A dewlap is a flap of skin hanging from the throat or neck of an animal, most famously the brightly colored, fan-like structure that lizards in the genus Anolis extend during territorial and courtship displays. But the term covers a surprisingly broad range of anatomy across the animal kingdom, from the inflatable red throat pouch of a frigatebird to the loose fold of skin swinging beneath a moose’s chin or a Brahman bull’s chest. What ties these structures together is their position on the ventral neck or throat, yet their internal anatomy, developmental origin, and primary purpose differ so much from group to group that biologists treat them as separate evolutionary stories that happen to share a common label.
How a Lizard Dewlap Actually Works
The best-studied dewlap in biology belongs to anole lizards, a group of several hundred species spread across the Caribbean and the Americas. At rest, the dewlap is folded flat against the throat and nearly invisible. When the lizard wants to signal, the whole structure fans outward in a fraction of a second, revealing a disc of stretched skin that can be vivid orange, yellow, red, white, or some combination. The display is brief, often paired with push-ups and head-bobs, and it can be repeated dozens of times in quick succession.
The engine behind this display is the hyoid apparatus, a set of small bones in the throat that function like a lever system. In the knight anole, for example, specialized elongated bones called the second ceratobranchials swing forward and downward when a muscle connecting two parts of the hyoid contracts, pulling the whole framework into a new position. That motion unfolds the dewlap skin the way an umbrella opens: the bones push outward from underneath, stretching the membrane taut.1PubMed. Functional morphology of dewlap extension in the lizard Anolis equestris (Iguanidae) Changes in the shape of these hyoid bones across species are what produce differences in how fast or dramatically different anoles can flash their dewlaps.2Functional Ecology. The biomechanical basis of evolutionary change in a territorial display
The dewlap skin itself is thin and translucent enough to be backlit by sunlight, which intensifies its color. It is laced with pigment cells and, in many species, carries a pattern of spots or bands that is distinct enough to tell populations apart. The combination of rapid mechanical deployment and eye-catching coloration makes the anole dewlap one of the most studied visual signals in animal behavior.
What the Dewlap Is For
Researchers have proposed several overlapping functions for the anole dewlap, and the honest answer is that it probably serves more than one purpose at once. The strongest evidence points to territorial signaling. Males extend the dewlap at rival males approaching their perch or territory, typically alongside push-ups and head-bobs. In staged encounters, males display more of these aggressive signals before escalating to a physical fight, as though the dewlap show is a way to resolve disputes without the cost of biting.3Behavioral Ecology. Female anoles display less but attack more quickly than males in response to territorial intrusions One hypothesis holds that dewlap size acts as a signal of bite force during long-distance encounters, giving a rival information about how dangerous a fight would be before anyone gets close enough to test it.4Functional Ecology. An experimental test of the effect of signal size and performance capacity on dominance in the green anole lizard
Species recognition is another frequently cited function, especially in the Caribbean where many anole species coexist on single islands. If a lizard can identify a rival as a member of its own species just from the dewlap flash, it saves the energy of confronting animals that pose no real territorial threat. Research on the brown anole, however, complicates this idea. Populations living alongside more anole species did not have consistently different dewlap sizes or colors, though males in those populations were more likely to have a spotted dewlap pattern, suggesting that at least pattern complexity responds to the presence of close relatives.5PubMed Central. The brown anole dewlap revisited: do predation pressure, sexual selection, and species recognition shape among-population signal diversity?
Sexual selection rounds out the picture. Females watch these displays, and a dull, infrequently displayed dewlap can signal poor condition. Research on a tropical anole showed that heavily parasitized males had duller dewlap colors and displayed less often than healthier males, suggesting the dewlap functions as an honest advertisement of quality: faking a bright, vigorous display is difficult when you are sick.6PubMed. Colorful displays signal male quality in a tropical anole lizard
Do Females Have Dewlaps?
Yes, and this is one of the more underappreciated aspects of dewlap biology. Female anoles across many species have dewlaps, though they are typically smaller and less vividly colored than those of males. For a long time, researchers focused almost exclusively on male dewlaps, treating the female version as a byproduct of shared genetics rather than something shaped by its own selective pressures. That view has started to shift.
