Schwarz (1927) placed all the currently recognised Patas Monkeys as subspecies of Erythrocebus patas (Schreber, 1775). Napier and Napier (1967) listed the Patas Monkey as comprising one species, with four subspecies: Western Patas Monkey E. p. patas (Schreber, 1775), Eastern Patas Monkey E. p. pyrrhonotus (Hemprich and Erhenberg, 1829), Southern Patas Monkey E. p. baumstarki Matschie, 1906, and the Aïr Massif Patas Monkey E. p. villiersi Dekeyser, 1950. Verheyen (1962), Dandelot (1968, 1974), and Kingdon (1971, 1997) also recognised these four subspecies, but listed Erythrocebus as a subgenus of Cercopithecus. This was based, evidently, on the indication of paraphyly with such species as Grivet Monkey Chlorocebus aethiops, Vervet Monkey Chlorocebus pygerythrus, and l'Hoest’s Monkey Allochrocebus lhoesti that were, at that time, all considered members of the genus Cercopithecus (Disotell 2000). This was resolved by the reinstatement of the genus Chlorocebus Gray, 1870, for the Grivet, Vervet, and other species of Savanna Monkeys, and the genus Allochrocebus Elliot, 1913, for l’Hoest’s Monkey and other species of Mountain Monkeys.

Grubb et al. (2003) recorded that Schwarz (1927) revised the systematics of the Patas Monkey and recognised three subspecies (those cited above except for E. p. villiersi, from the Aïr Massif, central northern Niger). They concluded that these subspecies “do not appear to be well-founded, so further research into their status is needed” (p. 1328) and listed them as synonyms of E. patas. Groves (2001, 2005) cited Loy (1987) as showing that the colours of the face and nose change during pregnancy and, believing that this covered “at least some of the supposed subspecific variation” (p. 199), listed the subspecies as junior synonyms of a monotypic E. patas, as did Kingdon et al. (2008). Isbell (2013) pointed out, however, that, while Loy (1974) believed that the captive Patas in his care were from Ethiopia (corresponding to the range then believed to be of E. p. pyrrhonotus), Goldman and Loy (1997, J. Loy. pers. comm. in Isbell 2013) subsequently reported they were, in fact, from Nigeria (corresponding to the range of E. p. patas). Isbell et al. (2009) found that the facial colour of female E. p. pyrrhonotus in Kenya does not change with reproductive status, and that the nose remains white once they are adult, distinguishing them from the black-nosed E. p. patas and E. p. baumstarki.

Isbell (2013), De Jong et al. (2008), Butynski and De Jong (2022), and De Jong and Butynski (2023) recognised E. p. pyrrhonotus as occurring in central and eastern Africa (Chad, Central African Republic, South Sudan, southern Sudan, northeastern Democratic Republic of Congo, western Ethiopia, northern Uganda, and western, northwestern, and central Kenya).

Erythrocebus baumstarki was described by Paul Matschie in 1906 (not 1905 as often stated) as a species (type locality: Ikoma, western Serengeti, Tanzania). Elliot (1913) appears to be the last authority to recognize baumstarki as a species. Subsequently, baumstarki was treated as either a subspecies or synonym of E. patas (e.g., Schwarz 1927, Allen 1939, Hill 1966, Dandelot 1974, Groves 2001, 2005, Grubb et al. 2003, De Jong et al. 2008, 2009, De Jong and Butynski 2012, 2018, Isbell 2013, Kingdon 2015). In 2021, De Jong and Butynski resurrected baumstarki as a species based on its considerable geographic isolation and the distinctive coloration and pattern of the pelage. They presented the first evidence for E. baumstarki in Kenya and reviewed its historic and current geographic distributions. Resurrecting baumstarki to species status agrees with Gippoliti (2017).

Erythrocebus p. villiersi, confined to the Aïr Massif in central Niger, has been recognized as a subspecies of E. patas (Dekeyser 1950, Hill 1966, Dandelot 1974), or as a synonym of E. patas (Groves 2001, 2005, Grubb et al. 2003). Isbell (2013) considered E. p. villiersi to be a synonym of E. p. patas because the only difference from E. p. patas is said to be body size, which is “heavily influenced by environmental conditions” (p. 257).

