Geographic variation in baseline innate immune function does not follow variation in aridity along a tropical environmental gradient

Abstract

Geographic variation in baseline innate immune function does not follow variation in aridity along a tropical environmental gradient chima J. nwaogu 1,2,3,4* , Will cresswell 2,3 & B. irene tieleman 1 Geographic variation in aridity determines environmental productivity patterns, including large-scale variability in pathogens, vectors and associated diseases. If disease risk decreases with increasing aridity and is matched by immune defense, we predict a decrease in innate immune function along a gradient of increasing aridity from the cool-wet forest to the hot-dry Sahel, from south to north in Nigeria. We sampled blood and measured five innate immune indices from 286 Common Bulbuls Pycnonotus barbatus between 6 and 13°N. We sampled in the dry season; we resampled the first location (Jos) also as the last sample location to test temporal change in immune function. immune indices did not decrease with aridity. One immune index, nitric oxide concentration showed a weak quadratic pattern. In Jos, ovotransferrin concentration, haemagglutination and haemolysis titres increased 12 weeks into the dry season, contrary to expectations that immune indices should decrease with increased dryness. In this tropical system, innate immune function does not decrease with increasing aridity but temporal factors within a location may influence immune function more strongly than spatial variation in aridity, suggesting that immune variation does not follow a simple environmental productivity pattern. Consequently, caution should probably be exercised in predicting effects of climate variability on immune function or disease risk. Geographic variation in aridity determines environmental productivity patterns 1,2 , including large-scale differences in infectious diseases 3,4 and/or important vectors for disease transmission 5-7. Consequently, animal distributions are hypothesized to be adapted to variation in disease risk 8-10. These patterns are largely driven by spatio-temporal variability in temperature and precipitation, and they underpin the mechanisms that together or independently shape interactions among infectious agents, vectors and hosts, including host contact rate, susceptibility, infectiousness and immunity 11-13. Assuming that innate immunity-the first line of defense against infection 14 , varies with the risk of infection 15 , we might expect immune investment to decrease with decreasing environmental productivity (or increasing aridity) due to reduced immune challenge 15 or limited resource availability for immune function 16 under similar life history circumstances. Both free living and parasitic species decrease with environmental productivity, and are more diverse in the tropics 7,17-19. This high biological diversity in tropical environments is not only associated with high environmental productivity, but also with the diversity of environmental conditions within the tropics. But despite the understanding that tropical environments are relatively disease prone and capable of imposing higher immune costs compared...

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“…Studies covering different physiological processes from different regions are needed to better understand the impact of urbanisation on wildlife. Ideally, such studies should incorporate other potential influences on physiology such as spatiotemporal environmental variables such as rainfall (Ndithia et al, 2017 , 2019 ; Nwaogu, Cresswell, & Tieleman, 2020 ; Tieleman et al, 2019 ), diet (Nwaogu, Galema, et al, 2020 ; Schultz et al, 2017 ) and individual variables like body mass (Hegemann et al, 2012 ) and should use robust analytical approaches capable of unravelling inter‐connectivity between potentially influential factors (Figure 1 ).…”

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