The eastern massasauga rattlesnake pictured is a female. We know this because she has 21 subcaudal scales, which are found on the underside of the tail. Males have 25 to 33 and females have 19-29. Believe it or not, most animals are sexually dimorphic. Sexual dimorphism, simply put, is a set of unique, observable characteristics that distinguish sex in a species. Observable is a key word here because there are many small details that often get overlooked. Laurence M. Klauber took the time to note many of these characteristics in his classic two volume book Rattlesnakes: Their Habits, Life Histories, and Influence on Mankind, published in 1972. I have always used this fantastic book as a reference. It has helped me to determine the sex of eastern massasaugas by counting the subcaudal scales.
– John Adamski, Assistant Curator