Large Mammals Have More Powerful Antibacterial Defenses Than Expected from Their Metabolic Rates

Terrestrial mammals span 7 orders of magnitude in body size, ranging from the < 2 g pygmy shrew ( Suncus etruscus ) to the > 3900 kg African elephant ( Loxodonta africana ). Although body size has profound effects on the behavior, physiology, ecology, and evolution of animals, how investment in immune defenses changes with body size is unknown. Here, we develop a novel 12-point dilution-curve approach to describe and compare antibacterial capacity against 3 bacterial species among >160 terrestrial species of mammals. We show that antibacterial activity in serum across mammals exhibits isometry, but the serum of large mammals is less hospitable to bacteria than would be predicted by their metabolic rates. Specifically, hypometric metabolic rates would predict that a large species should have disproportionately lower antibacterial capacity than small species, but body size is unrelated to killing capacity across species. Scaling of antibacterial immune defenses provides novel perspectives on the ecology of host-pathogen interactions, and on their co-evolutionary dynamics. These results have direct implications for effectively modeling the evolution of immune defenses and identifying potential reservoir hosts of zoonotic pathogens. ### Competing Interest Statement The authors have declared no competing interest.