The small, distinctive fold of skin often noticed on the outer edge of a domestic cat’s ear is part of the highly evolved feline auditory system. This subtle anatomical structure is present on all cats. The ear flap, known as the pinna, is designed to capture sound, and this extra fold contributes to the cat’s impressive ability to detect and localize sounds in its environment. Its presence suggests a shared evolutionary adaptation across various species that rely on acute hearing for survival.
Identifying the Cutaneous Marginal Pouch
This small flap of skin is formally known in veterinary anatomy as the cutaneous marginal pouch. Many people, however, refer to it by the more common name, Henry’s Pocket. This structure is located on the bottom-rear edge of the pinna, the visible outer ear flap. It appears as a slit or a small pocket formed by a fold of skin and cartilage.
The pouch is an area of increased flexibility compared to the firmer structure of the rest of the pinna. This pocket creates a secondary, distinct margin along the lower edge of the ear. The presence of this fold is entirely normal and is not an indication of injury or a health concern.
The Biological Advantages of the Pouch
The precise function of the cutaneous marginal pouch is still debated, but most theories center on enhancing the cat’s extraordinary hearing. Cats possess one of the broadest hearing ranges among all mammals, capable of detecting sound frequencies up to 85 kilohertz (kHz). This high-frequency acuity is useful for locating the ultrasonic squeaks made by their prey, such as mice and other small rodents, which emit calls between 22 and 70 kHz.
One theory suggests the pouch helps in the detection of these high-pitched sounds by acting as an acoustic filter. The pocket may cause lower-pitched sound waves to reflect or become slightly delayed, effectively muffling them. This mechanism allows the higher-frequency sounds of prey to become more pronounced and easier for the cat’s inner ear to isolate and process.
The mobility of the cat’s ear is a factor, and the pouch may also play a role in this mechanical action. Cats have 32 muscles in each ear, allowing them to rotate their pinnae a full 180 degrees to precisely pinpoint a sound’s source. The pouch’s flexible tissue allows the pinna to fold more tightly when a cat flattens its ears against its head, often as a sign of aggression or fear.
Comparative Anatomy: Presence in Other Species
The cutaneous marginal pouch is not exclusive to the feline family, suggesting it represents a widespread evolutionary advantage for directional hearing. A similar fold of skin is observed in several other mammalian species, such as dogs, although it is typically less prominent than it is in cats.
Other species that rely heavily on their auditory senses also possess this feature, including weasels and bats. The presence of the pouch across these diverse, highly-auditory predators implies a common anatomical solution for enhancing sound perception. In these animals, the fold is believed to serve a similar purpose: to assist with sound localization and possibly contribute to the flexibility of the outer ear.