Species differ in their susceptibility to cancer but the underlying mechanisms and evolutionary pressures are not yet well understood. In this paper we explore the possibility that greater reproductive competitiveness may be associated with greater cancer susceptibility as a result of life history tradeoffs between fast growth or other phenotypically extreme traits that may enhance reproductive competitiveness versus somatic maintenance in the form of cancer. Our predictions are based on a mathematical model that explores the effect of reproductive competitiveness on the evolution of cancer defenses. We model optimal cancer suppression under varying tradeoff intensity, extrinsic mortality and reproductive competitiveness. Model results suggest that greater intraspecific competitiveness (in the form of winner-take-all mating opportunities) can select for lower cancer defenses, especially when life history tradeoffs between investment in reproduction and somatic maintenance are more intense. We use this model to make predictions about cancer susceptibility among species and provide support for our predictions using case studies from the comparative oncology literature.