The term obligate can also be used in a biological context, in reference to species which must occupy a certain niche or behave in a certain way in order to survive. In biology, the opposite of obligate is facultative, meaning that a species is able to behave in a certain way and may do so under certain circumstances, but that it can also survive without having to behave this way. For example, species of salamanders in the family Proteidae are obligate paedomorphs, whereas species belonging to the Ambystomatidae are facultative paedomorphs.
In the Catholic Church, Holy Days of Obligation or Holidays of Obligation, less commonly called Feasts of Precept, are the days on which, as canon 1247 of the Code of Canon Law states, the faithful are obliged to participate in the Mass.
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