Tuesday, May 20, 2014

Can You See Me?

After a somewhat tortuous path, Dave's paper demonstrating signal modulation in response to predation pressure just came out in the Proceedings of the National Academy of Sciences. This study nicely illustrates the advantages of using a replicated design to conduct behavioral experiments under natural conditions. Such experiments are often characterized by unpredictability that can test the perseverance of any field biologist. In the case of this study, we had to endure tropical storms, the scorching sun, poisonwood, millions of sand-flies, and hundreds of horse-flies, in order to be prepared for the precise moment when a male Anolis sagrei would display. All of these nuisances were only exacerbated by a daily routine that included dealing with low tides that required walking in and out of islands carrying at least 20 pounds of video equipment. Kudos to Dave for collecting such a nice dataset. 

Dave feels fully hydrated after drinking two litters
 of his favorite "water"  

Dave walking to island (# 1) and wondering 
where is all the water
It has been long proposed that prey species should decrease the conspicuousness of their displays if predation risk is high. However, experimental data supporting this prediction are relatively rare, particularly for movement-based visual signals.  This study shows that A. sagrei can quickly decrease the amplitude of their head-bobs in the presence of the larger and predatory curly-tailed lizard. As illustrated by the figure below, anoles exposed to curly-tails decrease the amplitude of their displays by up to 60%. It should be noted that results presented in the study were collected in 2011, only three years after the beginning of the experiment. Therefore, the change has occurred relatively quickly. It is highly likely that this reduction in amplitude was already present in 2010; however, due to weather conditions we weren’t able to collect data for every island. 
Maximum amplitude of head-bob displays given by males Anolis sagrei  in the control and experimental islands
A second finding, which in my opinion can be a game changer in terms of how predation can impact individual fitness, is that a reduction in the amplitude of head-bob displays results in a much smaller signal active space (see figure). By integrating the motion detection parameters of the sensory system of A. sagrei, Dave was able to elegantly demonstrate that a reduction in the amplitude of head-bobs greatly reduces the signal active space of males on predator islands. For a territorial lizard that depends on signaling to attract females and to keep potential rivals away from its territory, a smaller active space should result in a reduction in reproductive success. This is an open question waiting to be answered! 
Differences in the active space of the signal between control and experimental islands. The inner circle corresponds to the active space in the presence of curly-tailed 
Another equally interesting implication of Dave's findings is the potential for changes in social dynamics resulting from smaller territory sizes.  For example, are males that were previously unable to secure territories now able to do so when the territory sizes of dominant males are reduced? Last but not least, are the results due to plasticity alone (i.e., modulation based on the perceived risk of predation) or is there some heritable variation in head-bob amplitude or accuracy of modulation that favors certain individuals? Our working hypothesis is that both of these likely interact. 

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