Tissue dependence of variation in metabolic rates of harvester ants

Metabolic rates vary tremendously among individuals and across species, and the causes of this variation remain controversial and challenging to identify. We utilized an Agilent Seahorse system to measure the metabolic rates of brains, flight muscles, leg muscles, jaw muscles, digestive tract, fat and reproductive organs of harvester ants of different species, sex and caste, and compared these to whole body metabolic rates. Across Pogonomyrmex species varying an order of magnitude in body size, mass-specific brain metabolic rates declined dramatically in larger species, such that the brain accounted for 43% of whole body metabolism in the smallest species and 14% in the largest. In Veromessor pergandei, males have much higher mass-specific metabolic rates than queens or workers, with queens being slightly higher than workers. The tissue specific bases to metabolic rates varied with sex and caste. Males are "flying gonads", with flight muscle, fat and gonads accounting for 44, 25 and 20%, respectively, of resting metabolic rate, and tissue-specific rates were generally higher in males than females. For queens, which have by far the largest fat content, the fat body contributes the largest fraction of the total tissue metabolic rate (38%), with flight muscle, femoral muscle, brain and gut each contributing 13 – 15% of total cost. For workers, brains are the most important contributors to total tissue metabolic rate (38%), followed by fat body (24%) and gut (19%). Together these results demonstrate that variation in whole body metabolic rate within and across closely related species is strongly affected by size, sex and caste-specific differences in tissue function, likely associated with life history variation. Supported by NSF IOS-1953419. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.