Dense fibers protect mammalian sperm against damage. The relative tensile strengths of the sperm of seven mammalian species and sea urchins have been measured by determining the minimum shear necessary to kill them (assayed by lack of motility) when they are suspended in a viscous fluid. In general, long sperm are killed by smaller shears than short sperm. However, the longer sperm are not as fragile as would be expected from theoretical predictions. Their additional tensile strength correlates well with the size of their dense fibers; a theory that includes the dense fiber contributions accurately predicts the sperm tensile strength for most of the species in which this has been measured. This added strength may be necessary to protect sperm from shear forces encountered during epididymal transport and especially during ejaculation, as these forces are strong enough to kill long sperm if they are not strengthened.