Sentence:  Other herpetologists have ascertained that in the <WSD>northern</WSD> United 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:  in or characteristic of a region of the United States north of (approximately) the Mason-Dixon line