The immunological variation of wild populations in relation to life-history traits has recently become a central topic in evolutionary biology, given the critical contribution of immunity to an individual’s fitness. Specific induced defenses, which require substantial time and resources and are mostly beneficial against repeated infections, are expected to be favored in “slow-living” species. To test this prediction, understanding the costs and benefits of immunity is essential. Here, we evaluated the energetic costs of activating two different arms of immune defense (humoral and inflammatory) in the Argentine subterranean rodent Ctenomys talarum (tuco-tucos), a “slow-living” species. While a significant increase in oxygen consumption was verified when tuco-tucos mounted an antibody-mediated immune response against sheep red blood cells (SRBC), no significant energetic cost was detected during the inflammatory response triggered by phytohemagglutinin (PHA), which in tucos has components of the innate as well as the adaptive response. However, PHA-induced inflammation was negatively affected by infection with naturally-occurring gastrointestinal parasites and in animals under food restriction, suggesting that currencies other than energy are mediating the costs of inflammation in tuco-tucos. When both arms of defense were activated simultaneously, we found no evidence that the humoral response was favored over the inflammatory response, despite tuco-tucos’ slow pace of life. The relevance of other factors, such as pathogen exposure in the subterranean environment and the low basal metabolic rate of tuco-tucos, is discussed in relation to the immune variability exhibited by these rodents, and the implications for natural populations of wild mammals.