Males of many species have multiple physical and behavioral traits which they display to females during courtship.  There has been a great deal of interest in how multiple traits evolve, and how they are maintained by sexual selection.  To understand the evolution of multiple traits, it is critical to understand how each trait influences female mating decisions.  One way that we may do this is by examining female behaviors during courtship—as females assess male display traits.  As part of my dissertation research, I examined female courtship behaviors and male display traits in the satin bowerbird (Ptilonorhynchus violaceus).  I found that that multiple male traits may interact to influence female choice—with attractive physical traits facilitating the expression of attractive behavioral traits.

            Satin bowerbirds are native to the rainforests and eucalyptus forests of Australia.  Male satin bowerbirds have an elaborate sexual display, involving both physical and behavioral traits.  Each displaying male builds a bower, which is a two-walled stick structure supported by a platform of sticks on the ground (Figure 1).  The bower is not a nest, it is used only as a site for courting females and—if males are successful—copulating.   Males decorate their bower platforms with objects collected from the forests, and often from the homes of local farmers and researchers.  They are especially fond of bright blue, such as blue parrot feathers, flowers, ball-point pens and clothespins.  During the mating season, female bowerbirds visit multiple males for courtship at their bowers, and they may visit each male multiple times.  Once the female chooses a mate and copulates, she raises the young in her own nest without any help from the male.  This leaves males free to mate with many females, and indeed attractive males may mate with up to 25 different females per year—though most males mate rarely or not at all.

            So female bowerbirds are extremely choosy about their mates, and males have much to gain by being attractive—but what makes a male bowerbird attractive to females?  During courtship, male satin bowerbirds must give intense and aggressive behavioral displays to be attractive (Figure 2), but these same displays may threaten females, startling them and disrupting courtship.  In an analysis of natural courtships, I found that a female behavior (“crouching”) signals the degree of intensity that females will tolerate without being startled—and that females tolerate increasingly intense displays during successive courtships with a male.  Thus by giving higher intensity displays as female crouching increases, males can display intensely enough to be attractive without threatening the female with displays more intense than she is ready to tolerate.  I tested the hypothesis that male satin bowerbirds reduce the threat of their courtship displays by modulating their display intensity in response to female crouching.  To do this, I collaborated with an engineer to build robotic female bowerbirds (Figure 3), which allowed me to experimentally control crouching and measure male response in the field.  I found that males respond to increased female crouching by increasing their display intensity, such that the highest-intensity displays are given when females are least likely to be threatened.  Further, I found that males more effective at responding to female signals startle females less often in natural courtships, improving their courtship success.

            These studies with the robot bowerbird showed that female crouching is critical in determining the intensity with which a male can display, and that a male’s display intensity and the rate at which he startles females are critical in affecting his courtship success.  I analyzed natural courtships to understand what factors affect female crouching and startling behaviors.  I found that females tolerate higher intensity courtship displays from attractive males than from unattractive males—they crouch more for attractive males and startle less during early courtships.  This allows attractive males to give higher intensity courtship displays that may further enhance their mating success.  I then examined why this is so, finding evidence that bower decorations reduce the likelihood that females will be startled, such that males with more decorated bowers can give higher intensity displays without threatening females.  These results suggest that  physical display traits may facilitate the expression of more attractive behavioral displays during courtship by reducing the threat to females.  This suggests a novel mechanism by which multiple physical and behavioral traits may interact to influence female choice.  Multiple male display traits are generally assumed to function independently in influencing female choice; these results suggest that by considering interactions among traits, we may gain new insight into the evolution and maintenance of multiple male display traits.

This research was conducted at the University of Maryland, in collaboration with my graduate advisor Gerald Borgia and with my colleagues J. Albert C. Uy and Gregory Walsh.

Figure 1.  A male bowerbird at his bower (photo by Seth W. Coleman).

Figure 2.  Sequential video stills of a male bowerbird performing an intense behavioral display to a female who is observing from between the walls of his bower.

Figure 3.  The robotic female bowerbird in a bower.  For experiments, the robot was placed in a male’s bower and controlled from a blind using the remote controller shown here.