Hill (1966) and Dandelot (1974) recognize the Blue Nile Patas Monkey E. p. poliophaeus (Heuglin in Reichenbach, 1863) while it is considered a synonym of E. patas by Groves (2001, 2005), Grubb et al. (2003), and Isbell (2013). In a review of the taxonomy of Erythrocebus, Gippoliti (2017) insisted that the diversity of the wide-spread Patas Monkeys was underestimated. Adopting the ‘Phylogenetic Species Concept’, he recommended that poliophaeus, baumstarki, and pyrrhonotus be reinstated as species, with pyrrhonotus having two subspecies, the nominate form and formosus Elliot, 1909, this latter from southwestern Ethiopia, Uganda, and, presumably, southeastern South Sudan. Erythrocebus poliophaeus of western Ethiopia and adjoining Sudan along the Blue Nile River occurs within the range formerly ascribed to E. p. pyrrhonotus. Butynski and De Jong (2022) follow Gippoliti (2017) in recognizing poliophaeus as a species.

In an earlier Red List assessment (Kingdon et al., 2008), Erythrocebus was taken to be a monotypic genus and Erythrocebus patas was taken to be as a monotypic species following Groves (2001, 2005) and Grubb et al. (2003). A review of the taxonomic arrangement of Erythrocebus is pending. Until then we provisionally recognise three species of patas (E. patas, E. baumstarki, and E. poliophaeus,) and three subspecies (E. patas patas, E. patas pyrrhonotus, and E. patas villiersi).

The Patas Monkey is listed as Near Threatened. Previously, under a different taxonomic arrangement (see Taxonomic Section), Erythrocebus patas was assessed as Least Concern (Kingdon et al. 2008). Although Erythrocebus patas has a wide geographic range and is sometimes locally common in West Africa and Central Africa, there is an observed population reduction throughout its range, particularly in eastern Africa. Erythrocebus patas does not meet the criteria for Vulnerable; however, the criteria for A2cd are nearly met as the population has likely declined by more than 20% over the last 30 years (three generations). Abundance, extent of occurrence (EOO), and area of occupancy (AOO) are expected to continue to decline as the causes of these declines (mainly habitat degradation, loss, and fragmentation, but also hunting) are ongoing and unlikely to be reversed.

Erythrocebus patas is endemic to tropical Africa. This species ranges in the Sahelian Region and Sudanian Region north of the equatorial forests and south of the Sahara, from western Senegal and The Gambia, southern Mauritania, Guinea Bissau, and northern Guinea, eastwards through northern Uganda, South Sudan, and southwestern Ethiopia, and southwards to the Laikipia Plateau in central Kenya (De Jong et al. 2008, 2009; Oates 2011; Isbell 2013; De Jong and Butynski 2017, 2023). Altitudinal range is from sea level to 2,050 m asl (Wallis 2022, De Jong and Butynski 2023).

Three subspecies are recognised:

1) Erythrocebus p. patas (Western Patas Monkey). North of the equatorial forests and south of the Sahara from western Senegal and The Gambia, southern Mauritania, southern Mali, Guinea Bissau, Guinea, and Sierra Leone eastwards to northern Cameroon, southern Chad, and western Central African Republic (Campbell et al. 2007, Oates 2011, Isbell 2013, De Jong and Butynski 2021, Chesney et al. 2023, Wild Chimpanzee Foundation pers. comm. 2024, C. Arong and J. Linder pers. comm 2024). The eastern geographic limits are unclear. Although E. p. patas ranges into the western Chad and Central African Republic, E. p. pyrrhonotus occurs in the east of these countries. A cline or region of hybridization may exist in the centre of both Chad and the Central African Republic. Altitudinal range is ~0–600 m asl. About 600 m asl at Dindefelo Falls, Reserve Naturelle Communautaire de Dindefelo, southeastern Senegal (A. Galat-Luong and G. Galat pers. comm. 2024).