A study of brown anoles across the Bahamas found that female dewlap characteristics varied among populations in ways that tracked environmental conditions, just as male dewlaps did. In drier environments, both sexes tended to have solid-colored dewlaps with higher reflectance in the ultraviolet range, while populations in wetter, more densely vegetated habitats had dewlaps that reflected more red light. One difference, though, was that display rate varied among populations in males but not in females, hinting that the behavioral use of the dewlap differs between the sexes even when the physical structure responds to the same environmental pressures.7Journal of Evolutionary Biology. Climate‐related environmental variation in a visual signalling device: the male and female dewlap in Anolis sagrei lizards The broader takeaway is that any complete understanding of dewlap evolution needs to account for both sexes, not just the flashier male signal.
How Habitat Light Shapes Dewlap Color
If you compare dewlap colors across anole species living in different habitats, a pattern emerges: the color of the dewlap often matches the light conditions where the lizard lives. Species in bright, open habitats tend to have red dewlaps, while those in darker forest understory environments tend toward yellow or white. This is not coincidence. Research across 17 Caribbean anole species demonstrated that dewlap color correlates with habitat light intensity, and that red dewlaps are most visible against vegetation backgrounds when the habitat is brightly lit, whereas lighter dewlaps perform better in dim conditions.8PubMed. Can Sensory Drive Explain the Evolution of Visual Signal Diversity in Terrestrial Species? A Test with Anolis Lizards
This idea, sometimes called sensory drive, essentially says that natural selection favors signal colors that stand out in a given light environment, because a signal that cannot be seen is useless. On Grand Cayman, for example, the local anole has a short-wavelength-reflective dewlap that appears to have evolved from a long-wavelength ancestor after the species colonized the island from Jamaica, likely in response to the different vegetation structure and ambient light on the new island.9Biological Journal of the Linnean Society. Habitat light, colour variation, and ultraviolet reflectance in the Grand Cayman anole, Anolis conspersus Within the green anole’s range along the Florida peninsula, dewlap and body color varied among populations, though the dewlap colors did not simply mirror local ambient light. The researchers proposed that the green anole’s unusually broad habitat range forces a compromise: no single dewlap color is optimal everywhere the species lives.10Journal of Herpetology. Color Variation, Habitat Light, and Background Contrast in Anolis carolinensis along a Geographical Transect in Florida
The practical result is that dewlap diversity among anoles is not random. It reflects a mosaic of pressures: the physics of light in different forests, the visual system of the lizards themselves, and the backdrop of vegetation colors. Sensory drive may also help explain how new anole species arise, because populations that evolve different dewlap colors in response to different light regimes could eventually stop recognizing each other as mates.
The Genetics Behind Dewlap Variation
Until recently, researchers could describe dewlap variation across populations in rich detail but had little insight into the genetic architecture producing that variation. A study of Hispaniolan trunk anoles began to change that. By examining crosses between populations with different dewlap colors, the researchers found that the proportion of orange in the dewlap appears to be controlled largely by a single gene with a dominant effect, rather than by many genes of small effect adding up. All offspring, including second-generation crosses and backcrosses, had less orange than would be expected if the trait were polygenic.11Evolution. The genetic architecture of dewlap pattern in Hispaniola anoles (Anolis distichus)
That finding matters because single-gene traits can shift more rapidly in a population than traits governed by dozens of interacting genes. If dewlap color can flip in a few generations rather than gradually blending, it helps explain how neighboring populations of the same species maintain strikingly different dewlap colors even when they are separated by only a short distance. It also suggests that the dramatic dewlap color boundaries seen across the range of certain anoles may not require elaborate genetic isolation to form.
Convergent Evolution and Different Internal Engineering
One of the most interesting findings from comparative studies is that dewlaps have evolved independently in multiple distantly related lizard groups. The anoles are the most species-rich example, but exaggerated throat fans have also appeared in lineages like draco flying lizards in Southeast Asia and sitana fan-throated lizards in India. All of these groups use the dewlap for similar purposes: territorial display, mate attraction, or both.
Yet when biologists looked beneath the skin, the internal machinery was different. The hyoid bones powering the dewlap have evolved along distinct trajectories in different lineages, meaning these animals arrived at the same functional outcome through different structural modifications.12PubMed. Repeated evolution of exaggerated dewlaps and other throat morphology in lizards Even within anoles, the hyoid apparatus varies more than you might expect given how similar the external dewlap looks across species. A detailed comparative study found multiple phenotypic pathways leading to a functionally convergent dewlap, suggesting that there is more than one engineering solution to the problem of “how to build a foldable throat fan.”13PubMed. Many Paths to a Common Destination: Morphological Differentiation of a Functionally Convergent Visual Signal
This pattern of convergent function with divergent anatomy is a textbook example of how natural selection shapes organisms. When a communication challenge is similar across environments, evolution arrives at similar-looking solutions, but it works with whatever anatomical raw materials are available in each lineage. The result is that two dewlaps that look and function alike on the outside can be assembled from quite different bones and muscles on the inside.
