When males compete through sperm competition, selection favours an increased investment in sperm production to streghten the male’s probability of siring offspring when females multiply mate. This increased investment in sperm quantity results in large testes relative to the species’ size. Thus, testes size can be used as a measure of reproductive fitness in males experiencing intense sperm competition. Despite this relationship, studies investigating testes size have focused on interspecific variation, leaving the factors driving intraspecific variation in testes size an underexplored area of study. I investigated the influence of age, body condition, rainfall, parasite abundance and testosterone concentration on the testes size of Cape ground squirrels, a sciurid known to be under intense sperm competition. Data was collected from males trapped on SA Lombard nature reserve in South Africa between the years 2013-2024 and the influence of each factor on testes mass was assessed using linear regression. Testes mass was found to increase with both age and body condition and body condition also increased with age. While the energetic demands of spermatogenesis appear to shape the relationship between body condition and testes size, declining oxidative stress with age and a terminal investment strategy may drive enhanced testes investment in older individuals. Parasite abundance negatively affected testes size suggesting parasites remove resources from their host that would otherwise be allocated to reproduction. Rainfall and testosterone concentration did not affect testes size. The lack of relationships between rainfall and testes size may have occurred due to the limitations of this study, while testosterone's lack of impact on testes size may be caused by the low testosterone demands of a year-round breeding strategy. These findings aid in our understanding of the factors influencing reproductive success in males experiencing sperm competition and the selective forces shaping intraspecific testes size variation.