Sensory Systems

Rhinos have small eyes for their body size and lateral placement, yielding a wide panoramic field but poor stereoscopic resolution. Acuity estimates (based on retinal ganglion-cell densities; Hall 2006) suggest effective recognition at ~20–30 m and reliable motion detection at ~50 m. Rhinos are thought to be dichromatic (S- and L/M-cone), similar to many ungulates. Under dim light rod-driven sensitivity is good, but form vision is limited.

The practical consequence is that rhinos rely on scent and sound to identify approaching objects; a still human observer can remain unnoticed at 20 m if downwind and silent.

Niimura 2014 and 2018 sequenced mammalian OR gene families and reported that the African elephant leads with ~1 948 functional genes. Using the Liu 2021 genomes, the white rhinoceros tops ~2 500 — the largest number reported for any mammal. The OR family has undergone lineage-specific expansions linked to dietary chemistry: rhinos encounter a vast spectrum of plant secondary metabolites, and OR diversification correlates with browser-grazer generalism.

Behaviourally, rhinos deposit scent in middens (dung piles) that function as social bulletin boards. A scent sample gives sex, reproductive status, identity, and recent diet. Territorial males over-mark female dung; females visit middens to assess mate quality. Olfactory communication is the principal route of reproductive signalling in all five species.

Rhinos have large, independently-rotatable pinnae capable of directional localisation to within ~10° across frequencies from ~20 Hz to ~10 kHz. Like elephants, rhinos produce low-frequency calls (down to 20–40 Hz) that propagate several kilometres through savanna grass; von Muggenthaler 2003 documented infrasonic vocalisations (<20 Hz) in Sumatran and black rhinos. Low-frequency carriers bend around vegetation and preserve their spectral character over distance, making them well suited to territory maintenance in dense habitats.

Field tests with recorded calls show that rhinos orient to conspecific vocalisations at >1 km and to alarm “snorts” at ~300 m — consistent with the measured sensitivity threshold of ~40 dB SPL at 200 Hz.

The prehensile upper lip of the black rhinoceros bears a dense array of Pacinian and Meissner corpuscles; browsers use the lip tip to select individual leaves. White rhinos have a squared, flat lip adapted for grass cropping; the hand-like tactile discrimination is reduced. Facial vibrissae around the muzzle provide close-range detection in the final phase of foraging. Dermal mechanoreception is less developed than in elephants; substrate vibration is detected primarily through the feet, with likely involvement of Pacinian corpuscles in the plantar pads.

Black rhinos are reported to avoid plants with high tannin or alkaloid content and to select species-specific mixes that balance nutrient intake against toxin load. Taste-receptor (TAS1R, TAS2R) gene counts are consistent with other herbivorous perissodactyls; the bitter-taste receptor family shows moderate expansion relative to carnivores. Anecdotal reports of Sumatran and Javan rhinos preferring mineral-licks suggest active ion-detection foraging for sodium and calcium supplementation.