The evolution of gestural display

How and why does gestural signaling evolve and diversify? What are the origins of gestural display? These questions remain largely unanswered, and we work to unravel them in a variety of study groups.

 

1. Gestural complexity from simple to elaborate

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This great-tailed grackle is one of many species that performs a wide array of gestural displays to communicate with rivals and potential mates alike.

One way that sexual selection drives phenotypic divergence is by acting on complexity. Indeed, gestural displays range from very simple -- a wave of the leg, for example -- to very complex. Our lab is therefore interested in studying why such signaling evolve in the first place, and how they become so elaborate. We have studied this topic in a variety of taxa, ranging from New World blackbirds (Icteridae), birds of paradise (Paradisaeidae), and toucans and barbets (Infraorder: Ramphastides). We’ve uncovered several interesting principles that help explain the emergence of complex gestural signals.

Schematic of a toucan performing an iconic tail-cock display, which exposes brightly colored plumage on the upper and under tail. This plumage can be ruffed out to enhance the signal.

Schematic of a toucan performing an iconic tail-cock display, which exposes brightly colored plumage on the upper and under tail. This plumage can be ruffed out to enhance the signal.

For example, in blackbirds, display complexity is best explained by biogeography rather than social mating system. This suggests that deep evolutionary history plays a fundamental role in shaping gestural displays. Likewise, in toucans, evolutionary routes to display complexity are relatively canalized, with display diversification arising through secondary loss of key display elements.

2. Evolution of neuromotor support for gesture

Sexual display in frogs is typically assumed to be purely acoustic, as almost all anuran species use calls to advertise for mates and threaten rivals alike. However, several frog species that inhabit acoustically noisy habitats (like high-flow streams) have evolved to use gesture as a visual signal when calling is ineffective. For the most part this takes the shape of a "foot flag", a gesture in which male frogs will raise and wave a hind or forelimb.

In order for novel signals to evolve, the neuromuscular system must be reconfigured to support the behavior. Therefore, we use frogs to understand what these modifications are, and thus to reconstruct the mechanisms of gesture evolution.

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The Bornean rock frog (Staurois parvus) is one of many anuans that uses foot-flag gestures.

Thus far, it appears that a major mechanism in facilitating foot flag production lies in locally increased androgen sensitivity; androgen receptor (AR) expression is elevated in the limb muscles of species that foot-flag, but not in species that only call. At the same time, AR expression is similarly high across frogs that call, regardless of whether or not they foot flag. This suggests that signal production is facilitated by receptor-side changes in androgen activity.