New World Monkeys

The formal term for the taxonomic group containing all the New World Monkeys (NWMs) is Platyrrhini. The NWMs diverged from the Old World monkeys and apes (Catarrhini) around 40 million years ago (Perelman et al. 2011). Ancestral species are thought to have migrated to the Americas on rafts of vegetation or via island hopping (Fleagle 2013). The extant species of NWMs are currently classified into five families, twenty genera, 156 species and 204 taxa (species and subspecies) (Mittermeier et al. 2013; Table 1). However, the taxonomy of several taxa is debated in the literature and, therefore, this classification is subject to change.

The NWMs exhibit striking diversity in their morphology, distribution, diet and behavior. Unlike some Old World monkeys, all NWMs are arboreal and, with the exception of the owl monkeys (Aoutus), diurnal. They are small to medium sized primates (ranging from 0.1 kg to 15 kg) and are distinguished from the catarrhines by several features. Firstly, NWMs have broad, flat nostrils, while catarrhines have narrow, downward pointing nostrils (Figure 1). Secondly, NWMs have three premolar teeth, while catarrhines have two. Thirdly, NWMs lack a bony ear tube (ectotympanic tube), which is present in catarrhines. Finally, there is a difference in the configuration of the bones of the skull between NWMs (the zygomatic and parietal bones are in contact) and catarrhines (the frontal and sphenoid bones are in contact) (Fleagle 2013). All NWMs have a tail, which is prehensile in some taxa. Prehensile tails have a naked patch of skin towards the end for the reception of tactile stimuli, which is suitable for grasping, suspension and arboreal locomotion (Figure 1).

Figure 1

All platyrrhines have broad, flat, outward pointing noses, like this bald uakari (Cacajao calvus), and some taxa have prehensile tails, like this northern muriqui (Brachyteles hypoxanthus).

Photos by Kevin O'Connell (left) and Petruss (right), licensed under CC BY 3.0.

The NWMs are found in Mexico, Central America and South America. Some countries have extraordinarily rich primate diversity, such as Brazil (with 139 taxa, 60% of which are endemic) and Peru (with 50 taxa, 14% of which are endemic). They fill a wide range of dietary niches, including insectivory, gumnivory, frugivory and folivory. However, most species tend to consume a mixture of foods from several dietary categories. Largely as a consequence of habitat destruction and, to a lesser extent, hunting, an alarming number of NWMs are threatened with extinction (IUCN, 2014).

A summary of the main features of each of the five NWM families is given below.

The owl monkeys, sometimes called douroucoulis, consist of a single genus, Aotus, containing 13 taxa (Table 1). They are distributed from Panama to northern Argentina. Owl monkeys are the only nocturnal anthropoids, though at least one species is cathemeral (Fernandez-Duque and Erkert 2006). They are relatively small bodied (0.7 - 1.2 kg), with a round head, large eyes and inconspicuous ears (the name is derived from the latin "a", meaning without, and "otis" meaning ear). Their forelimbs are shorter than their hindlimbs and they have long bushy (non-prehensile) tails, which help them balance during quadrupedal locomotion (Figure 2). Owl monkeys are primarily frugivorous, although they also consume leaves, insects and flowers. This genus is unusual in being socially monogamous. They live in small groups of 4 - 6 individuals, usually consisting of an adult pair, an infant, a juvenile and one or two subadults (Mittermeier et al. 2013).

Figure 2

Owl monkeys, like this gray-bellied owl monkey (Aotus lemurinus, left) are the only nocturnal anthropoids. Howler monkeys, like this mantled howler monkey (Alouatta palliata, right) are among the loudest terrestrial mammals.

Photos by dsasso (left) and Arturo Gonzalez-Zamora (right), licensed under CC BY 2.0.

The atelids are the largest NWMs (5 - 15 kg) and consist of two subfamilies, Alouattinae and Atelinae (Table 1). All five genera have a long, prehensile tails. Within the Alouattinae, there is just one genus, the howler monkeys (Alouatta). Howler monkeys are different from the other atelids in terms of their locomotion and morphology. They are more folivorous than most NWMs and minimize energy expenditure through slow, deliberate quadrupedal locomotion and very low levels of activity (Milton 1980). They live in relatively small groups, with different species exhibiting different mating systems (Di Fiore and Campbell 2007). Howler monkeys are among the loudest terrestrial mammals (Figure 2) and they have a highly modified larynx, with a greatly enlarged cup-shaped hyoid bone (Dunn et al. 2015). This is thought to function as a resonating chamber for their calls (Dunn et al. 2015). The subfamily Atelinae is more energetic than the Alouattinae, and atelines are largely frugivorous (though Brachyteles may seasonally consume more leaves than the other genera; Strier 1992). Atelines use their long limbs to travel rapidly over long distances by brachiation. They also have bigger brains than the Alouattinae, which is probably related to their patchy, ephemeral fruit diet (Milton, 1980). Within the Atelinae, there are four genera (Table 1). Spider monkeys (Ateles) live in fission-fusion societies in which individual animals from a large community associate on a daily basis in small, flexible parties that change size and membership frequently (di Fiore and Campbell 2001). Woolly monkeys (Lagothrix), yellow-tailed woolly monkeys (Oreonax), and muriquis (Brachyteles) live in large groups (up to around 50 individuals), which contain multiple reproductive males and females (di Fiore and Campbell 2001).

