Immune stress and diet influence reproductive fitness in male tuatara (Sphenodon punctatus)

The theoretical trade-off between immune and endocrine investment in mating animals has received mixed empirical support, particularly in reptiles. We investigated the relationship between male sexual characteristics, diet, and immune response to stress in an island population of tuatara (Sphenodon punctatus) across two mating seasons. Tuatara are promiscuous with a highly skewed mating system where males face significant competition for access to mates and post-copulatory competition for fertilisation success. We found that tuatara sperm viability and swim speed were negatively associated with the ratio of heterophils to lymphocytes and with male body condition. Additionally, sperm swim speed was negatively associated with spine area, mite load, and the total number of circulating white blood cells, but was positively associated with tick number, likely a function of social dynamics in this system where larger male size predicts greater spatial overlap with potential rivals and increased tick load. Because the production of sexual characteristics may be costly, we also investigated the effect of diet on sperm quality. We did not identify an association between diet and sperm viability. However, sperm swim speed was negatively associated with carbon-13 and positively associated with nitrogen-15. We suspect that these results reflect the influence of seabird-based nutrients in this island ecosystem, particularly poly-unsaturated fatty acids, and antioxidant damage on this tuatara sperm. In total, these results provide evidence of a trade-off between pre- and post-copulatory sexual characteristics and the immune and endocrine systems in male tuatara.