The Monkey and the Fig: A Socratic Dialogue on Evolutionary Themes

Specialized Anatomy and Physiology of New World Monkeys

The Monkey and the Fig  A Socratic Dialogue on Evolutionary Themes Stuart A. Altmann  Socrates: Look, Eusebius, what's that? Eusebius: Where, Socrates? Soc.: In that tree. Eus.: It's a monkey (1). Soc.: What is it doing? Eus.: Just eating figs, that's all. Soc.: Are you sure, Eusebius? Watch carefully! How is it getting at the figs? Eus.: Oh, is that what you meant! It's hanging by its tail. Soc.: Can all monkeys do that? Eus.: No, only some of the New World monkeys. You can't hang that way without a special gripping sur-face on the last part of the tail, something like the palm of your hand. A hairy tail would be just too slippery. Soc.: What else does he need in order to hang that way? Eus.: Strong tail muscles. Soc.:- And? Eus.: What do you mean, Socrates? Soc.: What controls those tail muscles? Eus.: Why, the brain, of course! These monkeys have large specialized areas in the sensory and motor centers of their brain, devoted just to controlling that fancy tail of theirs.

See how well the prehensile tail keeps the monkey from falling out of the tree as it plucks fruit. Eus.: It does far more than that! An ordinary mammal in the trees can feed only on the fruits it can reach from the tops of branches. But with the addition of a pre-hensile tail, it can also reach those underneath. It has a much larger "feeding sphere." Also, the monkey's  Stuart Altmann is a professor of ecology and evolution at the University of Chicago. This dialogue is based on his Charles Michener Lecture on Social Biology given in 1988 at the University of Kansas. Eusebius is a creation of the composer Robert Schumann, one of several fictitious characters who provided lively discussions in Schumann's journal of music criticism, Neue Zeitschrift fiir Musik. Socrates, by asking probing questions and making provocative generalizations as a way of clarifying ideas, is being Socratic. Address for Professor Altmann: Department of Ecology and Evolution, University of Chicago, Chicago IL 60637-1454. 

prehensile tail enables it to climb onto terminal branches that would otherwise be inaccessible, and that's where the fruit is. A prehensile tail, combined with unusually long arms and legs, means that there's hardly any fruit that these monkeys can't reach. Their prehensile tail also helps them when they are brachiating—swinging by their arms from one branch to another. Soc.: But doesn't that kind of suspensory locomotion take a lot of energy? Eus.: Yes, somewhat more than walking the same distance, but brachiating actually saves energy because a brachiator can take short-cuts through the forest: with prehensile tails, these monkeys can cross other-wise impassable gaps in the forest canopy. Brachi-ation also saves time. These monkeys routinely move through the forest at speeds that ordinary monkeys wouldn't dare. For an animal that exploits widely scattered food sources, like figs, rapid and efficient locomotion is essential. Otherwise, search time would take up the whole day! Such arboreal locomotion must require a lot of well-coordinated information from the monkeys' eyes. Eus.: Indeed! Monkeys are like owls and like us: their eyes are on the front of their heads, so they have good binocular vision. Soc.: It seems that a lot of this monkey's anatomy and physiology are specialized for fruit-eating.

Well, they use the same equipment in other activi-ties, too. Soc.: Which came first, Eusebius? Eus.: I don't know. Soc.: How could you find out? Eus.: I don't know that either. Soc.: Then you didn't understand the first question! To understand a question is to know how it could be answered (5). What else is special about the mon-key's eyes? Eus.: They can see colors. Soc.: So what? So can we. Eus.: Yes, but most mammals can't, or do so only poorly. Soc.: How does the monkey use its color vision?