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

Area of Science:

• Immunology
• Ecology
• Evolutionary Biology

Background:

• Mammals vary greatly in body size, from shrews to elephants.
• Body size impacts physiology and ecology, but its effect on immune defense scaling is unknown.

Purpose of the Study:

• To investigate how antibacterial capacity scales with body size across terrestrial mammal species.
• To test the safety factor hypothesis predicting hypermetric scaling of immune defenses with body mass.

Main Methods:

• Developed a novel 12-point dilution curve assay to measure antibacterial capacity.
• Assessed serum antibacterial activity against three bacterial species in over 160 mammal species.
• Applied three statistical approaches to analyze the scaling of immune defense with body mass.

Main Results:

• Antibacterial activity in mammal sera exhibited isometry, meaning killing capacity did not change with body size.
• Contrary to the safety factor hypothesis, immune defense did not scale hypermetrically with body mass.
• Serum of large mammals was less hospitable to bacteria than predicted by metabolic rates.

Conclusions:

• Mammalian antibacterial capacity is size-independent across species, challenging metabolic scaling predictions.
• These findings have implications for understanding immune evolution and pathogen reservoir dynamics.
• The study highlights a novel aspect of immune system adaptation across diverse mammal body sizes.