Introduction

Colobine or leaf-eating monkeys are a species-rich group of Old World primates with extant species mainly found in the tropical belt of Africa and throughout most of South and Southeast Asia. Today, 10 genera, 80 species and 125 taxa (species and subspecies) are recognized (following Mittermeier et al. 2013; Rowe & Myers 2016; Roos et al. 2020; for a full list of colobine taxa, see ESM, Appendix 2.1). However, the classification of colobine monkeys has changed considerably over the decades, mainly concerning the number of taxa recognized (genera, species), but also concerning the genus, species group and species affiliation of certain taxa. The number of genera has increased from 6 recognized in 1976 to 10 in 2013/2016. Part of this is due to a recent emphasis on the use of time since divergence to differentiate taxa above the species level, as opposed to the traditional method of using morphological differentiation. The number of species has increased even more drastically; from 24 recognized in 1967 (Napier and Napier Reference Napier and Napier1967) to 80 in 2020 (Mittermeier et al. 2013; Rowe & Myers 2016; Roos et al. 2020). This is partially due to the description of new species, e.g. Rhinopithecus strykeri or Trachypithecus popa, but mainly due to the elevation of various subspecies to species level as a result of adopting the Phylogenetic Species Concept (PSC; Cracraft Reference Cracraft, Johnston and Power1983; for a detailed discussion about species concepts, their pros and cons and their applicability, see Groves Reference Groves2012; Groves et al. Reference Groves, Cotterill and Gippoliti2017) and by applying molecular genetics. Comprehensive genetic data from colobines, however, are still rare and exist only for a few taxa, but available information provides nonetheless important insights into colobine phylogeny (see Chapter 4) and for instance, led to the taxonomic reassignment of several taxa; e.g. Nilgiri and purple-faced langurs are today recognized as species of Semnopithecus instead of Trachypithecus (Karanth et al. Reference Karanth2008; Mittermeier et al. 2013; Osterholz et al. Reference Osterholz, Walter and Roos2008; Rowe and Myers 2016; Zhang and Ryder Reference Zhang and Ryder1998).

In this chapter, I give an overview of the taxonomic classification of colobine monkeys. I follow the taxonomy proposed by Mittermeier et al. (2013) and Rowe and Myers (2016) and discuss taxonomic changes mainly since the seminal work by Napier and Napier (Reference Napier and Napier1967) and studies thereafter. Generally, taxonomy is a dynamic science and species delimitations should be regarded as taxonomic hypotheses. The taxonomy of colobines is still under debate and no overall accepted taxonomy is available, which is also reflected by the diverging opinions among the contributors of this book. Part of this debate is due to a lack of ecological, behavioural and genetic data for many colobine species, which has led to classifications largely based on phenotypical differences determined from museum specimens. But an important component is also a general lack of agreement among systematists regarding taxonomic philosophy and how classifications should be generated. The Mittermeier et al. (2013) and Rowe and Myers (2016) classifications generally follow a time-based classification at the genus level and the PSC at the species level, although some exceptions exist where there is a lack of data and/or consensus. Consequently, more work is needed to increase knowledge about colobine diversity and to come to an agreement on taxonomic philosophy towards categorizing this diversity in order to establish a refined and broadly acceptable colobine taxonomy.

