Free water in soil (%) Free water in soij_(%) 
34 
Psyche 
[Vol. 88 
Figure 3. Seasonal changes in the nest humidities of three species of 
Pogonomyrmex harvester ants. 
spring and low in the summer and fall. Throughout the winter, the 
soils receive relatively large amounts of rain or snow which raise the 
soil moistures to high levels. After this time, the surface and upper 
levels lose water rapidly by evaporation. The lower levels of the nest 
retain water throughout the entire season, although the percentage 
decreases. Soil moistures at levels below 30 cm are essentially the 
same for all three species. Summer showers rapidly increase soil 
moistures of the upper levels (note the peaks in the Figure 3), but 
have little effect on the levels below 30 cm. This water input into the 
soil is rapidly lost by evaporation. 
The soil moisture of the lower levels is generally higher than that 
of the upper levels, possibly forming a relative humidity gradient. 
There are more fluctuations in the higher levels, both in soil 
