Displays / Visual signals
- Sight well-developed
- Mother/calf bond is extremely close
- Mother licks, nuzzles, scrapes calf with lower incisors
- Discipline achieved by nudging or biting
- Calf responds by prostrating itself; prostration gesture continues into adulthood as sign of submission within herd
- Threat displays
- Yawning, water-scooping and head-shaking
- Rearing, lunging, chasing
- Roaring grunting, explosive wheezing
- Chasing
- Submissive displays
- Facing aggressor with open mouth
- Turning tail
- Urinating with slow tail-wagging
- Lying prone
- Flight
- Explosive exhalation of breath indicates alarm
Vocalization: Common Hippo (Barklow 1994, 1997) (Klingel 1990) (Vaughan et al 2011)
- 80% of hippo vocalizations are made underwater, in studies in Tanzania's Ruaha National Park
- Complex bellows, shrieks and grunts are made both in and out of water
- The exhaled bellow of a dominant bull often triggers a deafening chorus from other dominant hippos
- Sound carries at least a mile over the noise of the river
- When a hippo resting at the surface bellows, nostrils flare and sound is hummed through nose and nasopharynx to the air
- Contact call is a deep, choppy "o-o-o-o" often ending in a high "u"
- High-pitched squeals made during threatening frontal confrontations
- Produce at least three categories of sounds underwater (Barklow 1995, 1997)
- Rarely audible on surface; expel little air when made
- High pitched tonal whines that are somewhat similar to humpback whale song notes
- Pulsed croak between calves and sub-adults
- Click-like sounds (but no evidence yet for echolocation)
- Relatively silent on land; use exhaled breath to express threat and alarm
Olfactory signals
- Keen sense of smell
- Likely that individuals recognize each other by scent
- Dung middens do not mark territories on land, as formerly assumed; function of middens not clear to researchers
- Ritualized dung spraying
- By bulls who may deliberately defecate on subordinate animals in their territories
- Subordinate males often spray faeces in the face of dominant males
- Large males may also defecate in water without another male being near
- Syringe-like vomeronasal organ functions underwater drawing a sample of urine in water through ducts leading from the mouth (flehmen)
- Almost all male ungulates sample a female's urine to test for possible estrus hormonal levels prior to courtship