More than meets the eye

The online version of Dave's paper on signal modulation just came out in Animal Behaviour, here is link to the pdf. This study nicely illustrates the advantages of conducting behavioral experiments under natural conditions, which usually comes with plenty of bumps on the road. In the case of this study, those bumps included several years of trouble-shooting & hours of sitting in tropical downpours, waiting for those rare moments of sun to film the displays of A. gundlachi. Thus, KUDOS to Dave for sticking with the project and figuring out an elegant experimental design to demonstrate that distance between interactive individuals can account for differences in the amplitude of head-bob displays.

Two cameras, two lizards, and plenty of sun. Picture taken at El Verde, Puerto Rico

From an animal communication perspective, this study illustrates how sensory systems can constrain the physical properties of signaling displays and how signalers can modulate the physical properties of their signals to achieve signal efficacy. By integrating the motion detection parameters of the sensory system of A. gundlachi (see blog for discussion), Dave was able to demonstrate that lizards are producing displays within the motion parameters that would result in a higher probability of detection. As illustrated by the figure below, the majority of the displays were given within the predicted range for higher detection.

Correlation between receiver distance and maximum amplitude.  Most of the head-bobs displays have an amplitude that falls within the predicted values of maximal stimulation based on the sensory system of A. gundlachi (shaded region). 

The study also sheds light on an old observation that anoles give smaller displays as they interact at closer distance. This was previously suggested to result as a natural progression of social interactions.  Although the previous statement can certainly be one factor that contributes to small amplitude displays, the fact is that smaller displays are also necessary to maximize the probability of detection at the start of displays given at close-distance. Thus, changes in signal amplitude can also be driven by selection for efficacy of communication.  

Another equally interesting implication of Dave's findings is that lizards are capable of estimating the distance at which a potential receiver is located. Although this may not come as a surprise to anyone who has watched anoles, the mechanism(s) used to estimate distance, particularly at the scale used in this experiment, is an intriguing and unanswered question that we are currently working on.

The video shows a typical display of A. gundlachi, NOTE the highly stereotype head movements.