Leukocyte profiles reveal sex and age differences in immune investment in a polygynous bat

Developing and maintaining an immune system is critical for vertebrate survival, but when resources are limiting, investment in support of immune function has been hypothesized to trade off with the development and/or expression of traits that enhance reproductive success. Species that exhibit extreme polygyny typically have high reproductive skew, which can be expected to result in sex differences in immune investment particularly for those individuals that succeed in reproducing. Furthermore, life history theory predicts that investment in adaptive immunity should decline as an individual approaches maximum lifespan and result in immunosenescence. In this study, we evaluated these predictions by using differential white blood cell counts to assess relative investment in innate versus adaptive immunity over the lifespan of a sexually dimorphic neotropical bat that exhibits extreme polygyny. We also consider whether physiological stress or nutritional condition explains variation in leucocyte profiles. We captured and made blood smears from 511 greater spear-nosed bats, Phyllostomus hastatus, during periods of mating and lactation over 5 years and used a DNA methylation clock, recaptures of animals previously marked at a known age or sex-specific regressions of toothwear on age to estimate age. We measured cortisol concentration from urine samples using gas chromatography, tandem mass spectrometry or an enzyme-linked immunosorbent assay to measure physiological stress and sex-specific residuals of body mass relative to forearm length to measure nutritional state. We found that variation in the neutrophil-to-lymphocyte ratio (NLR) is associated with interactions between sex, age and season. NLR was higher in males than in females in all seasons and increased with age in both sexes, with the slope of age-associated variation steeper in males than in females. Both cross-sectional and longitudinal data revealed that NLR was highest in the mating season. Neither urinary cortisol nor body condition explained NLR variation within seasons. Our findings provide evidence from a long-lived bat that investment in innate immunity increases while adaptive immunity declines over the lifespan. Strikingly, this relationship occurs more rapidly in males than in females consistent with the higher mortality rate observed in males than in females of this highly polygynous species.