1976] 
Alcock — Populations of Centris pallida 
129 
females. Within a large emergence area there are patches from 
which a relatively large number of females emerge over a period 
of days. For example in Study Site 2, one area of about 10 m 2 pro- 
duced a large proportion of the total copulations we observed at 
this location. The tendency of patrolling males to return to these 
areas of higher-than-average productivity is clearly adaptive, al- 
though the underlying basis for the ability to locate such sites is 
not known (perhaps males are attracted to sites that would attract 
nesting females, or perhaps males can judge from the abundance 
of digging males or numerous contacts with females that a par- 
ticular location is superior and deserving of continuing inspection). 
The failure of males to defend searching areas is almost certainly 
a function of the high density of fellow searchers which would 
make defense of any but the smallest area extremely costly in 
terms of time and energy. Indeed, males become aggressive only 
when digging at a spot potentially containing a buried virgin female, 
repelling competitors from an area a few cm 2 around their body 
(Alcock et al., 1976; in press). 
Summary 
Males of the bee Centris pallida that patrolled emergence sites 
in search of emerging females possessed highly overlapping home 
ranges of 10-25 m 2 . Marked patrollers were found in the same 
general area for an average of 3.4 days. Males that hovered at 
spots while waiting for receptive females to fly near them pos- 
sessed weakly defended non-overlapping territories a few m 2 in 
size. Marked hoverers rarely returned to a specific hovering site 
and averaged only 2.6 days from the date of capture to time of 
last sighting. Relatively long-term occupation of a home range 
may be adaptive for patrollers because of the clumped distribu- 
tion of emerging females. In addition the high density of male 
competitors would make defense of a large area difficult and 
energetically expensive. Relatively short-term occupation of a 
hovering territory may be adaptive for hoverers because there 
are many potential hovering sites, all of which have the same low 
value to a male. This reduces competition for any one site making 
its defense feasible; at the same time, the low probability of mating 
