Animals communicate with an amazing variety of signals, and even closely related species often exhibit remarkable diversity. Understanding the causes of this diversity is a major challenge for biologists.  Typically the most dramatic and conspicuous animal signals are those used to attract mates or repel same-sex individuals.  Two key factors have been hypothesized to give rise to signal diversity among closely related species.  First, animals must find mates of the same species, and avoid mating with other similar species, since hybrid offspring often do not survive.  Thus, natural selection is expected to favor signals that can be easily distinguished from those of similar species in the same geographical area.  Second, to be effective a signal must transmit through, and be easily detectable in, the environmental conditions where it is deployed. Closely related species often occupy distinctly different habitats, characterized, for example, by differences in vegetation structure. This may favor different signal properties for optimal transmission and detection, which may lead to evolution of different signal properties for species occupying different habitats. 

Our research focuses on the evolution of signal diversity in lizards of the genus Anolis.  There are over 400 species, and in some locations 5-10 species are found in close proximity, with each species occupying a distinctly different habitat.  For example, some species prefer un-shaded grasslands, some prefer partially shaded forest edges, some prefer heavily shaded forest, while others occupy sunny areas near the top of the forest canopy.  Males utilize a colorful, expandable throat fan, called the dewlap, to attract females and to repel other males.  Dewlap colors differ among species.  In our research we explored two hypotheses about the causes of among-species differences in dewlap color.  First, we tested the idea that dewlap colors tend to evolve towards increasingly effective stimulation of the Anolis color vision system, and that the most effective color is different in each habitat.  Second we tested the idea that pairs of similar appearing lizard species that occupy adjoining habitats tend to evolve colors that are very different in appearance, in order to reduce the probability of accidental interspecies mating.

We studied habitat light conditions, dewlap colors and color perception in Anolis lizards from multi-species communities in Puerto Rico, Jamaica and the Dominican Republic.  We carefully observed each species, measured their dewlap colors, and recorded the light conditions at the precise locations where each species displays their dewlap. We carried out detailed analyses of color perception using behavioral experiments, detailed anatomical and physiological measurements, and mathematical models of perception. 

We found that the most important factor influencing dewlap color is the level of habitat shade.  In all habitats the visual background is dominated by green vegetation.  In brightly lit (un-shaded) habitats, red and orange coloration is most visible, because it contrasts strongly in color with the green background.  However in darker, more heavily shaded, conditions red and orange colors are not highly visible, because these colors reflect only a fraction of the available visible light spectrum, and therefore appear quite dark and difficult to see. In these low-light conditions, white and yellow dewlaps are much more effective at stimulating the anoline color vision system, because they reflect much more of the total available habitat light. Using behavioral experiments we demonstrated directly that, to anoles, red and orange are most visible in high light, while in low to moderate light, yellow or white are most visible.  As expected based on these results, we found that most species from low-light habitats possess yellow or white dewlaps, while dewlaps from bright, un-shaded habitats are usually orange or red.  In several species from low-light habitats, visibility and brightness is further increased by diffuse transmission of light through the dewlap: a property known as translucence (Figure 1).

In addition there is a tendency for similar-looking, closely-related, species that occupy adjoining habitats to diverge strongly in dewlap coloration.  In each case the species from the brighter habitat has evolved an orange or red dewlap and the species from the darker habitat has evolved a yellow or white dewlap (Figure 2).  Thus the dewlaps tend to diverge in a way that increases the visibility of each species in its own habitat, while making the males of the two species very easy for a female to distinguish.

Habitat light properties can critically impact the effectiveness of animal signals.  Habitat modifications, such as cutting of vegetation, may alter the light environment.  This may lead to dramatic changes in the visibility of anoline dewlap colors and other animal signals, which might substantially reduce their efficiency for communication. Reduced signalling effectiveness may reduce the viability of a population, with potentially devastating consequence for anoline species diversity. Globally, the loss of species diversity is a major threat.  Our studies demonstrate  that alteration of signaling effeciency through habitat modification may contribute to this process.

Last Modified: 04/12/2017
Modified by: Leo J Fleishman