To infer possible functions for the low gonadosomatic index and the high proportion of non-spermatogenic tissue of capybara testes, we analysed the relationship between testes mass and the size and ratio of the testes compartments and, indirectly, their activities. We found that the testes mass of the capybara is closely associated with the size of androgen-dependent scent glands and to the proportion of non-spermatogenic tissue in the testis. We further investigated the associations between variables related to dominance and to sexual characters. The results suggest that dominance in capybaras is more associated with androgen-dependent scent glands than with sperm production. We suggest that male capybaras compete for dominance with chemical and visual status signalling.