needing to do something or to have particular conditions in order to live

Horses are obligate nasal breathers during exercise. 马在运动时是强制性的鼻呼吸者。

As obligate parasites, viruses are dependent on their hosts.

certain to have something, such as a gene (= a part of the DNA in a cell that controls the physical development, behaviour, etc. of an individual plant or animal and is passed on from its parents)

She is an obligate carrier, absolutely certain to carry the gene. 她是确定携带者，绝对肯定会携带这种基因。