The Burden within: The Energy Cost of Load Carriage in the Honeypot Ant, Myrmecocystus

We measured the energy costs of locomotion and load carriage in two species of the honeypot ant, Myrmecocystus. The diurnal forager, Myrmecocystus mendax, ran voluntarily in a running tube respirometer at 40° C. After running for about an hour, foragers were removed and given access to honey water, which they drank. The energy cost of carrying this internal load was then measured. The nocturnalforager, Myrmecocystus mexicanus, ran voluntarily in the running tube at 30° C. These ants could not be persuaded to collect honey water. The minimum costs of transport (MCOT)-160 and 105 J kg⁻¹ m⁻¹ for M. mendax (6 mg) and M. mexicanus (14 mg), respectively-were substantially below values estimated from current allometric equations relating MCOT to body mass. Using our voluntary locomotion regime, we found the Y-axis intercept of the relation between metabolic rate and running speed to be equivalent to standard metabolic rate for all species and load conditions. The cost per unit mass of internal load carriage in M. mendax (the smallest animal to date for which we have data on load carriage costs) was equivalent to that of body mass carriage. Internal load carriage therefore does not appear to offer cost savings compared with external load carriage.