INTERINDIVIDUAL VARIATION IN ENERGY-COST OF RUNNING AND LOADING IN THE SEED-HARVESTER ANT, POGONOMYRMEX MARICOPA

The minimum cost of transport (MCOT) was measured in voluntarily runni ng unladen and experimentally laden Pogonomyrmex maricopa (mean mass = 8.9 +/- 1.4 mg). MCOT did not vary with temperature over a 10 degrees C range in which P. maricopa are normally active in the field. Y-axis intercepts varied significantly among individuals, and were greater t han the standard metabolic rate (SMR) for both unladen and laden ants. The mean intercept, or cost elevation above SMR, was greater for lade n ants (2.10 vs 1.06 J kg(-1)s(-1)). When speed was multiplied by load ratio, all ants shared the same MCOT, indicating the equivalence of u nit load and body carriage cost (load ratio range = 1.07-2.20). The co mbined overall MCOT for unladen and laden ants was 129 +/- 6 J kg(-1)m (-1), 61% less than predicted based on a current scaling equation. The energy transactions of P. maricopa and P. rugosus are compared.