2) Erythrocebus p. pyrrhonotus (Eastern Patas Monkey). North of the equatorial forests and south of the Sahara from southeastern Chad (e.g., Zakouma National Park and Ennedi Massif) eastwards through, eastern Central African Republic, northern Democratic Republic of Congo, South Sudan, and central and southern Sudan (presumably west of the White Nile River, perhaps as far north as Et Tura), southeastwards through northern Uganda (e.g., Murchison National Park, Kidepo Valley National Park, Pian Upe Wildlife Reserve) to northwestern and central Kenya (e.g., West-Pokot County, Laikipia County; De Jong et al. 2008; De Jong and Butynski 2012, 2013, 2014, 2023; Butynski and De Jong 2014a, 2014b, 2017). The western geographic limits are unclear. Altitudinal range is about 650 (Kakuma, northwest Kenya; De Jong 2004) to 2,050 m asl (Laikipia, central Kenya; De Jong and Butynski 2017, 2023).

3) Erythrocebus p. villiersi (Aïr Massif Patas Monkey). Endemic to the Aïr Massif, central northern Niger, where it occurs in several isolated pockets, but mainly in the larger valleys in the south (Dekeyser 1950, Dekeyser and Derivot 1959, Masseti and Bruner 2009). Altitudinal range 600 m asl (Dahaga) to 1,600 m asl (Baguezans Plateau; Dekeyser 1950).

An introduced population of E. patas occurs in Puerto Rico. Erythrocebus patas was intentionally released on Cueva Island and Guayacan Island between 1971 and 1981 by the La Parguera Primate Facility. During 1974 to 1981, some individuals moved from these islands to mainland Puerto Rico (González-Martínez 1998). Today, E. patas is considered a nuisance species on mainland Puerto Rico.

Erythrocebus patas is a widespread species which typically occurs at low densities. Probably more abundant in the western parts of the range (although there are no recent surveys to confirm this) than in the east (Isbell 2013). Typical density might be 1.5 animals/km². Hall (1965) recorded 110 Patas in a 3,112 km² area in northern Uganda. In Kidepo Valley National Park, northeastern Uganda, the rate of encounter with E. p. pyrrhonotus from a slow-moving vehicle was 0.05 groups/hr. (0.002 groups/km; Butynski and De Jong 2017).

In Kenya, the geographic distribution of E. p. pyrrhonotus declined about 56% from ~93,120 km² prior to 1995 to ~52,520 km² in 2014, and the size of gaps among populations increased (De Jong et al. 2008, Butynski and De Jong 2014b).

During 1979–1981, Laikipia Country supported a minimum of 415 E. p. pyrrhonotus (14–15 groups; Isbell and Chism 2007). The number in 2000 was 310–445 individuals (13–17 groups; Isbell and Chism 2007). In 2017, there were about 145–155 individuals (13 groups) in eastern Laikipia (De Jong and Butynski 2017). Although the 2017 survey did not include western Laikipia, the survey area was more than twice the size of that of the two previous surveys. Mean group size of E. p. pyrrhonotus in eastern Laikipia in 2017 was 12 individuals. Patas numbers have declined in eastern Laikipia through reduction both in group size and number of groups (De Jong and Butynski 2017, pers. obs. 2024). It is likely that Laikipia County supported <225 patas in 2017. The number in 2024 is suspected to be far less than this (Butynski and De Jong pers. comm. 2024).

Erythrocebus p. pyrrhonotus appears to no longer be present at Alupe, Busia District, western Kenya. There is a population of unknown size and geographic distribution in and around the Karasuk Hills region of central western Kenya (De Jong and Butynski 2013, 2014, Butynski and De Jong 2014a). There are reports of a significant number of E. p. pyrrhonotus in extreme northwestern Kenya. Surveys are required to assess the number of E. patas currently in Kenya.

In 2016, in an area of ~200 km² on the Ennedi Massif, northeastern Chad, S. Turner (pers. comm. 2016) encountered five or six groups of E. p. pyrrhonotus (total 80–100 individuals) during a brief visit.

In Bafing Faunal Reserve, southern Mali, E. p. patas occurred at 1.7 individuals/km² in 1993 (Pavy 1993 in Oates 2011).

In Comoé National Park, northern Côte d’Ivoire, ~1,150 E. p. patas occupied ~11,500 km² in 1991 (Poilecot 1991 in Fischer et al. 2000).

Erythrocebus p. patas is common in the Gadabedji Biosphere Reserve, central Niger (T. Rabeil pers. comm. 2019).

Estimated population of 500 E. p. villiersi in the central massifs and plateau of the Aïr Massif, central northern Niger (Magin 1990 in IUCN 2017).

