Prosimii: Tarsiioidea, Lorisoidea and Lemuroidea

The suborder Prosimii traditionally groups the most primitive living primates. They are characterized by a reliance on olfaction, nocturnal habits, and specific anatomical features that distinguish them from higher primates (Anthropoidea). Prosimians are found in Africa, Asia, and Madagascar. They are divided into three main superfamilies: Lemuroidea, Lorisoidea, and Tarsiioidea.

Lemuroidea

Lemuroidea includes all lemurs, which are endemic to the island of Madagascar. Their isolation from mainland Africa allowed them to undergo adaptive radiation, filling ecological niches occupied by monkeys and squirrels elsewhere.

• Lemurs possess a wet nose (rhinarium) and a dental comb for grooming.
• They exhibit high variation in social structures, ranging from the highly social Ring-tailed lemur to the solitary Aye-aye.
• The Aye-aye is a specialized lemur that uses a thin, elongated middle finger to tap on trees and extract wood-boring larvae.
• Most lemurs are nocturnal, but several species, such as the Ring-tailed lemur and Sifakas, are diurnal.
• They are primarily herbivorous, feeding on fruits, leaves, and flowers.

Lorisoidea

Lorisoidea comprises the lorises and galagos (bushbabies). These primates are native to the tropical forests of Africa and Asia. They are strictly nocturnal and rely on keen hearing and scent for navigation and foraging.

• Lorises are found in South and Southeast Asia. They move slowly through trees to avoid detection by predators and possess a strong, venomous bite in some species.
• Galagos are found in Africa. They are known for their leaping abilities, which are facilitated by elongated tarsal bones in their feet.
• Both groups possess large, forward-facing eyes to maximize light absorption at night.
• They use scent marking extensively to define territories and communicate with other individuals.
• Their diet primarily consists of insects, tree gums, and fruits.

Tarsiioidea

Tarsiioidea contains the tarsiers, a unique group of small primates found in the islands of Southeast Asia. Tarsiers occupy an evolutionary middle ground, sharing some traits with prosimians and others with higher primates.

• Tarsiers are the only entirely carnivorous primates, feeding exclusively on insects, lizards, and small snakes.
• Their eyes are enormous relative to their head size; each eye is larger than their entire brain.
• They possess a neck anatomy that allows them to rotate their heads nearly 180 degrees in both directions.
• Unlike other prosimians, they lack a reflective layer (tapetum lucidum) behind the retina, which is a trait shared with monkeys and apes.
• They are specialized vertical clingers and leapers, capable of jumping long distances between trees.

Comparative Anatomical Features

Evolutionary Trends and Characteristics

• Prosimians retain several ancestral mammalian traits. Their snout is generally longer than that of anthropoids, reflecting a greater reliance on the olfactory bulb. The postorbital bar provides structural support for the eyes, though they lack the full postorbital plate found in monkeys and apes.
• The dental comb is a diagnostic feature for Lemuroidea and Lorisoidea. It consists of the lower incisors and canines, which are closely packed and tilted forward to scrape bark or groom fur. Tarsiers have lost the dental comb, which is one reason they are often placed in a separate category closer to the Anthropoidea in some modern classifications.
• Reproductive strategies in prosimians differ from higher primates. Many species produce litters rather than single offspring. They often use nest-building behaviors for infant protection, a practice less common in monkeys and apes. Infants are frequently parked in vegetation while the mother forages, rather than being carried on the body constantly.
• The smallest living primate is the Madame Berthe’s mouse lemur, which weighs approximately 30 grams. It is native to Madagascar. In contrast, the largest living prosimian is the Indri, a lemur that can weigh up to 10 kilograms.

Tarsiers occupy a significant place in primate evolution. Their fossils appear in the Eocene epoch across North America, Europe, and Asia. Today, they are restricted to a few islands in Southeast Asia. Their unique combination of traits, such as dry noses and enclosed eye sockets, led to the development of the Haplorhini classification, which groups tarsiers with monkeys, apes, and humans.