The Private Natural Heritage Reserve–Feliciano Miguel Abdala, an Atlantic Forest fragment of c. 1,000 ha in Caratinga, Minas Gerais, Brazil, is well-known for its primates. These include one of the largest populations of the Critically Endangered northern muriqui Brachyteles hypoxanthus, which has been monitored since 1983 (Strier, 2021, Primates, 62, 861‒868), and three other species, the Critically Endangered buffy-headed marmoset Callithrix flaviceps, the Vulnerable brown howler monkey Alouatta guariba, and the Near Threatened black-horned capuchin monkey Sapajus nigritus. We have been monitoring these three species since 2017, following a severe yellow fever outbreak, with funding from the National Geographic Society and Re:wild, and in collaboration with the Muriqui Project of Caratinga and Preserve Muriqui.

One of the species most affected by the outbreak was the buffy-headed marmoset, whose population declined by over 80% compared to 2015 census data (Possamai et al., 2022, American Journal of Primatology, 84, 10–14). However, births in three of the four monitored groups suggest the population may be recovering. During May–November 2023 we counted 35 individuals in total, compared to the 17 individuals counted during October 2017–October 2021. Based on their physical characteristics we infer that nine of the 18 new individuals were born in three study groups, with one infant in group A (estimated birth in April 2023) and two successive sets of twins in group B (estimated births in January and August 2023) and group C (estimated births in July 2022 and January 2023). Of the other nine newly sighted animals, we suspect two could be from a missed birth of twins that matured during a gap in monitoring, up to four could be immigrants from neighbouring fragments or, along with the other three animals, were missed in previous censuses. However, even in the most conservative scenario the population appears to have increased by 35% (from 26 to 35 individuals) since July 2022. Genetic pedigree analyses are needed for a more accurate picture of the extent of the recovery, but the survival of some infants over the past year is encouraging.

The buffy-headed marmoset has the narrowest distribution of the genus Callithrix and lives in a highly fragmented landscape in the states of Minas Gerais and Espírito Santo. Its small, isolated populations are threatened by competition with invasive species such as the common marmoset Callithrix jacchus and black-tufted marmoset Callithrix penicillata, and by natural hybridization with the buffy-tufted-ear marmoset Callithrix aurita and white-headed marmoset Callithrix geoffroyi, which could cause the loss of genetic characteristics and lead to extinction. Although threats to the buffy-headed marmoset persist, the increase in our study population is promising for the species’ long-term recovery if the present trend continues, especially if the species is also recovering in other areas where it was decimated by yellow fever.