Colobine Monkeys

Colobine or leaf-eating monkeys constitute the primate subfamily Colobinae. Together with their sister clade, the cheek-pouched monkeys (subfamily Cercopithecinae), they form the Catarrhini family of Cercopithecidae (Old World monkeys) (Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves2001; Mittermeier et al. 2013; Rowe Reference Rowe1996; Rowe and Myers 2016). African and Asian colobines are traditionally regarded as reciprocally monophyletic clusters (Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves2001; Napier Reference Groves, Napier and Napier1970) and sometimes classified as subtribes based on morphology, the African Colobina and the Asian Semnopithecina (Szalay and Delson Reference Strasser and Delson1979) or Presbytina (Delson Reference Delson and Szalay1975; note: Presbytina has priority), or as tribes based on temporal divergence (Colobini, Presbytini) (Mittermeier et al. 2013; Perelman et al. Reference Perelman, Johnson and Roos2011; Rowe and Myers 2016). However, the reciprocal monophyly of African and Asian colobines was repeatedly questioned. Groves (Reference Groves1989) suggested Nasalis (with Simias as subgenus) as sister group to all other colobines and classified them as Nasalinae as opposite to Colobinae; both together formed the family of Colobidae. In contrast, Jablonski (Reference Jablonski1998a) suggested Procolobus as sister lineage to all other colobines, while Roos et al. (Reference Ebenau, Nadler, Zinner and Roos2011), based on retroposon integrations, found Colobus in this position. However, mitochondrial and nuclear sequence data support reciprocal monophyly of African and Asian clades (Finstermeier et al. Reference Finstermeier, Zinner and Brameier2013; Perelman et al. Reference Perelman, Johnson and Roos2011; Pozzi et al. Reference Pozzi, Hodgson and Burrell2014; Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; Sterner et al. Reference Sterner, Raaum, Zhang, Stewart and Disotell2006; Springer et al. Reference Koenig, Borries, Kappelar and Watts2012; Ting et al. Reference Kümpel, Milner-Gulland, Rowcliffe and Cowlishaw2008; Wang et al. Reference Huang, Cui, Scott, Wang and Xiao2012). Here, I recognize African and Asian colobine groups as reciprocal monophyletic clades and classify them as tribes, Colobini and Presbytini, as they separated roughly at the same time, ca. 12 million years ago, as did the two Cercopithecinae tribes, Papionini and Cercopithecini (Finstermeier et al. Reference Finstermeier, Zinner and Brameier2013; Perelman et al. Reference Perelman, Johnson and Roos2011; Pozzi et al. Reference Pozzi, Hodgson and Burrell2014; Springer et al. Reference Koenig, Borries, Kappelar and Watts2012).

African Colobines

Following Mittermeier et al. (2013) and Rowe and Myers (2016), African colobines are grouped into three genera with a total of 23 species. The three genera refer to black-and-white colobus (genus Colobus), olive colobus (genus Procolobus), and red colobus (genus Piliocolobus) (Groves Reference Groves2001; Mittermeier et al. 2013; Rowe and Myers 2016). However, in the past these three genera were combined into either a single genus (Colobus) with three subgenera (Napier and Napier Reference Napier and Napier1967) or into two genera, Colobus and Procolobus, with either the former (Napier and Napier Reference Napier and Napier1985) or the latter containing Piliocolobus as a subgenus (Brandon-Jones Reference Brandon-Jones and MacDonald1984a; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves1989; Grubb et al. Reference Grubb, Butynski and Oates2003; Kuhn Reference Kuhn, Starck, Schneider and Kuhn1967; Rowe Reference Rowe1996; Strasser and Delson Reference Leakey, Delson, Leakey and Harris1987). Genetic data confirm a sister group relationship between Procolobus and Piliocolobus, and show that the three major lineages diverged 6–10 million years ago (Finstermeier et al. Reference Finstermeier, Zinner and Brameier2013; Perelman et al. Reference Perelman, Johnson and Roos2011; Pozzi et al. Reference Pozzi, Hodgson and Burrell2014; Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; Springer et al. Reference Koenig, Borries, Kappelar and Watts2012; Ting Reference Locatelli, Lafay and Liegeois2008a).

Black-and-white colobus, genus Colobus, are distributed across equatorial Africa. In earlier times, only one (guereza; e.g. Schwarz Reference Schwarz1929) or two species (guereza, polykomos; e.g. Napier and Napier Reference Napier and Napier1967) were recognized, but nowadays there is consensus that Colobus contains (four to) five species (Dandelot Reference Dandelot, Meester and Setzer1971; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves2001; Groves et al. Reference Groves, Wilson and Reeder1993; Mittermeier et al. 2013; Napier and Napier 1994; Oates and Trocco Reference Oates and Trocco1983; Rowe Reference Rowe1996; Rowe and Myers 2016). Two of them, the King colobus (C. polykomos) and the white-thighed colobus (C. vellerosus), are monotypic, while the black colobus (C. satanas) contains two subspecies, the Angolan colobus (C. angolensis) seven subspecies and the guereza (C. guereza) eight subspecies (Mittermeier et al. 2013; Rowe and Myers 2016). It should be noted that the PSC has not been fully applied to this group, and that at least some of the Colobus subspecies could be elevated to species status upon re-evaluation of their diversity.

The genus Procolobus is monotypic and contains only the olive colobus, P. verus (Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves2001; Grubb et al. Reference Grubb, Butynski and Oates2003; Mittermeier et al. 2013; Napier Reference Napier and Napier1985; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994; Rowe Reference Rowe1996; Rowe and Myers 2016). The species has a disjunct distribution in western Africa, but there is no evidence for any subspecific variation (Davies and Oates Reference Bennett, Davies, Davies and Oates1994).

