Anole lizards, a diverse group of more than 400 species primarily inhabiting the Americas and Caribbean, are recognizable reptiles due to a striking feature beneath their chin. This structure, a brightly colored flap of skin known as the dewlap, is central to the daily life and social interactions of these small lizards. The dewlap functions as a dynamic, visual communication tool, rapidly deployed to convey complex messages. This expandable throat fan acts as a flag, signaling intent, identity, and status across species and individuals.
The Anatomy and Structure of the Dewlap
The dewlap is an elastic tissue fold ordinarily kept retracted under the lizard’s throat. Its deployment is an active, muscular process driven by a specialized skeletal element known as the hyoid apparatus. This structure, composed of bones and cartilage, is modified in anoles to serve as a lever for the display.
Extension is achieved through the coordinated contraction of specific muscles, notably the M. ceratohyoideus. This muscle pulls on the ceratohyal elements, causing the hyoid apparatus to rotate and act as a lever. This movement swings the elongated second cerabranchials forward and down, forcing the folded skin of the dewlap to unfurl rapidly and expose its full surface area.
The vibrant coloration is achieved through a combination of pigments and structural components within the skin. Pigments such as carotenoids, pterins, and melanin are responsible for the red, yellow, and orange hues frequently observed. Deeper in the skin, light-reflecting platelet arrays, called iridophores, produce structural colors and reflectivity, including strong ultraviolet (UV) reflectance easily perceived by anoles.
The dewlap’s color and pattern are generally considered an unchangeable trait once an individual matures, providing a reliable species-specific signal. Some anoles can regulate the color of the gorgetal scales, suggesting a limited ability for chromatophoric change. This combination of skeletal mechanics, muscular control, and coloration allows the anole to produce a highly visible signal from a small, retracted flap of skin.
Communication Roles and Display Behaviors
Anoles use their dewlaps in a complex signaling system that combines the visual flash of color with characteristic body movements. The display is used for establishing territory, attracting mates, and signaling to other species, including potential predators. The visual signal is often augmented by synchronized acts such as head-bobbing and push-ups.
In territorial defense, a male anole deploys its dewlap to warn rivals that they have encroached upon its home range. This aggressive challenge display is typically coupled with a precise sequence of head-bobs and push-ups, which makes the lizard appear larger and more intimidating. The cadence and duration of the display sequence communicate the male’s fighting ability and willingness to defend his territory.
The dewlap is also a central component of courtship rituals used to attract potential female partners. A male displays his brightly colored dewlap toward a female to advertise his species identity and individual quality. Females are attuned to these visual signals, using the dewlap’s color, size, and the vigor of the male’s display to choose a suitable mate. The quality of the display indicates the male’s overall health and genetic fitness.
Intraspecific signaling, which includes both aggression and courtship, relies heavily on the temporal pattern of the display. Each anole species has a characteristic Display Action Pattern (DAP), a unique combination of dewlap extension, retraction, and head-bobbing movements. This species-specific code ensures a lizard can recognize members of its own species, even when sharing a habitat with several other anole species. The rapid flash of the dewlap is also utilized as a pursuit-deterrence signal toward predators, startling them long enough for the anole to escape.
Dewlap Diversity and Ecological Significance
The variety of dewlap size, shape, and color across the genus Anolis is remarkable, with species exhibiting combinations of more than a dozen distinct colors. This variation acts as a primary mechanism for species recognition where multiple anole species coexist. The unique dewlap configuration prevents interbreeding and maintains species boundaries among sympatric populations, which is important in the diverse Caribbean islands.
The evolution of dewlap characteristics is strongly influenced by the specific light environment of an anole’s habitat, a process known as sensory drive. Species inhabiting dense, shady forest interiors often have larger dewlaps that are less brightly colored but possess higher UV reflectance. This ensures the signal is visible in the low-light conditions of the understory, where the contrast of a large, pale flap against the dark background is maximized.
Conversely, anoles living in open, sunny habitats, such as tree trunks or fence posts, often possess smaller, more intensely colored dewlaps. In these bright environments, the signal’s contrast relies less on size and more on the saturation and specific wavelength of the color. This prevents the message from being washed out by direct sunlight and ensures the communication signal remains effective across different ecological niches.
While structural habitat categories known as “ecomorphs” (e.g., trunk-ground, twig) are associated with body shape and limb length, the dewlap’s configuration is more closely linked to the local light environment and the number of co-occurring species. The diversity of the dewlap is a clear example of how a signaling structure can be shaped by both social pressures, like species recognition, and ecological constraints, such as the physics of light penetration in a forest.