54 
Psyche 
[Vol. 88 
occidentalis (Rogers et al. 1972). Chew (1960) estimated that no 
more than ! /2 of P. occidentalis workers were out of the nest at any 
one time. In a mark recapture analysis, Whitford et al. (1976) 
estimated the forager population at 2786 in P. rugosus. This 
estimate is higher than the one I determined which may indicate that 
the nest populations of P. rugosus in New Mexico are larger than 
those in southern California. My estimates are minimal: there may 
have been foragers which remained within the nest. Also the 
experimental channeling of the forager population may have 
affected the natural foraging activity. The whole work force may not 
have been activated because of a reduction of recruitment (Hbll-' 
dobler, Pers. Comm.). 
A comparison of the number of foragers given above and the 
number of foraging trips per day (Figure 10) indicates that 
individual P. montanus foragers make two or three trips per day, P. 
subnitidus foragers about nine, and P. rugosus foragers make more 
than ten trips per day. There are considerable differences between 
the three species in the numbers of foraging trips made (Figure 10), 
which compares with the differences in the sizes of the nest 
populations (Table 3). 
The seasonal changes in the daily amount of food brought to the 
nest are similar to those found in the numbers of foraging trips 
(Figure 11). As with the forager number, P. rugosus brings in food 
earlier in the spring and extends foraging later into the fall, 
compared to the other two species. Pogonomyrmex montanus 
abruptly increases the food input once foraging begins and de- 
creases it slowly until fall. Pogonomyrmex montanus is the only 
species of the three which does not store seeds in the nests. It may 
have to bring in large amounts of food once the larvae begin to 
appear in the nest. The other two species have seed reserves and may 
thus avoid such an abrupt increase in foraging in the spring. 
Comparisons of the food sources of the three species (Figure 12) 
indicate that the harvester ants utilize a wide variety of food items, 
although most materials are either seeds or plant parts. Pogono- 
myrmex rugosus relies almost exclusively on seeds. Pogonomyrmex 
subnitidus and especially P. montanus bring a much greater 
diversity of food items to the nest. Pogonomyrmex montanus relies 
more heavily on plant parts and insects than does P. subnitidus. 
Pogonomyrmex subnitidus brings in a greater proportion of feces 
than does P. montanus, although the ratio of bird to mammal feces 