The marmosets and tamarins are the smallest of the NWMs (100 - 700 g). The family consists of 7 genera and 62 taxa (Table 1), which range from Panama to southern Brazil. They are distinguished from the other NWMs by their small size and claw-like nails (on all digits except the big toe), which they use for vertical clinging on tree trunks (Figure 3). They have relatively long trunks, limbs, and (non-prehensile) tails, which they use for quadrupedal running and jumping. All callitrichids are frugivores and small animal predators, and they also consume plant exudates. Marmosets are specialist feeders and exhibit morphological adaptations to obtain, ferment and digest gum (i.e., gumnivory). Callitrichids live in social groups of up to about 20 individuals. However, marmosets tend to live in larger groups than tamarins (Mittermeier et al. 2013). Callitrichids are unusual in several aspects of their mating and reproduction. They vary widely in mating system, exhibiting monogamy, polygyny, polygynandry and polyandry (Dunbar 1995). Groups will often consist of extended family, with a single breeding female and several breeding adult males. Most groups also contain other non-breeding (subordinate) adult males and females, who assist in caring for the infants. Unusually among primates, all callitrichids (with the exception of Callimico) usually give birth to twins (Figure 3).

Figure 3

Callitrichids, like this common marmoset (Callithrix jacchus, top left), have claw-like nails, which allow them to use vertical clinging postures on tree trunks. They also give birth to twins, like this emperor tamarin (Saguinus imperator, top right). Robust capuchin monkeys, like this bearded capuchin (Sapajus libidinosus, bottom left), use stone tools to crack open palm nuts. Common squirrel monkeys (Saimiri sciureus, smaller monkey, bottom right) are often found in polyspecific associations with other primates, such as brown capuchin monkeys (Sapajus paella, larger monkey, bottom right).

Photos by birdphotos.com (top left) and Brocken Inaglory (top right), licensed under CC BY 2.0, and Tiago Falótico (bottom left) and Living Links (bottom right), used with permission.

The family Cebidae consists of two subfamilies, Cebinae (capuchin monkeys) and Saimiriinae (squirrel monkeys), ranging from Central America to Argentina. In total, there are 3 genera and 36 taxa. They are small to medium sized monkeys, though capuchin monkeys (1.5 - 5.0 kg) are significantly larger than squirrel monkeys (0.5 - 1.2 kg). Cebids are described as arboreal quadrupeds. The tail is semi-prehensile in capuchin monkeys (they use the tail to grasp, but cannot hang from it), but not in squirrel monkeys. Cebids are insectivore-frugivores, but consume a wide range of animal and plant foods. They have relatively large brains for their body size, which has been associated with their complex diets and relatively large home ranges. Uniquely among NMWs, robust capuchin monkeys (Sapajus) have been reported to use tools during foraging, such as cracking open palm nuts (Figure 3). Squirrel monkeys vary widely in their social organization and mating systems, but generally live in large groups of 15 - 75 individual. Capuchin monkeys tend to live in smaller groups of 10 - 27 individuals (Mittermeier et al. 2013). Squirrel monkeys and capuchin monkeys are commonly observed together in polyspecific groups, which may also include one or more other NMW taxa (Pinheiro et al. 2011). These associations may last hours, days or even years, and are thought to increase foraging efficiency and give better protection against predators (Terborgh 1983).

The pithecids represent a diverse group of some of the most unusual looking and morphologically diverse NWMs, with equally unusual behavior and ecological adaptations (Veiga et al. 2013). The family is formed of two subfamilies, 4 genera and 51 taxa (Table 1). Pithecids are predominantly herbivorous, with some taxa specializing in the consumption of seeds (granivory). They use quadrupedal locomotion and lack prehensile tails. The sakis (Chiropotes) range in size from 1.5 - 3 kg. They are elusive and little known primates that live in small groups and specialize on fruit and seeds (Figure 4). The bearded sakis (Pithecia) have extraordinary bouffant hair and a pronounced beard, which is especially pronounced in males (Figure 4). They are bigger bodied (2 - 4 kg) and are specialized seed predators, with wedge shaped canines and powerful jaws. They also live in larger groups of up to 20 - 30 individuals. The uakaris (Cacajao) are similar to saki monkeys in body size, group size and in having specialized teeth for a diet of seeds and hard fruits, but differ in some aspects of morphology. For example, the uakaris are the only NWMs to have a short tail. Two species groups of uakaris are distinguished, the distinctive bald uakaris, which are completely bald with a bright red face (Figure 1), and black-headed uakaris, which have black hair on their heads and bodies. The titis (Callicebus) represent the most species rich genus of NWMs, with 31 species (and counting), and are much smaller bodied than the other pithecines (0.8 - 1.5 kg). They also differ in their social organization, living in small family groups of a single monogamous adult pair (Mittermeier et al. 2013). They mainly eat fruits, although they also eat leaves, flowers, insects, bird eggs and small vertebrates (Figure 4).

Figure 4

The pitheciines represent a diverse group of unusual looking primates, such as the bearded saki monkey (Chiroptes, left), white-faced saki monkey (Pithecia pithecia, top right) and coppery titi monkey (Callicebus cupreus, bottom right).

Photos by chi King (bottom left), Anna Cotta (top right) and Davidwfx (left), licensed under CC BY 2.0.