumstances rather than by nature.

    ‘the difference between facultative skin pigmentation and constitutive pigmentation is a function of overall sun exposure’
    • ‘This paucity of experimental data makes it difficult to identify the ecological conditions that favor the evolution of the facultative response and of the particular environmental cues that may trigger it.’
    • ‘To date, no physiological mechanism for facultative sex ratio adjustment in birds has been identified.’
    • ‘Snakes of both groups ate readily, showing that reproduction-associated anorexia was a facultative response to lack of prey in the den.’
    • ‘These responses can and often are superimposed upon more programmed life history traits in ways that can suggest specific mechanisms by which facultative responses can become obligate.’
    • ‘Although the sample is small, this is a powerful test of the facultative response of females to changes in the composition of their group.’
    1. 1.1Biology Capable of but not restricted to a particular function or mode of life.
      ‘a facultative parasite’
      Often contrasted with obligate
      • ‘The filamentous fungus Fusarium oxysporum is a soil-borne facultative parasite that causes economically important losses in a wide variety of crops.’
      • ‘The functional significance of facultative sibling aggression is likely to depend on the magnitude of competitive asymmetry between siblings.’
      • ‘Other parasites will feed on any host available and are called facultative parasites.’
      • ‘Many species of Chlamydomonas are facultative auxotrophs, capable of utilizing acetate as their sole carbon and energy source.’
      • ‘Hughes argued that egg mimicry provides additional evidence that Black - billed and Yellow-billed cuckoos were obligate brood parasites in the past because such mimicry is unlikely to evolve in a facultative brood parasite.’


Early 19th century: from French facultatif, -ive, from faculté (see faculty).