Sentence:  Other herpetologists have ascertained that in the northern United States the prairie rattlesnake may not give first birth until it <WSD>is</WSD> four or even five years old , and that the young may be born every other year , rather than annually . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)