No Evidence of Trade-Offs in the Evolution of Sperm Numbers and Sperm Size in Mammals

Document Type


Publication Title

Journal of Evolutionary Biology

Publication Date



Post‐copulatory sexual selection, in the form sperm competition, has influenced the evolution of several male reproductive traits. However, theory predicts that sperm competition would lead to trade‐offs between numbers and size of spermatozoa because increased costs per cell would result in a reduction of sperm number if both traits share the same energetic budget. Theoretical models have proposed that, in large animals, increased sperm size would have minimal fitness advantage compared with increased sperm numbers. Thus, sperm numbers would evolve more rapidly than sperm size under sperm competition pressure. We tested in mammals whether sperm competition maximizes sperm numbers and size, and whether there is a trade‐off between these traits. Our results showed that sperm competition maximizes sperm numbers in eutherian and metatherian mammals. There was no evidence of a trade‐off between sperm numbers and sperm size in any of the two mammalian clades as we did not observe any significant relationship between sperm numbers and sperm size once the effect of sperm competition was taken into account. Maximization of both numbers and size in mammals may occur because each trait is crucial at different stages in sperm's life; for example size‐determined sperm velocity is a key determinant of fertilization success. In addition, numbers and size may also be influenced by diverse energetic budgets required at different stages of sperm formation.

Funding Source

This work was supported by the Spanish Ministry of Economy and Competitiveness (grants CGL2011‐26341 and CGL2012‐37423). J.d.‐T. held a ‘Ramón y Cajal’ fellowship (RYC‐2011‐07943) whereas M.T. has a ‘Juan de la Cierva’ fellowship (JCI‐2011‐10381) both from the Spanish Ministry of Economy and Competitiveness.


This article was originally published as Tourmente, M., delBarco-Trillo, Javier, Roldan, E.R.S. (2015) No Evidence of trade-offs in the evolution of sperm numbers and sperm size in mammals. Journal of Evolutionary Biology, 28(10): 1816-1827.