discovermagazine.com — Thus passed the bulk of three years. Milton found that most of the time the howlers ate leaves and fruit in almost equal measure, but when seasonal fruits were in short supply, the animals filled up on leaves. Howler monkeys were finicky, though. They ate only tender, young leaves, and only the tips at that....They appear to use a collective information pool to locate their foods. They’ll just set off in a straight line right to it....The howlers conducted these expeditions over 75 acres, searching out as many as 25 species of plants daily. Some, like the Ceiba pentandra tree, were edible for only a few hours a year; others were available more often. Unerringly, the howlers tracked them down. The ranges of various howler troops overlapped, so Milton would occasionally come upon a tree filled with monkeys, with other groups in adjoining trees politely waiting their turn at the table. All of which suggested that the animals had an extraordinary collective memory, an unfailing sense of direction, refined social manners, and a built-in barometer of what foods were good for them.This aggregate intelligence allows infant howlers to mature quickly. After 12 to 14 months, howler mothers don’t want to see their babies again, Milton says. The babies soon declare independence and rely on the group for support.Still, despite the obvious group intelligence, the monkeys individually didn’t seem particularly smart to Milton. They were relatively dull and placid - and unobservant. I ate lunch for months in full view of dozens of howlers, and not one ever seemed to realize that I was eating, much less that what I was eating might be something they would enjoy, too, she says. You could make noises and slurp and carry on - whatever cognitive processes are required to identify the act of eating, they don’t seem to use them.But spider monkeys did. I saw them all the time when I was studying howlers, says Milton. They’d go roaring by like greased lightning. Spider monkeys are the same size as howlers, and the two animals share parts of each other’s ranges on Barro Colorado. But there the similarities end. Whereas howlers travel through the canopy on all fours, spiders swing along like Tarzan. Unlike the placid howlers, spiders are playful and mischievous. They’re terrible teases, says Milton. And they’re mean little devils. They remind me of people, she confides with a laugh. Although not specifically any of my close friends.Spider monkeys had no trouble recognizing Milton’s lunch. ‘Food!’ they’d shout. ‘Let’s see if we can get it!’ They’d swing down toward you; they’d threaten you. They know what a banana is. They have a keen idea of what a peanut butter sandwich is. You simply cannot eat in front of them.Intrigued, Milton decided she’d add spider monkeys to her observations. She thought it might be interesting to compare how the two species evolved from a common ancestor. But while the comparatively sedate howlers were a researcher’s dream, dealing with the spider monkeys was something else again. They were too fast for me, says Milton. So I hired a young man to work with me. He would run through the forest as fast as he could, following the monkeys, and I would come behind. We communicated by calls. ‘Whooooo!’ Like that. The sound really carries through the forest.When the barnstorming spider monkeys found food, they’d finally screech to a stop, allowing Milton to catch up. They’d just stuff themselves. Then they’d lie around and take naps.Unlike the howlers, Milton discovered, the spider monkeys almost exclusively ate fruit, which often made up 90 percent of their diet. Even when fruit was out of season or in short supply, it constituted over half their food. But ripe fruit is even harder to find than tender leaves. To get enough, the 18 spider monkeys on the island would resort to splitting up and trying their luck on their own. During most of the year the distribution patterns of their foods are such that if they went around in a big group, there wouldn’t be enough at any one site to feed everyone, says Milton. So they’d spend almost the whole day foraging in small subunits or by themselves. Then around twilight they’d begin to call and coalesce, and then they’d spend the night together.As a result of this extended exploring, the spiders’ territory was huge, some 750 acres, ten times that of the howler monkeys. And that’s a conservative estimate, says Milton. Two thousand acres might be right. If the howlers displayed impressive feats of memory and direction by finding young leaves, the spider monkeys’ long-distance forays after fruit were astounding. Within an enormous area they had to remember at least 100 species of fruit and where to find thousands of fruit-bearing trees. They had to remember when each fruit was ripe, how best to approach the site, and how best to return home. If a howler forgot a food source or a travel route, the others were there to take up the slack. The spiders, though, had to fend for themselves.And they had to know how to stay in touch. Howler monkeys tended to be quiet, communicating through subtle clucks and rattles in the throat, except at daybreak, when their eerie howls declared. ...Spider monkeys, on the other hand, were conspicuously noisy. They’d yelp and cry, whinnying like horses, barking like dogs - sometimes for hours at a time. ...And in contrast to the howlers’ community messages, spider monkeys believed in individual expression. Spider monkey vocalizations are generally individualistic. ...All that variety and independence requires lots of training. As a result, infant spider monkeys mature slowly. They are nursed and carried by their mothers for two years, and they continue to associate almost exclusively with her until they’re about three or three and a half years old. ...Why were the two monkeys so dissimilar? Milton