Primates are mainly arboreal animals, with terrestrial species constituting secondary returns to almost permanent terrestrial locomotion. They vary greatly in size, the smallest primate in the world being Madame Berthe’s mouse lemur (Microcebus berthae), measuring just nine to 11 centimetres excluding its tail and weighing only 30 grammes as an adult! In contrast, the largest primates are gorillas (Gorilla spp.), which can stand up to 175 centimetres tall and weigh up to 275 kilos.
One of the unique features of primates is their ability to oppose their first digit with the rest of their digits on their feet and hands, enabling them to grasp objects. The big toe of humans and tarsiers is no longer opposable but are anomalies acquired secondarily due to locomotion, bipedal in humans and saltatory in tarsiers. They also have flat nails, are cephalised (i.e. their brain is more developed with an increase in cranial volume), have forward migration of the orbits, which results in binocular vision, and their ulna is independent of the radius, which allows the hands to pronate and supinate.
Some distinctive criteria:
- Strepsirhini: presence of a nose – Haplorrhini: presence of a nose ;
- Catarrhini: downward-facing nostrils separated by a septum – Platyrrhini: sideways-facing nostrils which are well separated ;
- Hominoids: presence of a coccyx – Cercopithecidae: molars have two cusps ;
- Hominids: development of frontal cortex – Gibbons: movement by brachiation.