Erythrocebus p. villiersi is common on Aïr Massif (T. Rabeil, T. Wacher, and J. Darbey pers. comm. 2019).

Between 2017 and 2050, almost 24% of the global increase in the human population will occur in 10 of the 11 countries of the Sahelian Region (UNDP 2017). Across the geographic range of E. patas, the rapidly growing human population has converted large parts of wooded savanna to agriculture, grassland, and bushland (UNEP 2012). The woodlands of the Sudanian Region are under similar pressure. During 1990 to 2000 alone, a massive 25,000 km²/yr of dry forest and woodlands were lost in the Sudanian Region (FAO 2005). Based on these levels of habitat loss, the global population of Erythrocebus patas is suspected to have declined by at least 20% over the last 30 years (three generations).

Erythrocebus patas is a medium-sized, diurnal, largely terrestrial, monkey. Occupies vegetation types ranging from grassland, to wooded savanna, to woodland (Isbell 2013). Most common in thinly bushed Acacia woodland. Appears to prefer woodland-grassland margins along rivers and lakes. Generally, avoids dense vegetation, including riparian forest (Hall 1965, Isbell 2013). In East Africa, E. p. pyrrhonotus is strongly associated with Whistling Thorn Acacia Acacia drepanolobium open woodland (Isbell 1998, De Jong et al. 2008). At Ennedi Massif, northeastern Chad, and Äir Massif, central northern Niger, E. patas occurs in subdesertic grassland, sparse Acacia woodland, and bushland (Dekeyser 1950, S. Turner pers. comm. 2016).

Although E. patas can climb trees when alarmed, it usually relies on its speed on the ground to escape.

Erythrocebus p. pyrrhonotus is omnivorous, feeding on gums, invertebrates, flowers, fruits, and seeds (Chism and Rowell 1988, De Jong et al. 2008, Isbell 2013). Visits to water are typically daily during the dry season. It often uses man-made water sources (e.g., dams, tanks, troughs), and uses fences to sit on and scan from. In all areas where it is encountered in Kenya, E. p. pyrrhonotus is somewhat habituated to humans, mainly to pastoralists, farmers, and 'monkey chasers' in crop fields. In Busia District, central western Kenya, it has adapted to a high human population and little natural vegetation. There, it eats maize and other crops (De Jong et al. 2008, Butynski and De Jong 2014a). In Kidepo Valley National Park, northeastern Uganda, E. p. pyrrhonotus raids garbage bins daily (Butynski and De Jong 2014a).

Erythrocebus p. pyrrhonotus lives in single-male groups of 13–56 individuals (Chism and Rowell 1988). Burnham (2004) reported longer day ranges for all-male groups (mean 7.3 km/day) compared with social groups (mean 4.7 km/day) in the same area of Laikipia County, central Kenya. There, home range sizes of social groups are 23–40 km², depending on group size (Chism and Rowell 1988, Enstam and Isbell 2004), but elsewhere can be as large as 52 km² (e.g., group of 31 individuals in Murchison Falls National Park, northwestern Uganda; Hall 1965).

Main threats to Erythrocebus patas throughout its geographic range, but probably particularly in East Africa, are habitat degradation, fragmentation and loss due primarily to agricultural expansion and intensification (both crops and livestock), charcoal production, and ‘development’ activities (e.g., settlements, roads, powerlines, dams, wind farms).

Erythrocebus patas is primarily distributed throughout the Sahelian and Sudanian regions. The Sahel is a region with major land degradation related to its rapidly growing human population (UNEP 2012). The Sahelian Region holds about 8 people/km², while the Sudanian Region has about 31 people/km² (Eva et al. 2006). Between 2017 and 2050, almost 24% of the global human population increase will occur in 10 of the 11 countries of the Sahelian Region (UNDP 2017). The rapid growing human population has converted large parts of the originally wooded savanna to agriculture, grassland, and bushland (UNEP 2012). The woodlands of the Sudanian Region are under similar pressure. During 1990–2000 alone, a massive 25,000 km²/yr of dry forest and woodlands were lost in the Sudanian Region (FAO 2005). This rapid and extensive loss of E. patas habitat over the Sahelian and Sudanian regions is expected to continue over the long term.

