WATER RELATIONS IN NOCTURNAL AND DIURNAL FORAGERS OF THE DESERT HONEYPOT ANT MYRMECOCYSTUS - IMPLICATIONS FOR COLONY-LEVEL SELECTION

The desert honeypot ant genus Myrmecocystus contains both diurnal and nocturnal species, which are subject to very different desiccation cha llenges when foraging. We measured water loss rates (WLRs) of live and dead M. mexicanus and M. mendax workers (nocturnal and diurnal forage rs, respectively) in still and moving air at 30 degrees C. In M. menda x, air flow and vital state significantly affected WLR; death increase d WLR, as did air flow (but only in live ants). The WLR of M. mexicanu s was unaffected by flow or vital state and was generally equivalent t o that of M. mendax. Cuticular permeabilities of both species 10-20 mu g H2O cm(-2) h(-1) Torr(-1)) are similar to values for other xeric in sects. The mean body water content of both species was very high at 84 % of live body mass. Coordination failed after losing 71% (M. mexicanu s), and 51% (M. mendax), of total original body water reserves. Parado xically, therefore, the diurnal M. mendex workers are operationally mo re sensitive to desiccation; accordingly, in a desiccating environment they lost coordination more rapidly than M. mexicanus workers (mean 2 7 vs. 59 hr). We therefore found no evidence that WLRs or dehydration sensitivities of these worker ants reflected the differing dehydration challenges imposed by diurnal vs. nocturnal foraging strategies and s uggest that this may reflect similar selective pressures not on worker s, but on female reproductives during colony foundation. (C) 1994 Wile y-Liss, Inc.