Why Can't all Males be Attractive? Inter-individual Variation in Male Spotted Bowerbird Display
Isden, Jessica Rose
Thesis or dissertation
University of Exeter
Reason for embargo
In order to publish material contained in the thesis
One of the greatest conundrums facing evolutionary biologists is how variation between individuals is maintained in situations where there is expected to be strong directional selection on an elaborate male trait. Sexual selection via female choice leads to the evolution of elaborate male traits, and consensus among females in their mate choice decisions can result in high reproductive skew. Such strong directional selection pressure may be expected to reduce genetic variation over time, yet high levels of inter-individual variation sustain such preferences. In this thesis I explored potential mechanisms that may maintain variation between individuals in one of the most unusual and exaggerated avian male traits; the bowers displayed by male spotted bowerbirds, Ptilonorhynchus maculatus. Choosy females exert strong selection pressure on males, yet males vary widely in their ability to exhibit a high quality display. My results showed that male bower owners were remarkably consistent in their display of decorations, a trait expected to experience a high level of fluctuation due to variation in the ecological and social environment. Given the range of factors that may introduce inconsistency, my results suggest that males actively maintain consistent displays, although I found no evidence for the adaptive benefits of doing so. I then explored three mechanisms that may be expected to maintain variation in bowerbird display. I found that attending the bower imposed physical costs on males, but these costs were only apparent in seasons of environmental stress. Males varied in their cognitive and problem-solving abilities, but I found no impact of higher cognitive performance on a male’s reproductive fitness. The final mechanism I tested was the impact of the social environment on male mating success. I found that males actively engage in marauding, a form of intrasexual competition targeting the bowers of rival males. Marauding was highly targeted and non-random within the population, and predominantly occurred between neighbouring bower owners. However, I was unable to determine what factors predicted these non-random interactions, and found no impact of the marauding rates experienced on male mating success. In the final part of this study, I looked at the novel occurrence of collaborative display between male bower owners and other non-bower-owning males. I found that these auxiliary males may gain delayed benefits from attending the bowers of experienced males, but was unable to determine what impact contributions from auxiliaries had on bower owning males and females attempting to assess them. I conclude by discussing the implications of my results for models of sexual selection.
PhD in Psychology