Figure 1.

The metabolic rate constraint hypothesis. Metabolic rate scales nonlinearly with body mass, so small mammals (such as rodents) have high mass-specific metabolic rates. Body size also influences cellular metabolic rate. Mammals with small body size have high mass-specific metabolic rates and presumably more metabolically efficient germ cells. As sexual selection intensifies, resources are turned into sperm rapidly enough to increase sperm size. By contrast, large mammals are metabolically less efficient. As a consequence, sperm size does not increase when sexual selection intensifies.