New World Monkeys

The Evolutionary Odyssey

Alfred L. Rosenberger

• Publisher: Princeton University Press
• Year: 2020
• ISBN:
  9780691189512
• Topic: Sciences biologiques
• Subtopic: Zoologie

A comprehensive account of the origins, evolution, and behavior of South and Central American primates New World Monkeys brings to life the beauty of evolution and biodiversity in action among South and Central American primates, who are now at risk. These tree-dwelling rainforest inhabitants display an unparalleled variety in size, shape, hands, feet, tails, brains, locomotion, feeding, social systems, forms of communication, and mating strategies. Primatologist Alfred Rosenberger, one of the foremost experts on these mammals, explains their fascinating adaptations and how they came about. New World Monkeys provides a dramatic picture of the sixteen living genera of New World monkeys and a fossil record that shows that their ancestors have lived in the same ecological niches for up to 20 million years—only to now find themselves imperiled by the extinction crisis. Rosenberger also challenges the argument that these primates originally came to South America from Africa by floating across the Atlantic on a raft of vegetation some 45 million years ago. He explains that they are more likely to have crossed via a land bridge that once connected Western Europe and Canada at a time when many tropical mammals transferred between the northern continents.Based on the most current findings, New World Monkeys offers the first synthesis of decades of fieldwork and laboratory and museum research conducted by hundreds of scientists.

CHAPTER 1

WHAT IS A NEW WORLD MONKEY?

This book is about the evolutionary odyssey of New World monkeys, the South and Middle American platyrrhines, though it is mostly about their evolution in South America where most of platyrrhine history was played out. Their odyssey appears to have begun 45–50 million years ago when an ancestral population of monkeys arrived in South America to found one of the most diverse and colorful adaptive radiations produced by the Order Primates. A robust view of what platyrrhines have become and how can be gleaned from the living animals today and the fossil record, which, though still limited, documents the major features of New World monkey evolution during roughly the last 25 to 35 or 40 million years of their existence, although the record is exceedingly sparse for periods older than 20 million years. Unlike other major primate groups, the history of New World monkeys is one in which the separate lines of descent leading to many of the 16 extant genera recognized herein can be traced back in time for millions of years by fossils and by molecules. This long-lineage pattern is what gives the structure of platyrrhine evolution its distinctive shape, and it is a centerpiece of this book. It also serves as a poignant point of reflection in thinking about the platyrrhines’ future. Fourteen of the 16 living genera include species that are now classified, according to the International Union for Conservation of Nature (IUCN), as Critically Endangered or Vulnerable.

There is an abundant record of fossil South American mammals that dates back nearly to the beginning of the Age of Mammals, about 66 million years ago. However, the oldest New World monkeys we know of date back only 36–40 million years. Given that South America was an island continent for most of the last 66 million years, as the world’s living mammals began to flourish and before Isthmus of Panama emerged to firmly connect North and South America 3 million years ago, the questions arise: Where did their ancestors come from, and how did they get there? Whether primates originally came from Africa by rafting across the Atlantic Ocean on a floating mat of vegetation, or mostly overland from North America, two scenarios detailed in chapter 10, they arrived as pioneers in a landscape where monkeys had never existed before.

The ways in which these animals evolved and thrived on the isolated continent, always in the trees, is a history of radical change and enduring stasis, novel adaptive solutions and predictable transformations. It is a story of giants, dwarfs, brainy predaceous tool users, dim vegetarians, fungus feeders, and bark-gnawing gum eaters. It is an account of cautious quadrupeds, acrobatic arm-and-tail swingers, quiet nocturnal denizens, and roaring diurnal howlers. Their mating strategies include codominant monogamists, and alpha males and alpha females living in large social groups. In some species females use scent to control the breeding success of their daughters; in another, males queue up on big branches waiting their turn to copulate with one female. By inhabiting a range of niches so varied in ecological and anatomical solutions to feeding and locomotion, or in social arrangements for group living, mating, and rearing offspring, platyrrhines have produced one of the most diverse adaptive radiations among the primates.

How did this happen? The present is key to understanding the past. There are two intertwined models describing how platyrrhine evolution has unfolded, the Long-Lineage Hypothesis and the Ecophylogenetic Hypothesis. What this means is that the many kinds of monkeys we see today have been around for millions of years and that some have existed for at least 20 million years with little change in their ecological situation, to the extent that their adaptations are documented in the fossil record. Furthermore, at another level, genetically related subgroups of New World monkeys, clusters of genera linked by their shared phylogenetic histories, have found success in various ecological niches defined by the particular sets of characteristics inherited from their remote common ancestors. Today, more than a dozen extant platyrrhine species belonging to all the six major subgroups can be found packed into a single rainforest locality, forming a harmonious monkey community. The fossil record suggests that this phylogenetic and ecological framework may have been in place for the entirety of the modern platyrrhines’ long-lived existence, setting the stage for the evolution of more refined divisions of niches by the procession of the living genera and species.

