Tusks & Bite Mechanics

Hippos carry four enlarged canines plus two elongate lower incisors. The lower canines are the most prominent, reaching 40–50 cm in a mature bull; upper canines are smaller but sharpened by wear against the lower. Lower central incisors are forward-thrust tusks used for breaking vegetation and for display. All grow continuously throughout life, compensating for wear.

Microstructurally, hippo ivory is a composite of cortical dentine with a thin enamel cap on the occlusal surface. Hardness comparable to elephant ivory but with a finer Schreger-line angle (Espinoza & Mann 1993), allowing unambiguous forensic distinction in seizures. CITES Appendix II governs hippo-ivory trade since 1995, and some post-1989-ivory-ban markets shifted to hippo teeth as a substitute.

Upper canines are offset slightly inboard of the lowers so that closing the jaws slides their medial edges across each other, grinding both to chisel points. The lateral face of the lower tusk is thicker enamel; the medial face is exposed dentine. Wear produces a self-sharpening bevel that maintains cutting sharpness over a lifetime. Rhizotomy (accidental tusk loss) is rarely observed in wild animals; when it occurs, the opposing tusk overgrows and impales the opposite jaw, producing classic pathologies seen in museum skulls.

Direct measurement of hippo bite force using instrumented bite bars (Thomason 1991, Christiansen & Wroe 2007) yields values of ~8–12 kN at the canine tip and ~13 kN at the molar row. Underlying anatomy: enormous temporalis and masseter musculature, a short out-lever (canine tip close to the joint), and a massive pterygoid complex. The skull is disproportionately heavy — the mandible alone weighs 15–20 kg.

\[ BF \;=\; F_{adductor}\cdot\frac{L_{muscle}}{L_{tooth}}\cdot\cos\theta \]

At a 150° gape the mechanical advantage falls by ~30% (cos term), so in peak territorial display the actual delivered force is ~8 kN at the canine tip — still sufficient to penetrate rival hide and 3–5 cm of muscle.

Dominant bulls defend 250–500 m stretches of riverbank against same-sex intruders. Displays escalate through threatens (gape display, water spray, defaecation display) to contact combat, in which tusks deliver slashing wounds that can kill. Intraspecific aggression is the leading cause of adult-male mortality in high-density pods (Owen-Smith 1988, Field 1970). Females with calves are also aggressive; the combination of bite power and underwater agility makes hippos one of the most dangerous mammals to human visitors to river-edge areas (M8).

Despite massive tusks, hippos are strictly herbivorous in the wild. Foraging uses the lips rather than teeth: the wide muzzle and prehensile lips crop grass close to the ground. Tusks and incisors are combat and display tools, not dietary tools; molars do the actual chewing of cropped grass. Occasional reports of carrion consumption (Dudley 2016) and very rare predation on conspecifics or juveniles have been documented, but hippos are not meaningful omnivores.