The ratio of protamine 2 to protamine 1 and transition nuclear proteins has been found to change the sperm head shape in various species of mice, by altering the expression of protamine 2 via mutations in its promoter region. A decrease in the ratio has been found to increase the competitive ability of sperm in Mus species. However, further testing is required to determine how this ratio influences the shape of the head and whether monogamy influences this selection.