Hippopotamuses are large, powerful semi-aquatic mammals. These creatures possess a unique ability: opening their mouths to an extraordinary degree. This remarkable feat serves several important functions within their complex social and ecological lives. Understanding the extent and purpose of this wide gape offers insight into the biology and behavior of these fascinating animals.
The Jaw-Dropping Measurement
A hippo’s mouth can open to an astonishing angle, typically reaching up to 150 degrees, and sometimes nearly 180 degrees, creating a straight line from top to bottom. This translates to an opening that can measure between three to four feet tall in adult hippos. Such an expansive opening makes the hippopotamus a contender for the widest gape among all land mammals.
For perspective, a human jaw can open only about 40 degrees, making a hippo’s gape nearly four times wider. This remarkable range of motion sets hippos apart in the animal kingdom. While some sources claim even wider angles, the 150-degree measurement is consistently observed across scientific studies.
Anatomy Enabling the Wide Gape
A hippo’s impressive mouth-opening capability is attributed to specific anatomical adaptations. Their jaw structure is uniquely designed, with the hinge positioned far back in the skull. This rearward placement allows for an exceptionally wide range of motion, enabling the lower jaw to drop significantly. The temporomandibular joint, which connects the jawbone to the skull, exhibits a specialized flexibility that facilitates this extreme gape without dislocation.
Powerful muscles further enhance this ability, providing the necessary force for both opening and closing the massive jaws. The large masseter and digastric muscles are particularly developed, contributing to the hippo’s formidable bite force, estimated at around 1,800 to 1,825 pounds per square inch. Additionally, the orbicularis oris muscle, located in their lips, can unfold like an accordion, preventing tissue tearing when the mouth is stretched so widely.
When a hippo opens its mouth, it reveals formidable dentition, including large, sharp tusks. These are primarily the elongated canine and incisor teeth, with canines capable of growing up to 20 inches (50 cm) long and incisors reaching 16 inches (40 cm). These tusks grow continuously throughout the hippo’s life and are naturally sharpened as the lower canines grind against the upper ones when the mouth closes. Although hippos are herbivores, these teeth are not used for grazing; instead, they are specialized weapons for combat and display.
Reasons for Such a Large Opening
The wide mouth opening serves as communication and intimidation within hippo social structures. This behavior is most commonly observed as a territorial display, particularly among male hippos, or bulls, establishing dominance over their stretch of water. By opening their jaws wide, hippos showcase their formidable tusks, signaling their strength and potential for aggression to rivals.
This impressive gape is often part of an aggressive posturing known as a “yawn-threat” or “gaping.” It is a clear warning to potential threats, including other hippos, crocodiles, or even humans, to maintain distance. The “yawn” is not a sign of fatigue but a deliberate display intended to ward off challenges without necessarily resorting to physical conflict.
Beyond territorial disputes, the wide mouth opening also plays a role in defense, especially for females protecting their young from predators. While seemingly aggressive, hippos also use their mouths for other forms of communication, such as signaling readiness for cleaning by fish, a mutualistic interaction where fish remove parasites. Despite their powerful jaws and teeth, hippos do not use this wide gape for feeding; they primarily use their broad, muscular lips to crop grasses.