In Africa, the ‘Rate of Natural Increase’ of the human population is 2.4%, compared to 0.9% worldwide (PRB 2022). Overall, of the range states for E. patas, the human population is doubling every 20–30 years. As such, there is ever increasing competition between people and E. patas for habitat and water (De Jong et al. 2008, 2009; Isbell 2013; Butynski and De Jong 2014b; De Jong and Butynski 2017). In none of the range states are natural resources, such as water, soil, bushlands, or woodlands, being used sustainably. Perennial water sources have been lost and competition with people and livestock over access to water is severe (De Jong and Butynski 2017). Among the results are widespread degradation, loss and fragmentation of E. patas habitat, and a rapid decline in the abundance and geographic distribution of this species.

Erythrocebus p. pyrrhonotus is in rapid decline in Laikipia County, central Kenya (De Jong and Butynski 2017, T. Butynski and Y. de Jong pers. obs. 2024). In Laikipia, this is the most threatened primate, even though this monkey is relatively compatible with livestock ranching, the primary economic activity in the county (Butynski and De Jong 2014b). Habitat degradation, loss, and fragmentation are predominantly caused by over-grazing and over-browsing by livestock, conversion of large areas to cropland, uncontrolled cutting of trees, charcoal production, and colonization of invasive plants, particularly Prickly Pears Opuntia spp. Damage is most severe in and around rural and communal areas – where the human population is most dense and where the extraction of natural resources is most intense, insufficiently managed, and unsustainable (De Jong and Butynski 2017). With the rapidly growing human population in the region, there is also increasing competition among people, livestock, and wildlife for water. Where perennial sources of drinking water continue to exist, they are sometimes inaccessible to E. p. pyrrhonotus due to heavy use by people and livestock, and interference, harassment, or killing by people and domestic dogs Canis familiaris (T. Butynski and Y. de Jong pers. obs., B. Ross and E. Kiperenge pers. comm. 2024). High densities of Savanna Elephant Loxodonta africana over large parts of Laikipia County, and concentrations of Black Rhinoceros Diceros bicornis and Reticulated Giraffe Giraffa reticulata in some areas, have greatly reduced the abundance of Whistling Thorn Acacia, a key food species for Patas in Laikipia County (Isbell 1998, De Jong et al. 2008, De Jong and Butynski 2017).

In West Africa, much E. p. patas habitat has been degraded and lost due to fire, desertification, and livestock grazing and browsing (Oates 2011). Perhaps because of hunting, E. p. patas is kept as pets and often remain so into adulthood. This has been observed in Sierra Leone, Nigeria, Ghana, and Côte d’Ivoire (Fischer et al. 2000, Chesney et al. 2023, A. Dempsey pers. comm. 2017). It is the most prevalent primate species in the pet trade in Ghana (A. Dempsey pers. comm. 2017).

Erythrocebus p. patas is hunted for food in Central Africa and West Africa (Kurpiers et al. 2016) and is widely persecuted as a crop pest (De Jong 2004, De Jong et al. 2008).

Erythrocebus p. villiersi raids crops (Dekeyser 1950) and is, most likely, persecuted in response.

Erythrocebus patas is hunted for bushmeat over all parts of its geographic distribution but, particularly, in Central Africa and West Africa. It is used as a research animal. There is a need to clarify how many research animals are captive bred and how many are wild caught. The CITES trade database indicates that hundreds of Erythrocebus patas are traded every year. In 2017, the origin of most wild-caught individuals was Sudan.

Erythrocebus patas is listed under CITES Appendix II and as Class B under the African Convention. Present in many protected areas across its range, including Aïr Natural Reserve and Ténéré Natural Reserve, Niger. Murchison Fall National Park in northwestern Uganda and Kidepo National Park in northeastern Uganda appear to be the main sites for the conservation of E. patas in East Africa.

Erythrocebus patas in Kenya is on the verge of extinction. Laikipia County probably once held the largest population of E. patas in Kenya (Isbell and Chism 2007, De Jong et al. 2008, Butynski and De Jong 2014b, De Jong and Butynski 2017). All Patas groups in Laikipia occur outside formally protected areas. Probably all live on private livestock ranches where there are large areas of Whistling Thorn Acacia woodland and artificial sources of water (Isbell and Chism 2007, De Jong et al. 2008, Butynski and De Jong 2014b, De Jong and Butynski 2017). These relatively well protected private ranches appear critical to the survival of E. patas in Kenya.