In terrestrial placental mammals, there is a well-known negative allometric relationship between body mass and relative investment in testes mass. Such a negative relationship means that males of relatively monogamous small species invest proportionately more in their reproductive tissues than males of more polyandrous larger species. The selective pressure responsible for this relationship remains unclear and is it not known if this is a general allometric relationship that is similar across all vertebrate lineages. To investigate this, we conducted the first comparison of relationships between body mass and testes mass (using percentage testes mass as the dependent variable) across a variety of vertebrate groups. In all amniote lineages examined, the allometric relationship between body mass and testes mass was relatively strong and negative. We show, for the first time, that reptiles, birds and terrestrial placental mammals followed the same allometric relationship and, contrary to previous expectations, this relationship is sigmoidal rather than linear. Within this data set, there was no significant difference between this general amniote relationship and any of the 13 orders of reptiles, birds and terrestrial placental mammals examined. As a result, we propose that a sigmoidal relationship should be considered the default assumption for the form of the body mass - testes mass relationship within the amniote lineage. However, we also identify significant differences within some additional mammal groups (marsupials, bats and cetaceans). In each of these cases, only some sub-groupings differed significantly from the general amniote relationship. In contrast to the amniotes, the relationship is relatively weak and positive in teleost fish and frogs suggesting that a negative allometric relationship is not universal in vertebrates. We explore whether variation in the body mass - testes mass relationships can be linked to sperm competition or a variety of ecological characteristics, either for amniotes in particular or vertebrates in general.