The intermembral index is a ratio used to compare limb proportions, expressed as a percentage. It is equal to the length of forelimbs (humerus plus radius) divided by the length of the hind limbs (femur plus tibia) multiplied by 100, otherwise written mathematically as:
The intermembral index is used frequently in primatology, since it helps predict primate locomotor patterns. For scores lower than 100, the forelimbs are shorter than the hind limbs, which is common in leaping primates and bipedal hominids. Quadrupedal primates tend to have scores around 100, while brachiating primates have scores significantly higher than 100. This information can also be used to predict locomotion patterns for extinct primates in cases where forelimb and hind limb fossils have been found.
Primate species + Intermembral Index of Extant Primate Species Species Intermembral Index Cheirogaleus major 72 Microcebus murinus 72 Phaner furcifer 68 Lemur catta 70 Hapalemur griseus 64 Eulemur fulvus 72 Lepilemur edwardsi 60 Avahi laniger 58 Indri indri 64 Daubentonia madagascariensis 71 Galagoides demidovii 71 Galago senegalensis 52 Otolemur crassicaudatus 70 Perodicticus potto 88 Arctocebus calabarensis 89 Loris tardigradus 90 Nycticebus coucang 88 Carlito syrichta 58 Cephalopachus bancanus 52 Callicebus moloch 74 Pithecia pithecia 75 Chiropotes sagulatus 83 Cacajao calvus 83 Alouatta seniculus 97 Lagothrix lagotricha 98 Brachyteles arachnoides 104 Ateles paniscus 105 Cebus capucinus 81 Sapajus apella 81 Saimiri sciureus 80 Aotus lemurinus 74 Callimico goeldii 69 Callithrix jacchus 76 Cebuella pygmaea 83 Saguinus geoffroyi 76 Leontopithecus rosalia 89 Macaca fascicularis 93 Macaca nemestrina 98 Cercocebus agilis 84 Mandrillus sphinx 95 Lophocebus albigena 78 Papio anubis 97 Theropithecus gelada 100 Miopithecus talapoin 83 Chlorocebus aethiops 83 Erythrocebus patas 92 Allochrocebus lhoesti 80 Cercopithecus mitis 82 Colobus guereza 79 Piliocolobus badius 87 Procolobus verus 80 Presbytis siamensis 78 Trachypithecus obscurus 83 Semnopithecus entellus 83 Nasalis larvatus 94 Pygathrix nemaeus 94 Hylobates lar 130 Symphalangus syndactylus 147 Nomascus concolor 140 Hoolock hoolock 129 Pongo pygmaeus 139 Pan troglodytes 106 Pan paniscus 102 Gorilla gorilla 116 Homo sapiens 72
Variation In a diverse ethnic sample of 314 modern human skeletons covering African Pygmies, Andaman Islanders, Khoesan, Zulu, African Americans, Sami and Inuit the intermembral index was found to vary between 64 and 74. A study published in 1937 found a range of variation between 64.5 and 79.2. This study found no link with humans of different groups with individuals from different ethnic groups showing similar scatter of variation. Variation has also beem found in chimapanzees (100.1 - 113.7), gorillas (110.3 - 125.0), orangutan (135.0 -150.9), siamang (145.0 - 155.2), gibbon (120.5 - 137.1), and macque monkeys (83.0 - 91.0).
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