The Unicorn Chameleons and their 1,2,3,4,5&6-Horned Allies

Among all chameleons in the world, only the genus Trioceros contains several species that exhibit morphological features what are known as true horns. These horns are characterized by their placement on the front part of the head, just in front of the eyes and on the snout. They consist of a needle/like bony base and are covered by a thick layer of keratin, which is often yellowish in color and often ringed. 

Various species of chameleons may possess one, two, three, four, or even six horns. Different numbers of horns are found in males and females: females are usually hornless, possess just an indication of horn (usually a warty protuberance or a conical scale) or have lower number of horns than males, or possess usually smaller horns than males, rarely same. The number and size of horns undergoes also a certain individual and population variability. 

Notably, abnormalities can occur due to the traumatic loss of one or more horns or through developmental pathologies. One specimen of wild Trioceros jacksonii is known possessing five horns.

A particularly interesting case involves chameleons with only one horn. The complete list of one-horned Chameleons is here: 

Trioceros conirostratus (TILBURY, 1998) (males)

Trioceros fuelleborni (TORNIER, 1900) (females with 1 rostral horn and with indications of 2 preorbital horns)

Trioceros jacksonii jacksonii (BOULENGER, 1896) (some females with one horn)

Trioceros jacksonii xantholophus (EASON, FERGUSON & HEBRARD, 1988) (rare females with 1 small horn)

Trioceros marsabitensis (TILBURY, 1991) (males)

Trioceros melleri (GRAY, 1865) (males, some females)

Trioceros merumontanus (RAND, 1958) (males)

Trioceros oweni (GRAY, 1831) (some males)

Trioceros werneri (TORNIER, 1899) (some females)

Undescribed species from Ol Doniyo Orok, Kenya (males with one horn, females unknown)

Undescribed species from S Tanganyica Territory (collected by A.C. Ionides, deposited in CAS, SF, USA) (males unknown, probably with one or three horns, female with one horn)

The exceptional tiny horn of male Trioceros conirostratus is not having the bony base, it is ringed, keratinous, but not on a needle-like bony protrusion. 

Unique among all these is the Owen's chameleon from the Congo Basin, which typically has three horns in males while females remain hornless. 

In 1906, Marquard described a chameleon named Chamaeleon unicornis, known as the one-horned chameleon, distinguished by a single, long, spiky rostral horn that curls slightly upwards. The single holotype found in "Ngomo, dans le bas Ogoue, Gabon" was deposited in the Natural History Museum of Paris (Muséum national d'Histoire naturelle) under the collection number MHNP 06.173.

In 1911, F. Werner listed it in his work "Chamaeleontidae" still as a separate species but mentioned that it differs merely in the absence of the preorbital horns and form of the occipital flaps. 

In 1959, D. Hillenius confirmed the observation of Werner (1911) concerning the difference in horn, found however, that the occipital flap size and form is very variable and not diagnostic. Additionally, he studied further 15 specimens of C. unicornis and found several specimens with intermediate developped preorbital horns. He concluded the C. unicornis should be considered a southern subspecies of Chamaeleo oweni.

 In 1965, this species was synonymized with the Owen's chameleon by DeWitt, who found out these two are conspecific. 

This opinion is respected until today, the mysterious Unicorn Chameleon, described as Chamaeleon unicornis, known from about twenty specimens, is considered a synonym of Trioceros oweni.

In 2008, B.G. Marcot shared few pictures of one specimen, found in the village of Monkoto between the two main parts of Salonga National Park in central Democratic Republic of the Congo.

Literature sources:

De Witte, G.F. (1965) Les Caméléons de l'Afrique Centrale. – Musee Royal de l'Afrique Centrale, Annales, Ser. 8, Sciences Zoologiques, 142: 1–215.

Glaw, F. (2015). Taxonomic checklist of chameleons (Squamata: Chamaeleonidae). Vertebrate Zoology, 65(2), 167–246

Hillenius, D. (1959) The differentiation within the genus Chamaeleo Laurenti 1768. Beaufortia, 8: 1 – 92.

Marcot, B.G. (2008) EPOW - Ecology Picture of the Week. 6-12 October 2008: The Unicorn Chameleon. https://www.plexuseco.com/EPOW/EPOW-Archive/archive_2008/EPOW-081006.htm</p>

Mocquard, F. (1906) Description de quelques espèces nouvelles de Reptiles. Bulletin du Muséum d'Histoire Naturelle, 12(7), 464–467. 

Werner, F. (1911) Chamaeleontidae. – Das Tierreich, 27: I – XI, 1–52.