SEXUAL SELECTION
Males
In the mating swarm, males outnumber queens and compete vigorously for access
to females. There is strong sexual selection on male body size, and
to less of an extent on male shape that is due to the increased mating success
of larger males. Mating takes place at the tops of hills following
a significant summer rain. Thousands of males and queens congregate
at the highest point on the hilltop, forming 'mating balls' with about 15-20
males surrounding a single female. At the center of the mating ball
there is usually a male and a queen 'in copula.' These males are nonrandom
subsets of all of the males at the mating swarm--they are larger.
Additionally, males who are successful at mating have different shapes than
'unsuccessful' males. Successful males tend to have a longer, narrower
body and they tend to have shorter mandibles with a smaller tooth surface.
These are differences in the shape of the males that are consistent in more than
one sample. Our interpretation is that successful males can compete with
males of similar body size if they can move the tip of their abdomen (and
genitalia) more effectively and if they can hold onto females more
tightly. Longer thorax and gaster relative to body size may make it easier
for males to engage in copulation. Shorter mandibles with a smaller tooth
surface means that the moment arm of the gripping tool is smaller and a smaller
surface mean that the greater force is applied over a smaller area. 
Mating ball Copulati
ng
pair
Fitness (mating success) of males 
as
a function of body size
Sperm counts of males (filled circles) and after (hollow circles) mating