The taxonomy of Cacajao was reviewed by Hershkovitz (1987), who recognised four subspecies of bald-headed uakaris: Cacajao calvus calvus; C. c. rubicundus; C. c novaesi; and C. c. ucayalii.  Based on new information on the geographic distribution, molecular phylogenetic analyses, and the examination of museum specimens, the four subspecies recognised by Hershkovitz (1987) were raised to the species level – therefore, Cacajao calvus, C. rubicundus, C. novaesi, and C. ucayalii (Silva et al., 2022). The population of white uakaris from the Tarauacá river basin, in Brazil, was described as a new species: Cacajao amuna. Therefore, five species of bald-headed uakaris are currently recognized (Silva et al., 2022).

Cacajao ucayalii is considered Vulnerable A4cd due to a suspected population reduction of at least 30% for three generations (30 years), including the current (2018–2028) and two subsequent generations (2028–2048). This decline is supported by Global Forest Watch data from regions of the Peruvian departments of Loreto and Ucayali in which this species occurs, and it is based upon an ongoing rate of deforestation that is likely to result in a 20% loss during that period (Móstiga et al. 2024). The additional threat of hunting, which is documented in this region, raises this species’ conservation status to Vulnerable.

Cacajao ucayalii is found mostly south of the Amazon River in Peru between the Ucayali and Yavarí Rivers. In the past, its range probably extended as far as the Urubamba River (Hershkovitz 1987), but surveys undertaken in the 1980s suggest that the southern limit is now the Sheshea River (Aquino 1988). The species may have a patchy distribution in some regions (e.g., south (right) bank of the Yavarí-Mirín River). Cacajao was recorded in the montane forests of Region Junín, Peru, in the Pampa Hermosa River basin. This population was considered isolated (Silva et al., 2021), but new records in the Region Ucayali (Shanee et al. 2023, Anca et al. 2023), along with records presented in Hershkovitz (1987) in the Río Tahuania and Río Urubamba confirm that the species occurs throughout the Ucayali River basin until the montane forests of Regíon Junín. Nevertheless, an isolated population of C. ucayalii was recorded in the Cordillera Cahuapanas, Region San Martin, a montane forest (>1,000 m a.s.l.) > 200 km west to the Ucayali River (Vermeer et al. 2013).

Hershkovitz (1987) claimed this taxon may also occur on the east bank of the lower Yavarí in Brazil, but field surveys are necessary to confirm the presence of bald-headed uakaris on the Brazilian side. In Brazil, Cacajao ucayalii was only confirmed in the Serra do Divisor National Park, in the State of Acre (Silva et al. 2021).

Densities estimates of C. ucayalii have varied in different localities in the Peruvian Amazonia. A preliminary study in the Ucayali River basin found a density of 0.11 troops/km² (Aquino 1988). An ongoing study in the same region estimated population densities using the distance sampling method at 25 individuals/km² (95% CI = 8.1–77.3, CV = 59.5), 0.6 groups/km² (95% CI = 0.2–2.0, CV = 59.3), and 37.4 individuals/group (95% CI = 33.7–41.5, CV = 5.2) (Silva et al., in prep). In the Sierras de Contamana, part of the Sierra del Divisor located on the right margin of the Ucayali River, near the border with Brazil, this species was recorded at an abundance of 47.9 individuals/10 km (Aquino et al. 2005).

In the Yavarí and Yavarí-Miri, Salovaara et al. (2003) estimated the population density of C. ucayalii at 4.94 individuals/km². Population densities of this species increased from 19.0 individuals/km² in 2001 to 116.8 individuals/km² in 2007 in the Lago Preto Conservation Concession – a growth at an average annual rate of 35.3% driven by the reduction in hunting as a result of conservation programs. In the Reserva Comunal Tamshiyacu-Tahuayo, Peru, densities of C. ucayalii were estimated at 2.5 individuals/km² (Puertas and Bodmer (1993). 

Cacajao ucayalii is suspected to be undergoing a population reduction of at least 30% over a three-generation (30 year) period. This includes the current (2018–2028) and two subsequent generations (2028–2048). This population decline is suspected as a result of forest loss and hunting.

Cacajao ucayalii occurs in a variety of habitats, including palm swamps (aguajal), flooded and unflooded forests of different altitudes (Heymann and Aquino 2010). As with other bald uakaris, C. ucayalii is specialized in seed predation, with fruits and immature seeds making up the bulk of the diet (Barnett et al. 2013). Mauritia flexuosa palm fruit was the most important species in the diet of this taxon at Lago Preto, accounting for 20% of the annual diet (Bowler 2007).

Habitat loss and hunting are the main threats to this species. Deforestation is also a significant threat.

This species is hunted for food and the local/national trade and is commonly found as pets in villages and markets (Shanee et al. 2017).

Cacajao ucayalii is a flagship species for conservation in Peru. The presence of this species was important for the creation of the regional Tamshiyacu Tahuayo Communal Reserve (TTCR) – an area of 322,500 ha close to Iquitos between the Amazon and the Yavarí rivers (Bowler et al. 2013). The reserve was created as a result of a strong alliance between local people and conservationists (Newing and Bodmer 2004). In addition, the species has been a flagship to support the conservation of Peruvian Amazonian forests in three other areas: Sierra del Divisor Reserved Zone, the Lago Preto Conservation Concession (LPCC), and the proposed Greater Yavarí Reserve.

Community conservation work is conducted by WCS-Peru and the Durrell Institute of Conservation and Ecology on the Yavari and Yavari-Mirin Rivers in Peru. This project aims to reduce the hunting of Cacajao ucayalii in this area (Bowler 2007). Cacajao ucayalii was confirmed in the Serra do Divisor National Park, Brazil (Silva et al. 2021).