Head and body length: 76-103 cm

Shoulder height: 45-52 cm

Tail length: 8-12 cm

Adult weight: 18-24 kg

Females are typically larger than males, and bay duikers from Central Africa (C. d. castaneus) are larger than those from West Africa (C. d. dorsalis). Both sexes are reddish-brown, with a very bold black stripe that runs along the spine from the back of the head to the tail. The legs are dark brown, and the tail is black above and white below. The underparts are the same color as the sides, but a dark stripe runs along the center-line of the belly. The reddish face has a dark blaze that extends from the nose to the top of the forehead. White spots are present above the eyes and on the lips and chin. The cheek muscles of this species are very large and powerful. Both sexes have a pair of short, spike-like horns that extend backwards from the forehead; in males, they usually grow 5-8 cm long, while the horns of females are shorter.

Similar species
In the field, "red duikers" are often difficult to tell apart based on fleeting glimpses. The bay duiker's nocturnal habits and bright white spots above each eye are distinctive.

• Ogilby's duiker (Cephalophus ogilbyi) can be recognized by its paler underparts and dorsal stripe that ends at the shoulders and is not distinct on the neck.
• The white-bellied duiker (Cephalophus leucogaster) has a less well-defined dorsal stripe that widens in the center of the back and a pale belly.
• The dorsal stripe of Peters's Duiker (Cephalophus callipygus) begins at the shoulders and becomes very wide, covering the entire rump.
• The bay duiker's dorsal stripe readily distinguishes it from the stripeless black-fronted duiker (Cephalophus nigrifrons) and Weyns's duiker (Cephalophus weynsi).

Gestation period: Approximately 240 days.

Litter size: 1.

Weaning: 3.5 months.

Sexual maturity: 18 months (for females).

Life span: Up to 17 years in captivity.

There is no specific breeding season, although in Central Africa there is a peak in births in January and February. Infants are born a uniform dark brown color, and begin to acquire the bright chestnut adult coloration at 5-6 months. For the first few weeks of life, an infant will remain hidden in dense vegetation while its mother forages.

The bay duiker is a nocturnal species, which provides some ecological separation from similarly-sized duikers that live in the same habitat. During the day, individuals rest in dense thickets or in the buttresses of trees. Females have home ranges 0.2-0.4 km² in size; males may range over twice that area. Population densities are typically 1·5–8·7 individuals per km². Bay duikers are very scent-oriented, using their noses to find food, detect danger, and communicate with others of their own species through the use of preorbital gland secretions, urine, and feces. If a threat is detected, bay duikers usually freeze in position and observe; if startled, they flee with a bounding gait into dense cover.

Family group: Solitary.

Diet: Primarily fruits and seeds, but also leaves, fungi, flowers, and even animal matter (invertebrates, eggs, and birds).

Main Predators: Leopard.

Tracts of moist primary forest interspersed with dense thickets in West Africa (C. d. dorsalis) and Central (equatorial) Africa (C. d. castaneus). The approximate range is depicted in the map below.

IUCN Red List: Least Concern (2008)

CITES Listing: Appendix II (2011)

Threats: Hunting for bushmeat, deforestation.

The estimated total population is approximately 725,000 animals.