Sentence:  Other herpetologists have ascertained that in the northern <WSD>United</WSD> States the prairie rattlesnake may not give first birth until it is 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:  North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776