A foundational and widely supported concept in behavioural ecology is that female reproductive success is primarily limited by access to resources while male reproductive success – by access to mates. Research programs (especially in the field of primatology) have thus emphasized the substantial costs of female reproductive effort but have tended to focus on variation in the benefits obtained by males. However, the processes necessary to achieve reproductive success carry high costs for males also, and the ability to sustain these costs may determine the success of some males and the failure of others. In this talk I use my work on chimpanzees and rhesus macaques, two primate species with contrasting social and mating systems, to demonstrate some hidden physiological costs of male mating effort. Measuring physiological markers of energy balance, immune function and oxidative stress allows us to investigate the relationship between males’ condition and their ability to invest in reproduction, as well as to assess the subsequent costs of doing so. Placing the physiological costs of male mating effort in their social context, I will also discuss how they might have shaped the unusual pattern of dominance rank acquisition among the particular provisioned population of rhesus macaques I worked with, where unlike in most other primate species males queue, rather than fight, for the alpha position.