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[Vol. 88 
(Marikovsky 1962). In general, the biomasses of Pogonomyrmex 
are much lower than those found in other genera. 
The species investigated, especially P. subnitidus and P. rugosus, 
are comparable to most of the North American representatives of 
the genus (Table 3). The South American species apparently have 
much smaller populations, but few nests have been excavated and 
most were partial excavations in which the queen was not found or 
after the excavation was finished, additional ants were found later. 
Species from arid regions tend to have larger colonies than those 
from mesic environments, with the exception of P. laticeps. The 
colonies of North American species live longer than South Ameri-* 
can species (Kusnezov 1951). Pogonomyrmex montanus is some- 
what atypical for the genus in occurring at higher altitudes, but is 
similar to other species in several aspects. The number of nests per 
hectare is comparable to several other species including P. badius, 
P. barbatus, P. occidentals, P. owyheei, P. rugosus, and P. 
subnitidus. The nest populations of P. montanus are smaller than 
those of most of the other species, but the number of workers/ m.sq. 
and/or the dry wt/m.sq. are comparable to P. badius, P. calif orni- 
cus, P. occidentals, P. owyheei, P. rugosus, and P. subnitidus. 
With regards to the populations, the three species investigated 
appear to be “typical” North American Pogonomyrmex harvester 
ants. It would be very interesting to do a comparable study of 
“typical” South American Pogonomyrmex harvester ants. 
Effect of altitude. 
It was anticipated that altitude would have three primary 
effects: 1) The higher altitude species, P. montanus, would be 
subjected to lower average temperatures. 2) The higher altitude 
species would be subjected to shorter foraging seasons, thus 
reducing the yearly food input into the nest, resulting in lower 
production. 3. The higher altitude’s shorter growing season would 
result in fewer available seeds from annual plants. 
Although P. montanus is subjected to the lowest seasonal 
temperatures of the specific populations of the three species 
investigated (Figure 2), it metabolically compensates for this by 
having higher respiratory rates than the other species (MacKay 
1981). Apparently altitude has an effect on foraging, although it was 
not as large as expected. The foraging season was somewhat 
reduced in P. montanus and P. subnitidus, when they are compared 
