1985] 
Matthes-Sears & Alcock — Hemipepsis ustulata 
261 
Table 3. The effect of visual contrast of artificial perches on the frequency with 
which male tarantula hawks approached and perched on them during 15-min trials. 
Perch 
Material 
Target 
Contrast 
Number 
of Paired 
Trials 
Mean 
Approaches 
Mean 
Perching Time (min) 
Number 
of Males 
Perching 
Plastic 
Dark 
14 
10.7 ± 4.6 
0.4 ± 1.5 
2 
Mesh 
Pale 
1.6 ± 1.7 
0 
0 
cial landmark was taller than the natural one and that flexible, 
structurally complex perching materials were available at the top of 
the pole. Perch switches occurred even though the natural sites often 
had a total volume much greater than the artificial ones. The results 
then are consistent with the hypothesis that males of H. ustulata 
prefer high, conspicuous points on the skyline as territories. 
We can speculate briefly on the adaptive significance of this pref- 
erence. We begin by dismissing the possibility that males are 
attracted to certain perches because these are the best sites from 
which to release pheromonal odor plumes. Although it might 
appear that males at the highest point would be able to generate the 
longest odor trail, heated air rising off the hillsides creates thermal 
updrafts that would carry the pheromonal signal upwards, render- 
ing it ineffectual. Instead, we believe that males select the tallest 
perches at the highest points in order to inspect their surroundings 
with a minimum of visual obstruction, and this may help them 
detect approaching females. If, in addition, receptive females more 
easily perceive tall, large, and dark objects outlined against the sky 
and therefore approach these objects more often, then male wasps 
perched in conspicuous landmarks should gain more opportunities 
to mate. The competition that occurs among males for preferred 
perches could then arise as individuals attempt to secure a clear view 
of incoming females and to advertise their presence to mate- 
searching females. 
To the best of our knowledge this is the first study to identify 
experimentally what makes a potential territory site attractive to 
males of a lekking species. Although it is well-established that cer- 
tain perch-display sites are more attractive than others in such 
animals as the white-bearded manakin (Lill 1974), the hammer- 
headed bat (Bradbury 1977), and sage grouse (Wiley 1973), it is 
