Sentence:  Laurence M . Klauber put length at maturity at two thirds the ultimate length for some rattlesnakes , and Charles C . Carpenter 's data on Michigan garter and ribbon snakes ( Thamnophis ) show that the smallest gravid females <WSD>are</WSD> more than half as long as the biggest adults . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)