Biogeography predicts macro-evolutionary patterning of gestural display complexity in a passerine family

Abstract

Gestural displays are incorporated into the signaling repertoire of numerous animal species. These displays range from complex signals that involve impressive and challenging maneuvers, to simpler displays or no gesture at all. The factors that drive this evolution remain largely unclear, and we therefore investigate this issue in New World blackbirds by testing how factors related to a species' geographical distribution and social mating system predict macro-evolutionary patterns of display elaboration. We report that species inhabiting temperate regions produce more complex displays than species living in tropical regions, and we attribute this to (i) ecological factors that increase the competitiveness of the social environment in temperate regions, and (ii) different evolutionary and geological contexts under which species in temperate and tropical regions evolved. Meanwhile, we find no evidence that social mating system predicts species differences in display complexity, which is consistent with the idea that gestural displays evolve independently of social mating system. Together, these results offer some of the first insight into the role played by geographic factors and evolutionary context in the evolution of the remarkable physical displays of birds and other vertebrates.

Keywords: Animal communication; New World blackbirds; biogeography; gestural signal; multimodal display; sexual selection.

Figure 1

Phylogeny of the New World blackbird family highlighting species included in the analyses, from Powell et al. (2004). At each species’ node in the phylogeny is its hemispheric designation, mating system, and name; display complexity score is denoted by the colored bar behind this information. Highlighted are examples of family members that represent a broad range of display complexity scores, as well as geographic distributions and mating systems.

Figure 2

Differences in gestural display complexity as a function of latitudinal distribution (temperate vs tropical range). On all indices, higher scores reflect more complex gestural displays. Data are presented as the mean index score, and error bars represent 1 SEM. Asterisks (*) indicate significant differences indicated by posthoc tests (P < 0.05). Double plus signs (⁺⁺) indicate marginally nonsignificant differences (total complexity: P = 0.068).

Figure 3

Differences in gestural display complexity as a function of social mating system (monogamous or polygynous). Species with variable mating systems were removed from the analysis due to low sample size (n = 3). On all indices, higher scores reflect more complex gestural displays. Data are presented as the mean index score, and error bars represent 1 SEM.