Red colobus monkeys, genus Piliocolobus, have a wide, but patchy distribution from Senegal in West Africa across the continent to Zanzibar in the East. The taxonomy of red colobus monkeys is debated and various classifications with different numbers of species and varying taxon assemblies were proposed. While Rahm (Reference Rahm, Napier and Napier1970), followed by Davies and Oates (Reference Bennett, Davies, Davies and Oates1994), lumped all taxa in the single species badius, Napier (Reference Napier and Napier1985) and Napier and Napier (Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994) recognized two species, kirkii and badius. Delson et al. (Reference Delson, Groves, Grubb, Miu, Wang, Honacki, Kinman and Koeppl1982), Groves (Reference Groves, Wilson and Reeder1993) and Rowe (Reference Rowe1996) listed four species (badius, pennantii, preussi, rufomitratus), Oates (Reference Oates1986, Reference Brandon-Jones, Edwards, Booth and Choy1996a) five species (badius, gordonorum, kirkii, pennantii, rufomitratus), Dandelot (Reference Dandelot, Meester and Setzer1971) split red colobus monkeys into eight species (badius, ellioti, kirkii, pennantii, preussi, rufomitratus, tholloni, waldronae), and Groves (Reference Groves2001) into nine species (badius, foai, gordonorum, kirkii, pennantii, preussi, rufomitratus, tephrosceles, tholloni). Recently, a total of 17 taxa, all recognized as monotypic species (Upper Guinea red colobus, P. badius; Bouvier’s red colobus, P. bouvieri; Niger Delta red colobus, P. epieni; Foa’s red colobus, P. foai; Udzungwa red colobus, P. gordonorum; Zanzibar red colobus, P. kirkii; Lang’s red colobus, P. langi; Oustalet’s red colobus, P. oustaleti; Lomami red colobus, P. parmentieri; Pennant’s red colobus, P. pennantii; Preuss’s red colobus, P. preussi; Tana River red colobus, P. rufomitratus; Semliki red colobus, P. semlikiensis; Temminck’s red colobus, P. temminckii; ashy red colobus, P. tephrosceles; Tshuapa red colobus, P. tholloni; Miss Waldron’s red colobus, P. waldronae), have been suggested (Mittermeier et al. 2013; Rowe and Myers 2016). The current classification is mainly based on investigations of museum specimens and limited data from the field. Much more work, including ecological, phenotypical, acoustic and genetic studies, is needed to better describe and understand diversity in red colobus monkeys. So far, only a single genetic study on intra-generic diversity in Piliocolobus has been published (Ting Reference Locatelli, Lafay and Liegeois2008a), which however, allows first intriguing insights into Piliocolobus phylogeny and suggests relatively deep splits among various taxa; the most recent common ancestor of the genus has been dated back to ca. 3 million years (Ting Reference Locatelli, Lafay and Liegeois2008a; see also Chapter 4).

Asian Colobines

Incorporating the recently proposed changes in Trachypithecus taxonomy (Roos et al. 2020), Asian colobines contain 57 species in seven genera. However, the classification of Asian colobines is in ongoing debate and needs further clarification. Although Groves (Reference Groves1989) suggested Nasalis (with Simias as subgenus) as a sister clade to all other colobines, a common ancestry of Asian colobines is generally accepted and confirmed by genetic data (Perelman et al. Reference Perelman, Johnson and Roos2011; Pozzi et al. Reference Pozzi, Hodgson and Burrell2014; Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; Springer et al. Reference Koenig, Borries, Kappelar and Watts2012; Sterner et al. Reference Sterner, Raaum, Zhang, Stewart and Disotell2006; Ting et al. Reference Kümpel, Milner-Gulland, Rowcliffe and Cowlishaw2008; Wang et al. Reference Wang, Forstner and Zhang1997, Reference Ang, Srivasthan, Md-Zain, Ismail and Meier2012; Zhang and Ryder Reference Zhang and Ryder1998). In most classifications, Asian colobines are divided into two groups, a langur group subsuming the genera Presybtis, Trachypithecus and Semnopithecus, and an odd-nosed monkey group with the genera Rhinopithecus, Pygathrix, Nasalis and Simias (Groves Reference Groves2001; Mittermeier et al. 2013; Rowe Reference Rowe1996; Rowe and Myers 2016). Genetic data clearly confirm the monophyly of the odd-nosed monkey group, but not of the langur group (Finstermeier et al. Reference Finstermeier, Zinner and Brameier2013; Liedigk et al. Reference Liedigk, Yang and Jablonski2012; Perelman et al. Reference Perelman, Johnson and Roos2011; Pozzi et al. Reference Pozzi, Hodgson and Burrell2014; Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; Springer et al. Reference Koenig, Borries, Kappelar and Watts2012; Sterner et al. Reference Sterner, Raaum, Zhang, Stewart and Disotell2006; Ting et al. Reference Kümpel, Milner-Gulland, Rowcliffe and Cowlishaw2008; Wang et al. Reference Huang, Cui, Scott, Wang and Xiao2012). In fact, mitochondrial and nuclear data revealed contradicting branching patterns among the three langur genera and the odd-nosed monkeys, and have been suggested to be the result of ancient hybridization events among the three langur genera (e.g. Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; Ting et al. Reference Kümpel, Milner-Gulland, Rowcliffe and Cowlishaw2008; Wang et al. Reference Huang, Cui, Scott, Wang and Xiao2012; see also Chapter 4).

