ANT%20151%2004

ANT%20151%2004 - T h e tr e n d s in p r im a te s :...

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1 Lecture 4 Why Did Primates Evolve? The trends in primates: • Enhanced reliance on vision – Eyes rotated forward – Post-orbital bar or closure – Orbital convergence • Brain enlargement • Grasping hands and feet • Nails instead of claws on at least one digit (other mammals have some but not all of these traits) Other Mammals Primates There are several hypotheses that propose to explain the origin of this suite of traits: 1. Arboreal theory (Smith, Jones, Le Gros Clark) 2. Visual predation hypothesis (Cartmill) 3. Angiosperm/omnivore hypothesis (Sussman) 4. Camouflage-breaking hypothesis (Crompton) 5. X-ray vision hypothesis (Changizi, Shimojo) 6. Snake detection theory (Isbell) The Arboreal Theory W.E. Le Gros Clark developed it in the early 1900s: • First primates lived high in the canopies of tropical trees where the sense of smell is not particularly useful. • So primates lost much of their olfactory ability. The Arboreal Theory • The sense of smell was replaced by expansion of the visual sense, which was needed because primates had to be able to – judge distances when leaping from branch to branch and – coordinate the hands with the eyes to grasp branches. • Successful judgment of distances between branches would favor evolution of the eyes closer together toward the front of the face. Photo: A. Melin The Visual Predation Hypothesis • The Arboreal Theory lasted a good 50 years! • Then Matt Cartmill developed the Visual Predation hypothesis in 1972. • He argued so successfully against the Arboreal Theory that now many people have rejected it. •He pointed out that many arboreal mammals have good smell and not such good vision so living in the trees shouldn’t be a reason for losing olfactory ability and strengthening visual ability. • Rodents like squirrels do just fine in trees without primate vision
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2 The Visual Predation Hypothesis • Cartmill proposed that the first primates were stalkers and grabbers of insects, and that they did this while on small- diameter branches in the lower levels of tropical forest trees. • A diet of insects would have favored the evolution of “visually guided reaching and grasping”. • The need to judge distances to insect prey would have favored rotation of the eyes forward away from the sides. The Visual Predation Hypothesis He pointed out that like primates, nocturnal predators such as owls and cats have convergent orbits The Visual Predation Hypothesis Later on, however, Cartmill noticed a problem: not all carnivores have forward-facing eyes. Mongooses are a good example. So he modified it by arguing that only those predators that are nocturnal would need orbital convergence and forward-facing eyes in order to see clearly what is in front of them. The Angiosperm/Omnivore Hypothesis
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This note was uploaded on 12/13/2009 for the course ANT 151 taught by Professor Lynn during the Winter '09 term at UC Davis.

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ANT%20151%2004 - T h e tr e n d s in p r im a te s :...

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