Urinary neopterin exhibits sex and age differences in wild greater spear-nosed bats

In many animals, old age is often associated with a decline in immune function and an increase in chronic inflammation. Neopterin is used as a biomarker of inflammation because it is a byproduct of macrophage activation. In humans and other primates, neopterin typically increases with age and is occasionally also elevated in young individuals, but it has not been studied in bats. Given that bats are particularly long-lived mammals for their size, studying neopterin across the lifespan may reveal whether individuals are resistant to age-related changes in immune activation. We used Enzyme Linked Immunosorbent Assay (ELISA) to estimate neopterin concentration in urine samples collected from wild greater spear-nosed bats ( Phyllostomus hastatus ) in Trinidad. Greater spear-nosed bats have an extremely polygynous mating system, with male mortality rates nearly double that of females. We estimated age from recaptures, an epigenetic clock, or tooth wear. We used ELISA and gas chromatography-tandem mass spectrometry to estimate urinary cortisol, a glucocorticoid that can influence and respond to inflammation, to assess how cortisol varies with neopterin concentration. Consistent with expectations, neopterin levels were positively associated with cortisol levels in both sexes. Unexpectedly, neopterin levels were negatively related to age in males, but independent of age in females and lower in males during the mating season. These results suggest that this species exhibits sex differences in age-related changes in immune activation that may relate to their female-biased survival.