anolisgundlachi3

Anolis gundlachi from The Reptile Database

Plasticity is the ability of one genotype to produce multiple phenotypes under different environmental conditions. Once considered a hindrance to the study of evolution, plasticity is now thought to be one basic way organisms may persist in novel habitats long enough for adaptation to occur. Because of its propensity for rapid change, behavioral plasticity is considered one of the most effective ways that organisms can adjust to new surroundings. In some circumstances, behavioral changes can even be immediate. In example, some populations of birds can alter their vocalizations when singing against the backdrop of city noise pollution (Potvin and Mulder, 2013). This ability to instantaneously respond to external stimuli is known as contextual plasticity, and it may be a powerful tool by which organisms adapt to novel conditions. 

A new paper by Ord et. al., explores the potential use of contextual plasticity in social communication using species of Puerto Rican Anoles. Anoles use visual signals (head bobbing, pushups, and dewlapping) to communicate with one another, and previous research has noted that these signals are in competition with background visual noise (i.e. presence/movement of leaves and visual obstructions) and their reception is more challenging under low light conditions (Ord et.al., 2007). Anoles must adjust their signaling behavior in order to compete with distractions in their environment. The point of visual signaling is to be seen, so a positive relationship exists between mean visual noise and signaling speed and a negative relationship between mean display duration and mean light levels. See the decibel chart  which is used  to measure the intensity of a sound.

In this study, fourteen male Anolis gundlachi were video recorded in their natural habitats over a succession of weeks and later meticulously observed. Their signaling behavior was documented along with the background visual noise and light levels of their environment while signals were being sent. In general, under low light conditions anoles lengthen the duration of their signals and against higher levels of noise they increase the speed of their signals; both attempts should increase the probability of being seen. The behavioral and environmental data were used to create a graph for each individual that uses a line to show how a trait (i.e. signaling speed or duration) changes across different environments (i.e. varying light levels). Such a graph is known as a reaction norm. Because previous studies have utilized brief observations on large numbers of anoles to generate reaction norms for signaling behavior, the authors of this study performed arduous observations on only a few individuals so that reaction norms could be generated on an individual basis. When using a large number of brief observations to generate a reaction norm, you don’t get a clear picture of how plastic a trait may be for an individual but for a population. Because of genetic variation (even in plasticity itself), the reaction norm for any given individual can often vary widely from the norm created for an entire group. Often times, including in the current study, population norms can mask the incredible variation exhibited across individuals.

Habitats in Puerto Rico vary in terms of canopy cover, which directly affects light levels, and spatial organization, which relates to background visual noise. As a result, anole species that occupy different habitats (i.e. shady vs open) must signal against varying environmental conditions; some are brighter or dimmer and some have more or less visual noise. The advantage goes to the anoles who can best be seen in each habitat; however, no individual habitat is completely uniform and thus contextual plasticity in signaling allows individuals to be seen across a variety of microhabitats in their home range. There are also tradeoffs that exist when signaling conspecifics: being conspicuous is useful during territorial confrontation but not so helpful when predators are around. We know from previous studies that signaling behavior is highly plastic, but this study sought to answer an important question: can contextual plasticity in visual signaling help organisms be seen in novel environments? If so, then this may support the notion that behavioral plasticity can help individuals persist in new environments.

ord figureTo answer this question the authors utilized the individual reaction norms of their fourteen male A. gundlachi and compared them to reaction norms (from other studies) of other populations of A. gundlachi and other Puerto Rican species that were measured in different habitats. A comparison between these reaction norms suggests that each male A. gundlachi’s signaling behavior was plastic enough to be effective in the habitats of these other populations. In other words, the contextual plasticity in signaling behavior possessed by each individual would allow them to send effective signals in environments across Puerto Rico that they do not currently inhabit (here it is important to note that this study assumes the typical signaling behavior a species exhibits in its own environment is sufficient to be effectively seen in that environment).

In addition, this study confirmed a few things that previous studies have demonstrated: the speed of headbob displays typically increase in response to visual noise and the duration of such displays lengthen under low light conditions. Though males are capable of adjusting the speed and duration of headbob displays, most males lack the ability to rapidly increase the use of dewlaps in their displays. Previous data suggest that the use of dewlaps in territorial displays is directly related to dewlap color and size (Ord et. al., 2013).It is logical then to assume that dewlap behavior would be less amenable to contextual plasticity since its effectiveness as a signal is a function of morphology. Overall, this study illustrates how plasticity in behavior can potentially allow organisms to invade and persist in new habitats, and the authors suggest that contextual behavioral plasticity may be an effective primary response to novel conditions.

Ord TJ, Peters RA, Clucas B, Stamps JA. 2007. Lizards speed up visual displays in noisy motion habitats. Proc Biol Sci. 274:1057–1062.

Ord TJ, Stamps JA, Losos JB. 2013. Convergent evolution in the territorial
communication of a classic adaptive radiation: Caribbean Anolis lizards.
Anim Behav. 85:1415–1426.

Potvin DA, Mulder RA. 2013. Immediate, independent adjustment of call pitch and amplitude in response to varying background noise by silvereyes (Zosterops lateralis). Behav Ecol. 24:1363–1368.

Joshua Hall
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