As further discussed below, I use the term lineage to mean a genus-level line of descent, an evolutionary stream carried in DNA that is embodied in a species, or a collection of intimately related species, and is manifested as a distinct ecological lifestyle. When examining an entire radiation such as the platyrrhines, the taxonomic level of genus, not species, is the most appropriate perspective. Genera exemplify and define the combinations of anatomical and behavioral characteristics that are of particular ecological relevance, and that separate all the significant lines of descent that compose an adaptive array.

FIG. 1.1. Cladogram of the major groups of living primates mapped with the distribution of external nose shapes. Primate images courtesy of Stephen Nash.

What is a monkey?

We regularly call platyrrhines monkeys, but the word monkey has no scientific significance. There are two groups of primates commonly called monkeys, the New World monkeys and the Old World monkeys. However, they are not grouped together in formal taxonomic language because they lack the evolutionary connection that is the main reason animals are classified jointly in particular groups: a genetic, or phylogenetic, relationship. The two groups we call monkeys are less closely related than the use of the word monkey suggests. In fact, the primates we call Old World monkeys, such as olive baboons and the rhesus macaques, are more closely related to apes than they are to New World monkeys (fig. 1.1). New World monkeys are a separate group entirely, an offshoot of the primate family tree that appeared about 25 million years before the earliest appearance of today’s Old World monkeys and apes documented in the fossil record. The sameness implied by the word monkey is an anachronism that may date back to the 14th century, according to the Oxford English Dictionary, an old-fashioned word based on an equally old, pre-evolutionary idea about the natural world. It was meant to distinguish these animals from apes and the other nonhuman primates, the lemurs, lorises, galagos, and tarsiers of Africa and Asia. They are all very different from monkeys and apes in many ways, including the structure of their skulls, their dentition and skeletons, sensory systems, and behavior, reflecting separate evolutionary histories.

Taxonomic groups that are formally recognized and named as units in classifications, such as species, genus, family, and order, are called taxa, the plural form of the word taxon. The term taxonomy, which means arrangement, is derived from the words taxon and taxa. The groups mentioned thus far—primates, platyrrhines and New World monkeys, Old World monkeys, apes, tarsiers, lemurs, lorises and galagos—are all taxa that have formal names in classifications as well as these common names. But monkey is not a taxon and has not been thought of in that way since Darwin introduced us to evolution and phylogeny, and reinforced the notion that classification should be based on relatedness, which previously was only a vague idea. The word is applied to two different groups of taxa that are actually not each other’s closest relatives.

Some labels for primate groups are like nicknames and have no scientific standing. Sometimes they are holdovers from the pre-Darwinian period when natural history was not a secular enterprise and scholars used such terms to express their ideas about how far a group was stationed along an imagined trajectory, a ladder of ascent, reflecting the Scale of Nature or the Great Chain of Being that emanated from Creation. Humans were considered the pinnacle of creation and all other animals were said to occupy standings below that high point, as lower grades or stages in the procession of life. The early naturalists arranged their classifications accordingly and their informal language sometimes expressed those views. Thus the term monkey referred to the group of primates grouped with the apes as “higher primates” and gradistically situated between apes and the “lower primates,” the tarsiers, lemurs, lorises, and galagos. The latter were called prosimians, meaning near monkeys and apes. Eventually, Darwin made it quite clear that the two great groups of monkeys were distinct: Old World monkeys are the closest living relatives of apes and New World monkeys are a separate line of evolution within the monophyletic group—the unique descendants of a common ancestor—we call Anthropoidea, informally anthropoids, the taxonomic equivalent of “higher primates,” composed of New World monkeys, Old World monkeys, apes, and humans.

Even in the Darwinian era grade-thinking persevered throughout biology, and particularly when it came to discussing nonhuman primates as human relatives. Darwin’s most effective scientific ally, Thomas Henry Huxley, wrote of primate diversity and evolution in 1863, in Man’s Place in Nature, four years after On the Origin of Species was published. He said, “Perhaps no order of mammals presents us with so extraordinary a series of gradations as this—leading us insensibly from the crown and summit of the animal creation down to creatures, from which there is but a step, as it seems, to the lowest, smallest, and least intelligent of the placental Mammalia.” In the next 100 years the gradistic mindset faded from research practice but it still endures in our everyday language as a convenience, hence the word monkey. As a way of viewing the world, however, gradistics failed with the onset of a methodological revolution known as cladistics that occurred in the 1960s, which sought to organize and classify groups according to their placement on the appropriate branch, or clade, of the phylogenetic Tree of Life, as will be fully discussed later. That failure had important consequences in spurring a wholesale re-thinking of platyrrhine evolution.