The genus-level classification of members of the (probably paraphyletic) langur group was one of the most controversial discussed topics in colobine taxonomy, but seems to be settled now. Today, most authorities recognize three genera, surilis (genus Presbytis), lutungs (genus Trachypithecus), and Indian langurs (genus Semnopithecus) (Brandon-Jones et al. Reference Brandon-Jones2004; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves1989, Reference Groves2001; Mittermeier et al. 2013; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe Reference Rowe1996; Rowe and Myers 2016), but in the past one to four genera were proposed. While Napier (Reference Napier and Napier1985) and Napier and Napier (Reference Napier and Napier1967, Reference Napier and Napier1994) combined all taxa of the langur group into the single genus Presbytis, Hill (Reference Hill1934) and Pocock (Reference Pocock1935, Reference Allen1939) divided them into four genera (Presbytis, Semnopithecus, Trachypithecus, Kasi). Already Reichenbach (Reference Reichenbach1862) recognized these four groups, but classified them as subgenera of the genus Semnopithecus. Alternative classifications suggest two genera, Presbytis and Semnopithecus, with Trachypithecus as subgenus of the latter (Brandon-Jones Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1995a,Reference Brandon-Jonesb, 1996; Strasser and Delson Reference Leakey, Delson, Leakey and Harris1987). Most recent classifications do not accept Kasi as a valid genus or subgenus, but Rowe (Reference Rowe1996) maintained Kasi as a subgenus of Trachypithecus. In fact, genetic studies have shown that the two species of Kasi, the Nilgiri and purple-faced langur, are paraphyletic and nested within the Semnopithecus clade, and consequently should be assigned to this genus (Karanth et al. Reference Karanth2008; Osterholz et al. Reference Osterholz, Walter and Roos2008; Zhang and Ryder Reference Zhang and Ryder1998).

Compared to the langur group, the genus-level classification of the members of the odd-nosed monkey group was historically less problematic. Generally, it is accepted that the odd-nosed monkeys contain four clearly distinct lineages that are commonly recognized as distinct genera, snub-nosed monkeys (genus Rhinopithecus), doucs (genus Pygathrix), the proboscis monkey (genus Nasalis), and the simakobu (genus Simias) (Brandon-Jones et al. Reference Brandon-Jones2004; Groves Reference Groves2001; Jablonski Reference Jablonski1998a; Jablonski and Peng Reference Jablonski and Peng1993; Mittermeier et al. 2013; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994; Rowe and Myers 2016). However, in the past Rhinopithecus was classified as a subgenus of Pygathrix (Brandon-Jones Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1996b; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Delson Reference Delson and Szalay1975; Groves Reference Groves, Napier and Napier1970, Reference Boesch and Boesch1989; Napier Reference Napier and Napier1985; Rowe Reference Rowe1996). Likewise, Simias was proposed as subgenus of Nasalis (Brandon-Jones Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1996b; Delson Reference Delson and Szalay1975; Groves Reference Groves, Napier and Napier1970, Reference Boesch and Boesch1989; Rowe Reference Rowe1996). Genetic data suggest a close phylogenetic relationship between Nasalis and Simias, and a basal position of Rhinopithecus among odd-nosed monkeys (Liedigk et al. Reference Liedigk, Yang and Jablonski2012; Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; see also Chapter 4), thus rejecting a monophyletic Rhinopithecus-Pygathrix clade. Jablonski (Reference Jablonski1998a) and Jablonski and Peng (Reference Jablonski and Peng1993) further divided Rhinopithecus into two subgenera, Presbyticus with R. avunculus and Rhinopithecus subsuming all other snub-nosed monkey taxa, which, however, is not well accepted. Interestingly, Simias and Nasalis diverged relative recently, just 1–2 million years ago (Liedigk et al. Reference Liedigk, Yang and Jablonski2012; Roos et al. Reference Ebenau, Nadler, Zinner and Roos2011; see also Chapter 4). This is well in the range of temporal divergences typically found among species and consequently, both genera would need to be combined into a single genus when a time-based classification at the genus level is applied.

