Strepsirrhini
The moist, hairless tip of a lemur's snout is called a rhinarium. This structure resembles the nose of a dog or cat more than that of a human. It serves as a tactile organ rather than an olfactory one. Touch receptors known as Merkel cells cover its surface. These cells allow the animal to feel vibrations and textures in the dark forest floor. A fold of mucous membrane connects this wet nose to the upper lip. This connection is called the philtrum. It creates a gap between the front teeth. This anatomical feature distinguishes strepsirrhines from other primates. They possess a vomeronasal organ below the nasal cavity. This organ detects fluid-based chemicals like pheromones. The main olfactory system handles airborne smells while the accessory system processes scents from the rhinarium. Strepsirrhini derives its name from Greek words meaning turning nose. French naturalist Étienne Geoffroy Saint-Hilaire coined the term in 1812. He described the nostrils as terminal and winding. British zoologist Reginald Innes Pocock revived the classification in 1918. Primatologists Paulina Jenkins and Prue Napier corrected the spelling error in 1987.
Strepsirrhine lineages diverged from haplorhine primates between 55 and 90 million years ago. Genetic analysis suggests an earlier split than fossil evidence indicates. The earliest true primates appeared during the early Eocene epoch around 55 million years ago. These creatures radiated across the Northern Hemisphere during a period of rapid global warming. Adapiforms thrived in Europe, North America, and Asia during this time. Genera such as Cantius and Donrussellia represent some of the most basal forms. Notharctus is another well-known genus found in North American fossil beds. These animals disappeared when the climate cooled at the end of the Miocene approximately 7 million years ago. Lemurs rafted to Madagascar between 47 and 54 million years ago. Lorises split from African galagos around 40 million years ago. Fossil lorisoids like Karanisia and Saharagalago were discovered in Egypt's El Fayum deposits between 1997 and 2005. Djebelemur from Tunisia dates back to the late early or early middle Eocene. Azibiids from Algeria are roughly the same age. Plesiopithecus from Egypt represents a stem lemuriform. Molecular clock estimates suggest lemurs and lorisoids diverged in Africa during the Paleocene about 62 million years ago.
Swedish taxonomist Carl Linnaeus grouped strepsirrhines under the genus Lemur in 1758. Dutch naturalist Pieter Boddaert divided this group into two genera ten years later. É. Geoffroy and Georges Cuvier grouped tarsiers with galagos based on hindlimb similarities. German zoologist Johann Karl Wilhelm Illiger placed them in the family Macrotarsi. English comparative anatomist William Henry Flower created the suborder Lemuroidea in 1883. British anatomist William Charles Osman Hill wrote an entire volume on strepsirrhine anatomy in 1953. The debate over tarsier placement has persisted since the 1970s. Tarsiers are often placed in Haplorhini with simians or Prosimii with strepsirrhines. Prosimii is based on evolutionary grades rather than phylogenetic clades. American paleontologist Philip Gingerich proposed Simiolemuriformes in 1975 to link adapiforms to simians. Media attention surrounding Darwinius masillae dubbed Ida in 2009 highlighted confusion about terminology. Cladistic analysis of Ida was flawed but sparked public interest. Most experts now accept Strepsirrhini-Haplorhini as the preferred division. Three suborders including Prosimii, Tarsiiformes, and Anthropoidea remain a less common arrangement. The term prosimian is no longer used in official taxonomy but persists in behavioral ecology discussions.
All lemuriforms possess a specialized dental structure called a toothcomb. This consists of two or four procumbent lower incisors and canine teeth. The aye-aye modified this into two continually growing incisors similar to rodents. Lemurs groom orally using their mouth and tongue. They also possess a grooming claw on the second toe for scratching inaccessible areas. Strepsirrhine orbits contain a postorbital bar but lack postorbital closure. A reflective layer behind the retina called tapetum lucidum improves night vision. This layer consists of riboflavin crystals. They lack a fovea which improves day vision. Their brain-to-body size ratio is smaller than that of simians. The mandibular symphysis is unfused in most living species. Notharctus exhibited fusion of the jaw halves. Some extinct giant lemurs also showed fused jaws. Many nocturnal species have large independently movable ears. Lorisoids have a smaller tympanic cavity compared to lemurs. Both lorisoids and cheirogaleid lemurs replaced the internal carotid artery with an enlarged ascending pharyngeal artery. Strepsirhines produce their own vitamin C unlike haplorhines who must obtain it from diet. Extant strepsirhines have an epitheliochorial placenta where maternal blood does not contact fetal chorion directly.
Strepsirhine primates face an elevated risk of extinction due to human activity. Deforestation in tropical regions has converted much of their habitat for agriculture and pasture. The International Union for Conservation of Nature announced in 2012 that lemurs were the most endangered mammals. Illegal logging and hunting followed a political crisis in Madagascar in 2009. Slow lorises in Southeast Asia are threatened by the exotic pet trade and traditional medicine. Laws protecting these animals are rarely enforced despite being prohibited. Local taboos known as fady sometimes help protect lemur species in Madagascar. Both lemurs and slow lorises are protected from commercial international trade under CITES Appendix I. Hunting occurs for bushmeat or traditional medicine purposes. Live capture happens for export or local exotic pet trade. Much of the diversity and habitat of lemurs has been lost due to recent human activity. Prior to human arrival some forests had nearly double the current diversity of species.
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Common questions
What is the rhinarium of a strepsirrhine and how does it function?
The moist, hairless tip of a lemur's snout is called a rhinarium. This structure serves as a tactile organ covered in Merkel cells that allow the animal to feel vibrations and textures in the dark forest floor.
When did Strepsirrhini derive its name from Greek words meaning turning nose?
French naturalist Étienne Geoffroy Saint-Hilaire coined the term Strepsirrhini in 1812. He described the nostrils as terminal and winding based on the anatomical features of these primates.
How many million years ago did strepsirrhine lineages diverge from haplorhine primates?
Strepsirrhine lineages diverged from haplorhine primates between 55 and 90 million years ago. Genetic analysis suggests an earlier split than fossil evidence indicates for this divergence event.
Which Swedish taxonomist grouped strepsirrhines under the genus Lemur in 1758?
Swedish taxonomist Carl Linnaeus grouped strepsirrhines under the genus Lemur in 1758. Dutch naturalist Pieter Boddaert divided this group into two genera ten years later.
What specialized dental structure do all lemuriforms possess called a toothcomb?
All lemuriforms possess a specialized dental structure called a toothcomb consisting of two or four procumbent lower incisors and canine teeth. The aye-aye modified this into two continually growing incisors similar to rodents.
When did the International Union for Conservation Nature announce that lemurs were the most endangered mammals?
The International Union for Conservation of Nature announced in 2012 that lemurs were the most endangered mammals. Deforestation in tropical regions has converted much of their habitat for agriculture and pasture since that time.