Hypothesis
Because disease risk falls with aridity, birds' baseline innate immune function would decline from the wet forest of southern Nigeria to the dry Sahel in the north.
What failed
Five innate immune indices measured in 286 Common Bulbuls sampled between 6 and 13°N did not decrease with aridity; only nitric oxide showed a weak curved pattern. At the resampled first site, several indices rose 12 weeks into the dry season, the opposite of expectation.
Broken assumption
Immune investment tracks environmental productivity in a simple, predictable way.
Why
The authors suggest that seasonal change within a site may influence immune function more strongly than spatial differences in aridity.
From the published paper: Nwaogu et al. Scientific Reports, 2020. ↗CC BY