Langur Group

Surilis, genus Presbytis (sensu stricto), occur in the Sundaland region of Southeast Asia with main distributions on Sumatra, Borneo, Java, the Mentawai Islands and the Malay Peninsula. Napier and Napier (Reference Napier and Napier1967) divided surilis into five species, aygula (=comata), frontata, melalophos, rubicunda and potenziani, of which the former four were grouped into the P. aygula (=comata) group, referring to today’s genus Presbytis, while potenziani was assigned to the P. cristatus group, which refers to today’s genus Trachypithecus. Later on, Napier (Reference Napier and Napier1985) and Napier and Napier (1994) correctly assigned potenziani to what is now referred to as the P. melalophos group and recognized, following Groves (Reference Groves, Napier and Napier1970), a total of seven species, melalophos, frontata, rubicunda, potenziani, comata, hosei and thomasi. Davies and Oates (Reference Bennett, Davies, Davies and Oates1994) and Rowe (Reference Rowe1996) followed this classification, but Rowe (Reference Rowe1996) in addition separated femoralis from melalophos. Brandon-Jones (Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1995a, Reference Brandon-Jones1996b) recognized also seven species, but he used a different species assembly. He also accepted frontata, potenziani and rubicunda as distinct species, however, he divided the melalophos complex into three species, melalophos, femoralis and siamensis, and separated fredericae from comata, while he followed Napier and Napier (Reference Napier and Napier1967) by integrating thomasi and hosei into comata. Although there is now general consensus concerning the distinct species status of comata, hosei, frontata, thomasi and potenziani, there is ongoing debate about the taxonomy of the melalophos complex. Traditionally all taxa were combined into the single species melalophos (Chasen Reference Chasen1940; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Napier Reference Napier and Napier1985; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994), but it was proposed to split them into two (Aimi and Bakar Reference Aimi and Bakar1992, Reference Aimi and Bakar1996; Pocock Reference Pocock1935; Rowe Reference Rowe1996; Wilson and Wilson Reference Wilson and Wilson1977), three (Brandon-Jones Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1996b; Brandon-Jones et al. Reference Brandon-Jones2004), five (Groves Reference Groves2001), or even eight species (Mittermeier et al. 2013; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe and Myers 2016). Recently, also the two subspecies of potenziani and the three subspecies of hosei were elevated to species (Mittermeier et al. 2013; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe and Myers 2016). Following the most recent classification (Mittermeier et al. 2013; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe and Myers 2016), the genus Presbytis comprises a total of 17 species. Twelve of them, the black-and-white langur (P. bicolor), Miller’s grizzled langur (P. canicrus), white-fronted langur (P. frontata), Hose’s langur (P. hosei), black-crested Sumatran langur (P. melalophos), mitred langur (P. mitrata), Natuna Islands langur (P. natunae), Pagai langur (P. potenziani), Sabah grizzled langur (P. sabana), Siberut langur (P. siberu), black Sumatran langur (P. sumatrana), and Thomas’s langur (P. thomasi), are monotypic, while the cross-marked langur (P. chrysomelas) and Javan langur (P. comata) contain two subspecies each, the banded langur (P. femoralis) three subspecies, the pale-thighed langur (P. siamensis) four subspecies, and the maroon langur (P. rubicunda) five subspecies. The herein adopted classification of Presbytis is highly disputed and should be treated as preliminary, but it reflects best phylogenetic relationships among taxa (Md-Zain Reference Md-Zain2001; Md-Zain et al. Reference Md-Zain, Morales and Hasan2008; Meyer et al. Reference Meyer, Rinaldi, Ramlee, Perwitasari-Farajallah, Hodges and Roos2011; Vun et al. Reference Dong2011; see also Chapter 4).

