Multiple signals in chameleon contests: designing and analysing animal contests as a tournament

Traditionally, studies of intrasexual selection have focused on single traits that are more exaggerated in males. Relatively little is known about systems in which traits are larger in females or the role of multiple traits in male contests. We used a tournament design in which each male encounters a series of different opponents, in conjunction with the structured Bradley-Terry model, to examine the role of multiple male traits in contests between male Cape dwarf chameleons, Bradypodion pumilum. Females are larger but males have relatively longer tails, larger and more ornamented heads and a larger central flank patch, all of which are emphasized during agonistic displays. We found no evidence that larger body size confers an advantage in male contests, despite high levels of aggression and escalated encounters. However, both the height of the casque (head ornament) and relative area of the flank patch were positively associated with fighting ability, and not correlated with each other, suggesting that they may represent independent sources of information about an opponent's ability. We discuss these results in relation to the role of male contest competition in the evolution of multiple male signals and sexual dimorphism in dwarf chameleons. In addition, we show that the use of tournament designs, in conjunction with the structured Bradley-Terry model, has important advantages over traditional designs and methods of analysing animal contests.