Dissertation
Research

Postdoctoral
Research

Crotaphytus
Project

Research Interests 

General

I am interested in the structure and function of threat displays, the signals exchanged in aggressive interactions. I study the types of information that is expressed in threat displays, and attempt to elucidate the relationship between signal form and content. I strive to integrate between the ultimate ecological and evolutionary forces that influence signal design and the proximate physiological mechanisms that determine signal costs.  I am particularly interested in the capacity of signals to advertise locomotor performance, in displays that interrupt the respiratory cascade, and in the potential of locomotor impairment to act as a cost of signaling.

Dissertation Research
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My dissertation focussed on the threat displays of Side-blotched lizards (Uta stansburiana). I examined the potential of push-up displays and of threat posturing to advertise endurance capacity and individual identity. I developed  a novel and widely applicable approach for examining these important topics, utilizing a motorized treadmill to measure and to manipulate endurance.

First, as in earlier studies, treadmill endurance provided a direct measure physical prowess. Direct measures of physical prowess are lacking in many of the classic signaling studies, such as roaring by Red deer stags, or stotting by gazelles. I found  a robust relationship between endurance and the duration of threat posturing in lizards, whereas push-up frequency was only weakly related to endurance.

Second, by imposing treadmill exercise, endurance can be directly manipulated. Compared with baseline displays, threat posture duration was significantly reduced after treadmill exercise, whereas push-up production was not. A detailed analysis of push-up attributes, conducted with the help of J. Ryan Allen, revealed individually distinctive signatures that persist in spite of experimentally induced fatigue, suggesting that push-up structure can advertise individual identity.

Third, treadmill performance can be used to measure signal costs, by comparing baseline endurance to endurance levels after display production. Lizards experienced a significant endurance decrement as a consequence of display production. I hypothesized that threat posturing functions as a quality handicap, a signal that uses up the very attribute that is being signaled, perhaps by interfering with respiration. Consistent with my hypothesis, I found that lactate levels were higher in the post-display condition than in the baseline condition.

Barry Sinervo and I examined the effect of familiarity on the relative importance of endurance capacity and throat color badges in predicting dominance. We found that among un-familiar individuals, throat color predicted dominance, whereas among familiar individuals, endurance predicted dominance. These results suggest that lizards can flexibly switch between assessment tactics, based on the availability of previous information.

Postdoctoral Research
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In my dissertation research I found that the laterally compressed threat posture employed by lizards acts as quality handicap (sensu Zahavi), simultaneously advertising and expending locomotor endurance [PDF]. I hypothesized that the locomotor impairment imposed by threat posturing is caused by interfering with respiration.

I am conducting a quanitative  test of the effect of threat posturing on respiration in Mark Chappell's lab. Mark and I are measuring respiratory variables  in lizards before, during, and after threat posturing, to gauge the  magnitude of respiratory restriction caused by threat posturing. Further  work will test whether display costs, measured by locomotor impairment, vary between individuals as a function of their baseline locomotor performance, as predicted by models of honest signaling. Eventually, I would like to to dissect the signal value of each component of the threat display using playbacks of digitally manipulated video clips.