With 20 species currently recognized, the genus Trachypithecus is the most speciose among all Asian colobines. The genus is widely distributed on the mainland and Sundaland region of Southeast Asia, from Bhutan and Assam in the West, to Vietnam and Southern China in the East, and South to Java. Napier and Napier (Reference Napier and Napier1967) classified taxa of today’s genus Trachypithecus as members of the Presbytis cristatus group and recognized a total of seven species, cristatus, francoisi, geei, pileatus, obscurus, phayrei and potenziani. One of these species, potenziani, was later correctly reallocated to the P. aygula (=comata) group (i.e. genus Presbytis) (Napier Reference Napier and Napier1985; Napier and Napier 1994). Davies and Oates (Reference Bennett, Davies, Davies and Oates1994) in principle followed this classification, but included also the two species of the Presbytis senex (=vetulus) group (Napier Reference Napier and Napier1985; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994), vetulus and johnii, in Trachypithecus, and separated T. auratus from T. cristatus according to Weitzel and Groves (Reference Weitzel and Groves1985). Rowe (Reference Rowe1996) adopted the classification of Davies and Oates (Reference Bennett, Davies, Davies and Oates1994), but additionally recognized delacouri as a full species. In contrast, Brandon-Jones (Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1995b, Reference Brandon-Jones1996b) divided Trachypithecus (as subgenus of Semnopithecus) into 12 species, vetulus, johnii, pileatus geei, cristatus, auratus, obscurus, barbei, francoisi, hatinhensis, laotum and delacouri. Compared to other classification, he placed phayrei in obscurus, and recognized barbei as distinct species. Moreover, he separated the francoisi subspecies delacouri, hatinhensis and laotum from the nominate form, and reclassified poliocephalus as subspecies of the South Indian johnii. In 1992, he described a new taxon, ebenus, from northern Vietnam as subspecies of the Javan auratus (Brandon-Jones Reference Brandon-Jones1995b). Roos et al. (Reference Roos, Thanh, Walter and Nadler2007) subsequently reallocated ebenus to the francoisi complex and recognized four species within the group, francoisi, poliocephalus (including leucocephalus), delacouri, and laotum (with hatinhensis as subspecies and ebenus as synonym of hatinhensis). Brandon-Jones et al. (Reference Brandon-Jones2004) eventually accepted Trachypithecus as a full genus and recognized the following nine species: auratus, villosus (=cristatus), barbei, obscurus, francoisi (including ebenus and hatinhensis), laotum, poliocephalus (including leucocephalus), geei and pileatus. In contrast, Groves (Reference Groves2001) proposed a classification of Trachypithecus into 17 species, arranged in five species groups. According to his arrangement, the pileatus group consists of pileatus, geei and shortridgei, the cristatus group of cristatus, auratus, germaini and barbei, the obscurus group of obscurus and phayrei, the francoisi group of francoisi, hatinhensis, poliocephalus, laotum, delacouri and ebenus and the vetulus group includes vetulus and johnii. The taxonomy of Groves (Reference Groves2001) was generally adopted by Mittermeier et al. (2013), Roos et al. (Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014) and Rowe and Myers (2016), although with some minor changes. First, the two species of the vetulus group were reassigned to Semnopithecus (Karanth Reference Karanth2008; Karanth et al. Reference Karanth2008; Osterholz et al. Reference Osterholz, Walter and Roos2008; Wang et al. Reference Huang, Cui, Scott, Wang and Xiao2012, Reference Abood, Lee, Burivalova, Garcia‐Ulloa and Koh2015; Zhang and Ryder Reference Zhang and Ryder1998). Second, barbei was reallocated to the obscurus group (Geissmann et al. Reference Brandon-Jones, Eudey and Geissmann2004). Third, leucocephalus was separated from poliocephalus on species level (Liu Z et al. Reference Li, Grueter and Ren2013a). Fourth, phayrei crepusculus was elevated to species level (Liedigk et al. Reference Liedigk, Thinh, Nadler, Walter and Roos2009; Roos et al. 2019). Fifth, a new species as described from central Myanmar (T. popa) and phayrei shanicus was elevated to species level and renamed into melamera (Roos et al. 2020). Sixth, within the T. cristatus group, mauritius and margarita were elevated to full species and a new taxon, selangorensis, was described (Nadler et al. Reference Nadler, Walter and Roos2005; Roos et al. Reference Osterholz, Walter and Roos2008). By incorporating these changes, the genus Trachypithecus contains four species groups and a total of 22 species: T. pileatus group with golden langur (T. geei), capped langur (T. pileatus with four subspecies) and Shortridge’s langur (T. shortridgei), T. cristatus group with silvered langur (T. cristatus with two subspecies), Selangor silvery langur (T. selangorensis), East Javan langur (T. auratus), West Javan langur (T. mauritius), Germain’s langur (T. germaini) and Annamese langur (T. margarita), T. obscurus group with dusky langur (T. obscurus with seven subspecies), Phayre’s langur (T. phayrei), Shan States langur (T. melamera), Popa langur (T. popa), Indochinese grey langur (T. crepusculus) and Tenasserim langur (T. barbei), and the T. francoisi group with the François’s langur (T. francoisi), white-headed langur (T. leucocephalus), Cat Ba langur (T. poliocephalus), Delacour’s langur (T. delacouri), Hatinh langur (T. hatinhensis), Laos langur (T. laotum) and black langur (T. ebenus). Similar to Presbytis, the classification of Trachypithecus is not fully clarified and needs additional work.