The Frigatebird’s Inflatable Throat Pouch
Birds offer a different take on the throat-display concept. Male magnificent frigatebirds possess a bare, red gular pouch that they inflate to the size of a balloon during courtship. Sitting in treetop colonies, males tilt their heads back, puff out the pouch, and vibrate their spread wings while producing a rapid drumming sound by clacking their beaks. The drumming resonates inside the inflated pouch, turning it into both a visual signal and an acoustic one.
Research on frigatebird colonies showed that males with larger gular pouches produce drumming at lower fundamental frequencies, and that males who successfully attracted mates drummed at lower frequencies with a steadier cadence than unsuccessful males.14PubMed. Morphology and ornamentation in male magnificent frigatebirds: variation with age class and mating status The pouch grows with age, so females selecting for lower-frequency drumming are effectively choosing older, presumably more experienced partners. Crucially, while a male can manipulate how he visually presents the pouch by adjusting inflation, the fundamental frequency of the drumming sound is a physical consequence of pouch size and cannot be faked. This makes the acoustic signal an honest indicator of the trait in a way that the visual signal alone might not be.15Ornithological Applications. Bimodal Signaling of a Sexually Selected Trait: Gular Pouch Drumming in the Magnificent Frigatebird
The frigatebird gular pouch is not homologous to the lizard dewlap; the two structures have completely different developmental and anatomical origins. But functionally, both are extensible throat structures used to communicate status or quality, and both demonstrate how evolution has repeatedly settled on the ventral neck as prime real estate for visual signaling.
Mammalian Dewlaps and the Thermoregulation Puzzle
The word “dewlap” also applies to mammals, though the structures look nothing like a lizard’s colorful fan. In cattle, especially tropical breeds like the Brahman, the dewlap is a heavy fold of loose skin hanging from the chest and throat. In moose and other ungulates, it appears as a bell-shaped flap of skin and sometimes cartilage dangling from the chin. Female rabbits develop a pronounced fur-covered dewlap under the chin that they use to line their nests with plucked fur.
The function of mammalian dewlaps has been debated. In cattle, early physiological work associated the dewlap with heat tolerance. Indian breeds with larger dewlaps showed higher heat tolerance than European breeds, seemingly because the extra skin surface area and associated vasculature help dissipate heat, along with other traits like shorter hair and lower metabolic rates.16Journal of Dairy Science. Climatic Physiology of Cattle In wild ungulates, a comparative analysis across species concluded that the dewlap’s primary function is likely thermoregulation rather than sexual selection or predator deterrence, contrasting with the communication-centered role of dewlaps in lizards and birds.17PubMed Central. Evolution of the ungulate dewlap: thermoregulation rather than sexual selection or predator deterrence?
Squamate reptiles, too, use vascularized body surfaces for thermoregulation. Iguanas and other squamates have highly vascularized sites of thermal exchange that help fine-tune body temperatures beyond what behavior alone can achieve.18PubMed Central. Vascular Patterns in Iguanas and Other Squamates: Blood Vessels and Sites of Thermal Exchange Whether the anole dewlap itself serves a meaningful thermoregulatory role on top of its signaling function remains unclear, but the vascular infrastructure is there.
Why One Word Covers So Many Different Things
It is worth stepping back to appreciate how wildly different the structures called “dewlap” really are. A male anole’s dewlap is a thin, translucent membrane deployed by a bony lever mechanism, tuned by evolution for maximum visual contrast in its specific light habitat. A frigatebird’s gular pouch is an inflatable balloon that doubles as a resonance chamber. A moose’s dewlap is a cartilage-stiffened flap of skin that may help shed body heat. A rabbit’s dewlap is a fatty, fur-covered chin roll whose main use involves nest-building.
The shared name persists because all of these structures occupy roughly the same anatomical neighborhood: the ventral throat or upper chest. But they are not derived from a common ancestral structure, and they serve different primary functions across the groups that have them. Biologists working on any one of these systems are usually careful to specify what they mean, but in casual usage the word “dewlap” gets applied to all of them, which can make the topic feel more unified than it really is. If anything, the diversity of dewlaps across vertebrates is a showcase for how the same body region can be repurposed in dramatically different ways depending on the ecological pressures an animal faces.