Sentence:  The best way to determine the correct figure ( in captives ) <WSD>is</WSD> by direct observation of pairs isolated from birth , a method that produced surprising results : maturing of a male Indian python in less than two years , his mate in less than three ; data on the boa constrictor about match this . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)