1985] 
Cokendolpher & Francke — Fire ants 
99 
the ants can move the brood to optimize the Darwinian fitness of 
the colony. The “optimum” temperature for fast brood development 
is often associated with reduced relative humidities which endanger 
brood survival. Alternatively, saturated air might be present only at 
soil depths where temperatures are less than optimal for develop- 
ment and survival. Thus, the ants apparently monitor these two 
environmental factors continuously, and move the brood accord- 
ingly to optimize colony growth and productivity. In addition, by 
shifting the seasonal peak of reproduction, and the daily peak of 
foraging, the ants can avoid the extremes in temperature and rela- 
tive humidity which are detrimental to their survival. 
The differences in the ecogeographic distribution of the four spe- 
cies of fire ants studied can not be explained simply by differences in 
their responses in a temperature gradient. Interspecific comparisons 
among species indicate that the most xeric species, S. aurea , has the 
lowest preferred temperatures at both 0% and 100% R. H. The two 
mesic species, S. geminata and S. invicta , follow in that order, 
respectively. The widespread species, S. xvloni , has the highest pre- 
ferred temperature at both 0% and 100% R. H., and it also showed a 
significant acclimation effect. Ants acclimated at 32° C had the 
lowest preferred temperatures, followed by those acclimated at 12°C 
and 22° C, respectively. These results suggest that ants held at 32° C 
might have been under some type of water stress, and that the 
brood-tending workers of S. xvloni responded faster to this envi- 
ronmental factor than the brood-tending workers of the other species. 
Perhaps this faster response time to the needs of the brood is corre- 
lated with the broader ecogeographical distribution of S. xvloni. 
Comparison of the temperature preferences of fire ants with those 
of other ant species have limited value because of the paucity of 
data, and the differences in experimental protocols. The only pre- 
vious study in which both moisture gradients and effects of phero- 
mones (from earlier experimental trials) were adequately controlled 
was that of Brian (1973). He reported that Myrmica rubra L. from 
England, when tested at 100% R. H., preferred temperatures in the 
19 to 21°C range. It is interesting to note that the temperature 
preferences of M. rubra are about 8°C lower than those of the fire 
ants collected at lower latitudes. Additional species need to be stud- 
ied before any generalizations can be made about relationships 
between latitudal/ altitudinal distributions of ants and their temper- 
ature preferences. 
