The human neck features a noticeable protrusion, often recognized as a sign of maturity in males, prompting questions about its presence across the animal kingdom. Exploring this structure requires understanding the mechanics of breathing and sound production, and how it compares to the vocal apparatus found in other species.
Anatomy of the Human Laryngeal Prominence
The visible bulge in the front of the human throat is formally known as the laryngeal prominence. This prominence results from the shape of the largest cartilage in the throat, which forms a shield-like structure. This cartilage is composed of two flat plates, or laminae, that join together at an angle in the midline of the neck.
This junction forms the exterior angle that is clearly visible and palpable, particularly in adult males. The primary function of this cartilage is to surround and protect the delicate vocal cords housed within the voice box. It is an integral part of the overall laryngeal structure, governing airflow and sound production, and allowing for the manipulation of the vocal cords to change pitch and volume.
Laryngeal Structure in Non-Human Animals
Every mammal and all tetrapods possess a structure analogous to the human voice box to facilitate breathing and sound creation. Animals like dogs, cats, and horses all have cartilaginous frameworks that include a component equivalent to the human thyroid cartilage. This voice box is necessary for vocalization and acts as a protective sphincter for the airway.
In non-human mammals, this cartilage rarely forms the sharp, externally visible protrusion seen in humans. For example, in many domestic animals, the equivalent cartilaginous structure is less angular, more fused, or positioned differently within the neck, making it essentially invisible from the outside. The specific external prominence is generally absent, even though the overall structure is present.
Evolutionary variations show specialized structures for different vocal needs. Some artiodactyls, such as the Takin, exhibit an enlarged thyroid cartilage that forms a voluminous, hollow structure called a bulla. This bulla increases the mass of the vocal folds to lower fundamental frequency. Rodents also possess a cartilage that is bent rostro-ventrally and sometimes mineralized to anchor vocal folds, often supporting ultrasonic vocalizations. These examples demonstrate that while the protective cartilage is universal, its shape and outward appearance are highly specialized and differ significantly from the human form.
The Role of Sexual Dimorphism in Vocal Anatomy
The laryngeal prominence is noticeable in human males due to sexual dimorphism driven by hormones. During male puberty, a surge in testosterone causes the thyroid cartilage to undergo significant and rapid growth. This hormonal influence leads to the angular reshaping and enlargement of the cartilage.
This growth extends the front-to-back dimension of the voice box, lengthening and thickening the vocal cords housed inside. The physical lengthening of the cords causes the male voice to drop significantly in pitch, resulting in the characteristic deeper tone of the adult male voice. The outward projection of the cartilage is the external manifestation of this internal change.
While many animal species display sexual dimorphism in their vocal anatomy, it does not result in a visible neck prominence. For instance, male South African clawed frogs develop larger laryngeal muscles and motor neurons under the influence of androgens to control their sex-specific calls. Male rats also show larger laryngeal muscle fibers and lower frequency ultrasonic vocalizations compared to females. These species show hormonally driven changes that affect vocalization, but the degree of external projection seen in the human male is a unique physical adaptation.