Fig. 10. The relationship between the ram–suction space and the ram–suction index, RSI (Norton and Brainerd, 1993). RSI values are isoclines in this space. Paired values of ram and suction distance along each line give a common value of the RSI. Thus, RSI does not reflect the absolute amounts of ram distance or suction distance, and a given value of the RSI can be formed by any value of ram distance or suction distance paired with the appropriate partner value. These relationships indicate that the RSI will not perform well as a measure of suction- or ram-feeding performance.