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
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. To understand how gestural signaling evolves, we first must figure out why some species have a more complex gestural repertoire than others.
We first explored this topic in New World blackbirds (Icteridae), which exhibit a wide variety of gesture complexity. While some blackbirds don't gesture at all, others perform highly complex routines.
While exploring the evolution of gestural complexity in blackbirds, we found that 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. We are now exploring the link between gesture and other displays-- including vocalizations-- in this group.
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.
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.