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Psyche 
[Vol. 88 
was recently challenged by Smallwood and Culver (1979). These 
investigators conducted a study in which they found that Tapinoma 
sessile and Aphaenogaster rudis emigrated frequently. Their study 
differs from ours in that colonies were marked and rechecked only 
after intervals of 11-21 days and no attempt was made to follow the 
behavior of individual colonies. Because T. sessile and A. rudis 
choose different nesting sites and have different life styles these 
investigators deduced that emigrations occur more frequently 
among ants than had been previously thought. However, the fact 
that T. sessile, as mentioned above, is an opportunistic nester which 
is expected to emigrate frequently weakens their argument. 
It is difficult to apply any of the known or previously hypothe- 
sized causes of colony emigrations to explain the frequent emigra- 
tions of P. desertorum and P. hyatti. The nests of both species are 
excavated in the soil to a depth of 30 to 40 cm (based on excavations 
in oak-juniper woodland), and hence are not easily disturbed. 
Shading is obviously not a factor in the desert-grassland, and is 
negligible in the oak-juniper woodland where the canopy is not 
extensive. Permanent deterioration of the nest as a cause is elimi- 
nated by the fact that colonies return to former nests. Indeed almost 
half of all emigrations for both species resulted in a return to a 
former nest. This fact, and the patterns of emigrations which tended 
to keep a colony in the same area, argue against any hypothesis 
which involves a deterioration of some local condition such as might 
be due to interference competition or to a decrease in the local food 
supply. 
However, in one instance the possibility that an emigration may 
have been the result of intraspecific competition should be men- 
tioned. This involved colony H-Jnl4-1 which was observed emigra- 
ting a distance of 4.9 m the day after it was found. This distance is 
considerably larger than the mean emigration distance of 1.8 m 
found for P. hyatti. Four days later, colony H-Jnl9-1 was dis- 
covered emigrating 2.3 m into the nest vacated by colony H-Jnl4-1. 
Although large-scale conflicts between these species were never 
observed, workers will attack any alien workers of either species 
discovered near their nest. Frequent encounters of this sort may 
have caused colony H-Jnl4-1 to make its unusually long emigra- 
tion. 
In discussing the causation of any behavior a distinction should 
be made between those hypotheses that invoke a proximate cause 