wondered about the differences in their diets. Howler monkeys ate mainly leaves, sometimes exclusively leaves, a low-quality source of nutrition. Leaves are plentiful and relatively high in protein, but they’re low in energy-rich carbohydrates. They also consist of some 60 percent indigestible fiber and sometimes contain toxic chemicals. How in the world did howlers get enough energy from this unpromising diet? And why did they stick to it even during seasons when there was plenty of ripe fruit in the forest?Fruits are loaded with easily digested carbohydrates and are relatively low in fiber - they’re high-quality, nutritious food. They mean instant energy. On the other hand, fruits provide little protein. So, Milton wondered, how did spider monkeys get enough protein? And why, when fruits were scarce, didn’t they fill up on leaves, as howlers did? Why did they go to such extremes to find fruits?Milton began finding some answers to these questions in 1977, when she returned to Barro Colorado after completing her doctoral thesis. She soon conducted an experiment measuring how long it took the monkeys to process their food. I needed to look at internal features of the monkeys, she says. I thought that perhaps the structure of their guts or efficiency of their digestion might be influencing their behavior.She trapped howler and spider monkeys, confined them in pens, and fed them food in which she had concealed tiny plastic markers. I used a type of thin plastic material that I cut with very fine manicure scissors into little colored plastic worms, she explains. When the monkeys excreted the remains of their food, out came the markers. Milton could therefore measure the time it took any one meal to pass through a monkey’s digestive tract. The results were dramatic: howlers took 20 hours to digest their food, five times as long as spiders. ...When Milton came upon monkeys that had died in the forest, she took them back to the research station, dissected them, and measured their gastrointestinal tracts. She then confirmed her figures against published material on differential gut measurements in various primates. She found that the colons of howlers were considerably wider and longer than those of spider monkeys. Food had to travel much farther and remained much longer in howler guts, and the monkeys had room for much more bulk. As a result, bacteria had a chance to ferment masses of fibrous leaves in the monkeys’ colons, producing energy-rich fatty acids. Milton eventually found that howlers receive more than 30 percent of their daily energy from such fatty acids.... Spiders were far less efficient at extracting energy from the fiber in their diet - but they didn’t have to be efficient. They ate easily digestible fruits. By moving a steady stream of fruit through their gastrointestinal tracts every day, they obtained all the carbohydrates they needed and some of the protein. The rest came from supplements of young, tender leaves.It was a striking example of evolutionary adaptation. Each monkey’s physiology fit its particular diet. Spider monkeys couldn’t get away with eating a howler diet of mostly leaves. With their smallish guts, they’d never keep enough bulk around long enough for fermentation to provide energy. And howlers wouldn’t manage for long if they used the spider monkey tactic of eating fruit - their slow digestive tracts couldn’t process nearly enough of it.
Besides, it took smarts to track down sufficient fruit, and Milton thought it unlikely that the howlers were up to the job. Nor was the howler diet of leaves up to the job of fueling the amount of brainpower necessary. The brain, a big, hungry organ, requires a disproportionate amount of energy, and leaves just don’t provide enough....The more I thought about it, the more it seemed to make sense that if you have a high-energy diet and widely distributed foods, you’re going to need a certain amount of ability to locate those foods. ...A scientist named Daniel Quirling had published extensive statistics about the sizes of primate brains. Spider monkey brains, he had determined, weigh twice those of howlers, 107 grams compared with 50.4. No wonder spiders are smarter....Compared with the howlers, spider monkeys were brighter and more lively. They matured more slowly and had more to learn; they made more ruckus, with a greater variety of vocalizations; they ate widely dispersed, high-energy foods that were harder to find--and their brains were twice as large. Why?As far as Milton was concerned, diet was the key to these discrepancies. Eating fruits fueled the evolution of the spider monkeys’ large brains. Says Milton, It would have been a feedback process in which some slight change in the monkeys’ foraging behavior conferred a benefit, which in turn permitted a modest improvement in the quality of their diet, which led to an excess of energy. Over generations, the monkeys that spent the energy on making their brain slightly bigger and more complex had an evolutionary advantage. Their improved brain allowed for more helpful changes in their behavior, and so on.Milton realized that if such a scenario was correct, similar differences in brain size should show up in other primates with similar differences in diet - monkeys and apes that eat fruits should have larger brains than their leaf-eating counterparts. Sure enough, when Milton checked the literature, she found the pattern held true. For example, of the three great apes, lively, quick chimpanzees, our closest animal relatives, have a bigger brain for their body size than do the slower, more placid gorillas and orangutans. Chimps take some 94 percent of their diet from plants, largely in the form of ripe fruits. Gorillas and orangutans eat 99 percent plant foods, but mainly lower-quality leaves, pith, even bark. Diet had to be the key to their disparate evolution.