The geographic modifier in the name New World monkey is also an anachronism. Since the Age of Discovery, in the 15th century, European writers have referred to the Western Hemisphere as the New World, ostensibly discovered by Columbus, in contrast to the Old World, comprising Eurasia and Africa. Similarly, platyrrhines are also often called Neotropical primates, meaning primates of the New World tropics. In an ecological sense, that term may conjure up a misunderstanding about the habitats where platyrrhines live, and what the relevant environments of South America in particular look like. It delimits the wide swath of South and Central America straddling the equator, the tropical zone, where the climate is moist, warm or hot all year round and supports dense, evergreen, jungle vegetation. But that landscape is not all continuous rainforest, and platyrrhines are not strictly jungle dwellers.

South America is a vast continent that is two-and-a-half times the size of the Amazonian rainforest, where most platyrrhines are found. Another vitally important tropical and subtropical region, the Atlantic Forest of southeastern Brazil, supports a smaller, unique ensemble of monkeys including several endemic forms, meaning they are found nowhere else in the world (fig. 1.2). Most of them are presently endangered as a result of the wholesale decimation of the Atlantic Forest that occurred during the last 500 years which, as discussed in chapter 11, has reduced their habitat to disconnected, relict forest fragments about one-tenth the size it was when European colonists first arrived in Brazil half a millennium ago.

FIG. 1.2. Map of South America and its major ecological zones.

The full geographic range encompassed by monkeys in South America extends from the northern edge rimming the Caribbean Sea and the Atlantic and Pacific Oceans, to northern Argentina in the distant south. The habitats mapped out in this enormous expanse are predominantly evergreen rainforests, semideciduous forests where trees lose their leaves seasonally, and open-country savannas, grasslands, and shrublands. Primates can be found in all these areas, though the greatest concentration of species and the most densely packed communities of platyrrhine species occur in the rainforests. In drier, more sparsely vegetated zones, only a few generalist species of monkeys, or those with a special set of adaptations to procure food from a limited, local supply, manage to get by. There they are often found in narrow strips of forest situated alongside water courses. Of all things, New World monkeys need trees no matter where they live.

Why is this so? Comparing the vegetation map of South America with the distribution maps of the living species highlights an intensely interesting question about platyrrhine evolution: Why are there no terrestrial species? In Africa, another enormous continent with a similarly varied distribution of habitats, Old World monkeys have evolved an impressive array of terrestrial and arboterrestrial species, living in forests and even extending into bone-dry, near-desert areas. In contrast, while platyrrhines are obviously an exclusively arboreal radiation, there is nothing about the design of their bodies or their dietary needs that makes it impossible for a New World monkey to habitually visit the ground and benefit from it. Actually, some species do so occasionally in order to cross large gaps in the forest or obtain drinking water in drier places when the forest does not provide them with enough because watery fruits are in short supply.

Juvenile monkeys sometimes play on the ground. Clever capuchin monkeys living in swampy areas have even learned to collect clams on the ground when the tide recedes. Yet, no living platyrrhines have evolved terrestrial adaptations or a terrestrial lifestyle. Given their long evolutionary history, however, and knowing that South American forests have waxed and waned over the entire continent, it may be that the fossil record will at some point turn up a ground-dwelling New World monkey. In fact, there is already a hint of this in the few remains of an extinct Caribbean platyrrhine, Paralouatta, to be discussed in a later chapter. With all that biologically built-in ecological flexibility and a vast area of the continent as potentially exploitable habitat, under the forest canopy and beyond, the absence of living terrestrial platyrrhines seems quite the mystery.

What is a platyrrhine?

The technical name for New World monkeys is Platyrrhini; platyrrhines, colloquially. It means flat- or wide-nosed. The name was given to them in 1812 by the French naturalist Étienne Geoffroy Saint-Hilaire, who was then sorting and cataloging specimens of mammals held in the collections of the Muséum National d’Histoire Naturelle in Paris. He found that the shape of the nose turned out to be a useful way to identify several groups of primates. In platyrrhines the nostrils are widely spaced and laterally facing, separated by a broad fleshy strip between the openings (fig. 1.1). In some, such as the Saki Monkey, the expression of this characteristic is rather extreme. A contrasting pattern occurs among Old World monkeys and apes, which have nostrils that are closely spaced and separated by a thin band of flesh. They are classified as Catarrhini; catarrhines, informally, meaning downwardly facing nose.

These distinctions, like many others used in identifying and classifying primates, are exhibited consistently among platyrrhines, but not universall...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Dedication
• Contents
• List of Illustrations
• Preface
• Chapter 1. What Is a New World Monkey?
• Chapter 2. Diverse Lifestyles
• Chapter 3. What’s In a Name?
• Chapter 4. Evolutionary Models
• Chapter 5. How to Eat like a Monkey
• Chapter 6. Arboreal Acrobats
• Chapter 7. Many Kinds of Platyrrhine Brains
• Color Plates
• Chapter 8. The Varieties and Means of Social Organization
• Chapter 9. 20 Million Years: Every Fossil Tells a Story
• Chapter 10. South America Was Once an Island: How Did Platyrrhine Ancestors Get There?
• Chapter 11. After 20 Million Years of Existence, New World Monkeys Face Extinction
• Acknowledgments
• Glossary of Terms
• Recommended Reading
• References
• Index