The genus Semnopithecus encompasses the langurs of the Indian subcontinent. Most commonly, the genus (or Presbytis entellus group) was considered to contain only a single species, the Hanuman langur, entellus, with 10–16 subspecies (Brandon-Jones et al. Reference Brandon-Jones2004; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Ellermann and Morrison-Scott Reference Ellermann and Morrison-Scott1951; Groves Reference Groves1989; Napier Reference Napier and Napier1985; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994; Pocock Reference Pocock1928, Reference Allen1939; Roonwal Reference Roonwal1981a; Roonwal and Mohnot Reference Roonwal and Mohnot1977; Roonwal et al. Reference Johnson, Roonwal, Mohonot and Rathore1984; Rowe Reference Rowe1996). However, Hanuman langurs have sometimes been divided into two (either entellus and hypoleucos; Brandon-Jones Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1996b, or entellus and priam Brandon-Jones Reference Brandon-Jones2004), four (entellus, schistaceus, priam, hypoleucos) (Hill Reference Hill1939), or seven species (schistaceus, ajax, hector, entellus, hypoleucos, dussumieri, priam) (Groves Reference Groves2001). Moreover, two species, vetulus and johnii, previously assigned to the genus Kasi or Trachypithecus have been recently integrated into Semnopithecus based on genetic evidence (Karanth Reference Karanth2008; Karanth et al. Reference Karanth2008; Osterholz et al. Reference Osterholz, Walter and Roos2008; Wang et al. Reference Huang, Cui, Scott, Wang and Xiao2012, Reference Abood, Lee, Burivalova, Garcia‐Ulloa and Koh2015; Zhang and Ryder Reference Zhang and Ryder1998). In general, the current taxonomy of Semnopithecus follows Groves (Reference Groves2001) with the exception that dussumieri is not recognized as valid species (Ashalakshmi et al. Reference Ashalakshmi, Nag and Karanth2015; Nag et al. Reference Nag, Pramod and Karanth2011) and by including vetulus and johnii. Accordingly, Semnopithecus contains a total of eight species, Bengal sacred langur (S. entellus), Chamba sacred langur (S. ajax), Terai sacred langur (S. hector), Nepal sacred langur (S. schistaceus), Malabar sacred langur (S. hypoleucos with three subspecies), tufted grey langur (S. priam with three species), Nilgiri langur (S. johnii), and purple-faced langur (S. vetulus with four subspecies). As for the other two langur genera, also for Semnopithecus more work, particularly for the northern taxa is required.

