THE BURDEN WITHIN - THE ENERGY-COST OF LOAD CARRIAGE IN THE HONEYPOT ANT, MYRMECOCYSTUS

We measured the energy costs of locomotion and lend carriage in two sp ecies of the honeypot ant, Myrmecocystus. The diurnal forager, Myrmeco cystus mendax, ran voluntarily in a running tube respirometer at 40 de grees C. After running for about an hour, foragers were removed and gi ven access to honey water, which they drank. The energy cost of carryi ng this internal load was then measured. The nocturnal forager, Myrmec ocystus mexicanus, ran voluntarily in the running tube at 30 degrees C . These ants could not be persuaded to collect honey water. The minimu m 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 bo dy mass. Using our voluntary locomotion regime, we found the Y-axis in tercept of the relation between metabolic rate and running speed to be equivalent to standard metabolic rate for all species and load condit ions. The cost per unit mass of internal load carriage in M. mendax (t he smallest animal to date for which we have data on load carriage cos ts) was equivalent to that of body mass carriage. Internal load carria ge therefore does not appear to offer cost savings compared with exter nal load carriage.