A dewlap is a flap of skin or flesh that hangs beneath the lower jaw or neck of a vertebrate animal. This structure appears across a wide array of species, including mammals, reptiles, and birds, demonstrating convergent evolution. While the specific purpose of the dewlap varies among different animal groups, it serves a range of biological functions, often playing a role in survival, communication, or reproduction.
Defining the Dewlap: Anatomy and Appearance
The physical composition of a dewlap is primarily loose, flexible skin, allowing it to hang or be extended and retracted. In many large mammals, this flap contains underlying layers of fatty tissue, contributing to its mass and pendulous shape. The loose skin provides a large surface area relative to its volume, which is important to its function in many species.
In reptiles, particularly lizards, the dewlap is a thin membrane supported by specialized skeletal structures. The hyoid apparatus, a bone and cartilage structure in the throat, allows the dewlap to be rapidly extended and retracted for display. Unlike the fleshy, permanent flap of a cow, a lizard’s dewlap is often folded and hidden when not in use. The external appearance varies significantly, ranging from being covered in the animal’s natural fur or scales to being completely bare and brightly colored.
Essential Biological Functions
The dewlap performs several distinct biological functions, primarily thermoregulation and communication. In large animals living in warm climates, the dewlap acts as a specialized radiator for dissipating heat. Because the flap has a high surface area and is often well-vascularized, blood flowing through the dewlap cools down more efficiently before returning to the body’s core. This mechanism is important for large-bodied species that struggle to lose heat due to their high volume-to-surface-area ratio.
Communication and signaling represent the other major function, especially in reptiles and birds. The structure is used in visual displays for courtship, territorial defense, and species recognition. Rapid movement, size, and color convey information about the animal’s mating suitability, dominance, or aggressive intent.
The appearance of the dewlap is frequently linked to sexual dimorphism, meaning it differs noticeably between males and females. Males often exhibit larger, more prominent, or more colorful dewlaps than females, highlighting a role in mate attraction or competition. In some mammalian species, however, the female dewlap is more developed, indicating a different, non-sexual primary function.
Species-Specific Variations
The dewlap’s function is highly specialized depending on the species and its environment. In the male anole lizard, the dewlap is a signaling device used for intraspecific communication. The lizard rapidly extends and retracts this membrane in a species-specific pattern, often accompanied by head-bobbing movements. These displays attract mates and warn off rival males, with the dewlap’s bright coloration making the signal highly visible against varied backgrounds.
For Zebu cattle, a breed common in hot, arid regions, the dewlap is a substantial, permanent flap of skin that hangs from the neck and brisket. Its primary role is thermoregulation, as the large surface area helps the animal cool down in high temperatures. This adaptation allows the cattle to thrive in environments that would cause heat stress in breeds without this feature.
Female rabbits, particularly mature does, develop a prominent dewlap on their neck that serves a reproductive purpose. When a pregnant female is preparing a nest, she plucks fur from this fleshy flap to line the nesting area. The dewlap acts as a portable reserve of soft, insulating material, ensuring the newborns have a warm and protected environment.