Evidence concerning occurrences of multiple-partner matings by female mammals was reviewed in Chapter 4. There, the conclusion reached was that sperm competition is widespread among extant members of the Monotremata, Marsupialia and Placentalia. Because sperm competition arises when the gametes of two or more males are situated in contention for access to a given set of ova, males that are able to produce and store larger numbers of sperm prior to mating may then gain a reproductive advantage via post-copulatory sexual selection. Greater sperm production may be achieved by increasing the mass of gamete-producing (seminiferous) tissue in the testes, or by increasing the rate of spermatogenesis. Evidence regarding both these mechanisms is discussed in this chapter. When sperm leave the testis, they pass via the excurrent ducts into the epididymis and are stored in its terminal region (cauda) prior to copulation. Transit times and numbers of sperm stored in the cauda are discussed here, as they may also be affected by post-copulatory sexual selection.