Unlike many coolwater, freshwater species it is possible to visually determine the sex of mature muskellunge. In adult female muskie the urogenital region resembles the shape of a pear, while in adult male muskellunge the shape of the region resembles a keyhole (Figure 2) (Lebeau and Pageau 1989). Sexually dimorphic growth occurs with muskellunge, as females reach larger ultimate sizes than males. Another interesting fact about the muskellunge is that there is evidence that muskie have a different sex-determining system than northern pike. While northern pike appear to use an XY system similar to ours (with males being XY and females being XX), research by Dabrowski et al. (2000) uncovered evidence of a WZ system (with males being ZZ and females being WZ) in muskellunge. Simply stated, in northern pike, males determine the sex of offspring, while in muskie, females determine the sex of offspring.