Graham supposed that the size difference was caused by environmental factors. Furthermore, Graham reported that Mead assumed the smaller mink specimens to be the American mink, and the larger mink specimens outside the range of the American mink to be sea minks; this may have been a case of sexual dimorphism wherein all specimens were sea minks, the larger being male and the smaller being female.[6] A 2007 study compared the dental makeup of the sea mink to the American mink, and concluded that they were distinct enough to be considered separate species.[5]