First records of leucism in Leontocebus nigricollis graellsi (Primates: Callitrichidae)
Authors: Tirira Diego G. , Prado-Lita Diego 
Published: May 2023

Leucism is an atypical condition of recessive genetic origin that occurs as a consequence of mutations that affect the generation, distribution, and aggregation of the melanin pigment in the skin of many vertebrates and is particularly noticeable in mammals (Lamoreux et al., 2010). Although this condition has been frequently mentioned in various orders of mammals (Tirira & Arbeláez, 2020), reports in primates are extremely rare (Aximoff et al., 2020; Barros-Diaz et al., 2022; Ramos-Luna et al., 2022), even though they comprise a conspicuous group that is frequently sighted (Mittermeier et al., 2013).
The few known reports of leucism in primates have been documented mostly in large species, particularly in the genus Alouatta (Aximoff & Vaz, 2016; Barros-Diaz et al., 2022; López-Platas et al., 2021), but also in Ateles (Guzmán Caro et al., 2018); in small primates, there are only a few reports for the genus Callithrix (Aximoff et al., 2020; do Vale et al., 2018). Therefore, in this short communication, we present the first reports of leucism in the genus Leontocebus.
In November of 2021, we observed two black-mantled tamarins (Leontocebus nigricollis graellsi) with leucism in the Reserva de Producción Faunística Cuyabeno, Sucumbíos Province, Ecuadorian Amazon. We sighted the first individual (Figure 1) on the November 21, 2022, at Waita Amazon Lodge, located along the middle of the Cuyabeno River course (00°12’17” S, 75°55’31” W, 205 m a.s.l.); the specimen was observed while it was feeding in a tree together with 12 other individuals. The second record occurred on November 26, 2022, at the mouth of the Aguas Negras river in Cuyabeno (00°04’55” S, 76°09’09” W, 220 m a.s.l.), 29 km NW in a straight line from the previous record; the primate with leucism (not photographed) was moving through the forest along with other five individuals.
Leontocebus nigricollis graellsi is a species with a blackish brown coloration (Tirira, 2017); therefore, the two individuals with leucism that we report here were clearly observed by presenting white areas on their bellies. L. n. graellsi is one of the three species of tamarins that inhabit Ecuador; it is present in the northeastern Amazon, north of the Napo River (Tirira, 2017).
According to the scientific literature, leucism in monkeys may be caused by two processes: inbreeding because of isolated populations (Aximoff et al., 2020; Aximoff & Vaz, 2016; do Vale et al., 2018; Guzmán Caro et al., 2018; López-Platas et al., 2021), environmental pressure caused by pollution, or the interaction of both (Barros-Diaz et al., 2022), conditions that we believe do not correspond to the records presented here. L. n. graellsi is a species frequently sighted in the Cuyabeno Reserve, which means that its population is large. In addition, this protected area is one of the largest in Ecuador (5,901 km2) (MAE, 2014) and its relative distance from areas of contamination suggests that the area remains largely pristine.
It has also been indicated that individuals with leucism have a negative survival outlook (Parsons & Bonderup-Nielsen, 1995; Rodrigues et al., 1999), especially in species with diurnal habits, since they can be sighted by their predators with greater ease (Geiger & Pacheco, 2014). These arguments indicate the need to continue reporting observed cases of leucism so that we can understand the origin of this genetic anomaly and the consequences for the individuals with leucism.