Model answer & approach
- What are Strepsirhini Primates? Describe their geographical distribution and mention the characteristics that distinguish them from Haplorhini Primates. (Answer in 250 words)
Introduction
Strepsirrhini is one of the two suborders into which living primates are divided, the other being Haplorhini. It comprises the lemurs of Madagascar and the lorises and galagos of Africa and Asia.
Body
1. What the group is: the lemuriforms, including Lemuridae, Indriidae, Cheirogaleidae and the aye-aye, together with the lorisiforms, Lorisidae and Galagidae, some 140 species. Pocock's division replaced the older Prosimii because tarsiers, though prosimian in grade, share derived characters with monkeys and apes.
2. Distribution: Madagascar holds the lemurs, an isolated adaptive radiation from mouse lemurs to the indri; sub-Saharan Africa holds the galagos, potto and angwantibo; South and Southeast Asia hold the lorises, including the slender loris of the Western Ghats and Sri Lanka and the Bengal slow loris of north-east India. Almost all are forest-dwelling and arboreal, and most are nocturnal.
3. Distinguishing characters: a moist rhinarium bound to the upper lip by a philtrum, a functional vomeronasal organ and heavy reliance on olfaction and scent marking, against the dry nose and visual dominance of haplorhines. They retain a tapetum lucidum, an unfused postorbital bar without the postorbital septum, and unfused mandibular and frontal symphyses. The lower incisors and canines form a toothcomb, with a grooming claw on the second toe. The placenta is epitheliochorial rather than haemochorial, gestation shorter, litters and nesting common, and they can synthesise vitamin C, which haplorhines cannot.
Conclusion
The division is cladistic, not one of grade: tarsiers are haplorhines despite their appearance. Strepsirrhines are best defined by the retained rhinarium and toothcomb rather than by being primitive.
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