To tradeoff or not to tradeoff? Is this the question?

TOURMENTE, MAXIMILIANO; ROLDAN, EDUARDO R. S.

Bakewell

Congreso; 13th Biology of Spermatozoa Meeting; 2015

It has been hypothesized that sperm competition would lead to tradeoffs between sperm numbers and sperm size if both traits share the same energetic budget. But, is this the right framework to understand the evolution of numbers and size of spermatozoa? It may not be for three reasons. First, energy budget may not be fixed. Second, tradeoffs may not be the sole factor affecting these variables since developmental constraints may also play a role. Third, sperm traits may vary depending on timing and spacing of sperm function in the female tract. We addressed, in mammals, if sperm competition maximizes sperm numbers and sperm size, and whether there is a tradeoff between these traits. Our results showed that sperm competition does maximize sperm numbers among mammals. But there was no relationship between numbers and size once the effect of sperm competition was taken into account, suggesting the absence of a tradeoff. Maximization of numbers and size of mammalian spermatozoa may occur because (a) these traits are crucial at different stages in sperm´s life, (b) are defined at different stages of sperm formation, and (c) depend on diverse energetic budgets.