Meeting Abstract
Body mass is likely to affect the way organisms evolve, develop, and use immune defenses. We investigated how variation in microbicidal capacity of serum scales with body mass among >175 species of terrestrial mammals spanning 7-orders of magnitude in size. Specifically, we tested whether predictions derived from existing theories (e.g., Protecton Theory) best-predicted slope coefficient of the microbiocidal capacity of serum collected from healthy, zoo-housed adult animals against Escherichia coli (EC). We measured microbicidal capacity at 12 serum dilutions and fit a non-linear regression to the data to describe the full shape of the microbicidal capacity. We used the curve parameters as the response variables in our scaling models. A preliminary analysis showed that phylogeny explained less than 17% of the variation of each curve parameters for EC. We then used a mixed-effects, multivariable model to simultaneously estimate the interspecific scaling exponents (b) for the curve parameters. Our response variables had repeatabilities of 1-80%. Low repeatability for some parameters was a statistical artifact partially explained by species that switched from 100% killing to 0% at a dilution close to our least concentration samples. Large species needed less concentrated serum to kill 50% of EC (b = -0.22), had higher maximal killing capacities (b = 0.52), and had steeper killing slopes than small species (b = 8.0). These results indicate that the strength of constitutive microbicidal capacity increased disproportionately with body mass. They are consistent with the performance-safety hypothesis, but additional analyses of other forms of microbicidal activity are ongoing.
