Oxygen and the upper limits to animal design and performance

Area of Science:

• Comparative physiology
• Metabolic scaling
• Vertebrate and invertebrate respiration

Background:

• Mass-specific metabolic rates (VO2/Mb) inversely correlate with body mass (Mb).
• Hummingbirds and flying insects exhibit exceptionally high VO2/Mb, exceeding other vertebrates.
• Understanding the physiological and biochemical underpinnings of these high metabolic rates is crucial for explaining size limits in animals.

Purpose of the Study:

• To investigate the physiological and biochemical adaptations enabling high mass-specific metabolic rates in hummingbirds.
• To compare the oxygen transport and metabolic machinery of hummingbirds with flying insects.
• To explore the relationship between functional capacities, metabolic rate limits, and the minimum size of vertebrate homeotherms.

Main Methods:

• Analysis of oxygen (O2) transport through respiratory and cardiovascular systems.
• Control analyses of metabolic fluxes in energy metabolism pathways.
• Comparative studies of hummingbirds and honeybees, focusing on biochemical flux capacities and physiological rates.

Main Results:

• Hummingbirds possess enhanced lung O2 diffusing capacities, cardiac outputs, capillary-to-muscle fiber ratios, mitochondrial densities, and metabolic enzyme concentrations.
• Oxygen transport and metabolic pathways show coordinated, multi-step control rather than single rate-limiting steps.
• Hummingbirds and honeybees demonstrate a closer match between biochemical capacities and physiological rates compared to animals with lower VO2/Mb.

Conclusions:

• The high metabolic rates in hummingbirds are supported by a comprehensively up-regulated system for O2 delivery and utilization.
• Matched functional capacities across multiple physiological and biochemical steps explain the high VO2/Mb observed.
• These upper limits on functional capacities likely contribute to setting the lower size limit for vertebrate homeotherms.