A method and tool for comparing mammals, such as horses, the tool including an electrode; an electronic heartbeat measurement instrument; an analyzer for determining an output indicative of adaptability, reactivity, or equanimity; and a plotting system for plotting the temperament parameter or temperament quotient on a grid. The tool is used to provide a value for the sympathetic nervous system index (SNSI), the parasympathetic nervous system index (PNSI), or the standard deviation of average mode normal to normal intervals (SDMNN). This value is correlated to a selected characteristic of the mammal.