Odd-Nosed Monkey Group

Snub-nosed monkeys, genus Rhinopithecus, occur only in China, Vietnam and Myanmar, with all species having relatively small geographical distributions (Groves Reference Groves2001; Mittermeier et al. 2013; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe Reference Rowe1996; Rowe and Myers 2016). Originally, only two species, avunculus and roxellana, the latter with three subspecies, have been recognized (Ellermann and Morrison-Scott Reference Ellermann and Morrison-Scott1951; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994). However, Groves separated first brelichi as distinct species (Groves Reference Groves, Napier and Napier1970), which was followed by Napier (Reference Napier and Napier1985). Later on, Groves (Reference Groves1989) also elevated bieti to full species, a classification that is now widely accepted (Brandon-Jones Reference Brandon-Jones and MacDonald1984a, Reference Brandon-Jones1996b; Brandon-Jones et al. Reference Brandon-Jones2004; Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Jablonski Reference Jablonski1998a; Jablonski and Peng Reference Jablonski and Peng1993; Mittermeier et al. 2013; Roos et al. Reference Roos, Thanh, Walter and Nadler2007, Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe Reference Rowe1996; Rowe and Myers 2016). In contrast, a separation of avunculus in its own subgenus Presbyticus from its congenerics as proposed by Jablonski (Reference Jablonski1998a) and Jablonski and Peng (Reference Jablonski and Peng1993) has not been adopted and can also not be justified with available genetic data (Liedigk et al. Reference Liedigk, Yang and Jablonski2012; Roos et al. Reference Roos, Thanh, Walter and Nadler2007; Yu et al. Reference Guo, Dang and Chen2016; see also Chapter 4). In 2011, a new species of snub-nosed monkey, strykeri, was described from the border region of Myanmar and China (Geissmann et al. Reference Geissmann, Lwin and Aung2011). Although closely related to bieti, its distinctiveness was confirmed by genetics (Liedigk et al. Reference Liedigk, Yang and Jablonski2012; Zhou et al. Reference Zhou, Wang and Pan2014). By adding strykeri, the genus Rhinopithecus contains a total of five species. Four of them, the Tonkin snub-nosed monkey (R. avunculus), the Guizhou snub-nosed monkey (R. brelichi), the Yunnan snub-nosed monkey (R. bieti), and the Myanmar or black snub-nosed monkey (R. strykeri), are monotypic, while for the golden snub-nosed monkey (R. roxellana) three subspecies are listed.

Doucs, genus Pygathrix, are found in Vietnam, Laos and Cambodia, east of the Mekong River. Traditionally, only one species, nemaeus, with two subspecies has been recognized (Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves, Napier and Napier1970; Jablonski and Peng Reference Jablonski and Peng1993; Napier Reference Napier and Napier1985; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994), although sometimes both taxa have been classified as separate species (Brandon-Jones Reference Brandon-Jones and MacDonald1984a; Brandon-Jones et al. Reference Brandon-Jones2004; Rowe Reference Rowe1996). An additional taxon, cinerea, was described as a subspecies of nemaeus (Nadler Reference Nadler1997). Brandon-Jones et al. (Reference Brandon-Jones2004) followed this subspecies arrangement, while most other authorities support a species level recognition for cinerea (Groves Reference Groves2001; Mittermeier et al. 2013; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe and Myers 2016). The distinctiveness of all three douc taxa is supported by genetic data (Liedigk et al. Reference Liedigk, Yang and Jablonski2012; Roos and Nadler Reference Roos and Nadler2001; Roos et al. Reference Roos, Thanh, Walter and Nadler2007). Accordingly, the three taxa of Pygathrix are currently classified as separate species, the red-shanked douc (P. nemaeus), the black-shanked douc (P. nigripes), and the grey-shanked douc (P. cinerea).

The genus Nasalis contains the single species N. larvatus, the proboscis monkey from Borneo. The species is commonly regarded as monotypic (Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves, Napier and Napier1970, Reference Groves2001; Mittermeier et al. 2013; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994; Roos et al. Reference Roos, Boonratana, Supriantna, Fellowes, Groves, Nash, Rylands and Mittermeier2014; Rowe Reference Rowe1996; Rowe and Myers 2016), but sometimes two subspecies are listed (e.g. Brandon-Jones et al. Reference Brandon-Jones2004).

The genus Simias comprises the single species S. concolor (Davies and Oates Reference Bennett, Davies, Davies and Oates1994; Groves Reference Groves2001; Grubb et al. Reference Grubb, Butynski and Oates2003; Mittermeier et al. 2013; Napier and Napier Reference Napier and Napier1967, Reference Bennett, Davies, Davies and Oates1994; Rowe Reference Rowe1996; Rowe and Myers 2016). The simakobu or pig-tailed langur is endemic to the Mentawai Islands and contains two subspecies.

Future Directions

The taxonomic classification of colobines is one of the most disputed among primates with different opinions concerning the number of genera and species to be recognized, and also concerning how to best group taxa, i.e. their assignment to species, species groups and genera. In recent years, our knowledge about colobines has expanded considerably and many taxonomic issues were settled. However, compared to the cercopithecines, colobines are still a neglected group of primates and we have to learn more about their taxonomic diversity and evolutionary history, particularly regarding geographically widespread and taxon-rich genera such as Piliocolobus, Colobus, Presbytis, Trachypithecus and Semnopithecus. Accordingly, much more work in the field and laboratory providing information on ecology, behaviour, morphology and genetics is needed. However, to establish a well-grounded and broadly acceptable colobine taxonomy, we need, besides additional biological data, also an agreement among systematists regarding taxonomic philosophy and how classifications should be generated.