Plecturocebus oenanthe

Geographic Distribution and Habitat

The San Martin titi (Plecturocebus oenanthe), also known as the Rio Mayo titi or Andean titi monkey, is endemic to the San Martin region of Peru, and it is considered Peru’s most endangered primate. They make their homes in low secondary forests and are often seen in bamboo groves, viney thickets, fruit groups, and palm-dominated forests. They seem to have a slight preference for being near water, such as rivers and streams. Their exact altitude range is unknown, but the maximum is believed to be 3,280–4,600 feet (1,000–1,400 m), with low population densities at high altitudes. They are often found in fragmented forest, with unsustainably high numbers of individuals forced into increasingly smaller pieces of habitat as their home is deforested. More than 55% of their habitat has been lost due to human encroachment. Only about a third of the remaining habitat is rated as “good” condition. This is expected to continue due to deforestation and climate change.


San Martin titis were first recognized as a species in 1990. Before 2016, they were considered a member of the genus Callicebus. That genus was then split into three: CheracebusPlecturocebus, and Callicebus. San Martin titis share their genus with 21 other species.

San Martin titi range, IUCN 2020

Size, Weight, and Lifespan

The exact size of San Martin titis is not recorded, but based on other titi monkeys, the females are likely between 11 and 17 inches (29–42 cm) in length, while males are likely between 12 and 18 inches (30–45 cm). The average female likely weighs about 2 lbs (0.9 kg), while the average male weighs slightly more at 2.2 lbs (1 kg). They live to about 25 years of age in the wild.​

What Does It Mean?

Inbreeding depression:
A reduction in the overall health and reproductive fitness of a population due to inbreeding.

Visit the Glossary for more definitions


​At first glance, San Martin titis appear to have a round, stout body. This is because their dense fur masks their true body shape, making them look at times like a growth on a tree branch—very effective for hiding from predators! They have a long, non-prehensile tail that they use to help balance on branches. They also have a variety of coloring types: individuals in the northern part of their range tend to have a brown body with a white mask, while those in the south more often have an orange to dark brown body and lack a mask. Sometimes, pairs of different colorings have offspring together. Infants are born a uniform agouti color, and begin developing their white mask, if they have one, at about five weeks of age. Their orange undersides begin to become apparent sometime between 9 and 18 months of age. San Martin titis exhibit some subtle sexual dimorphism, such as the male’s larger size, the female’s brighter coloring on the inner parts of her chest, limbs, and belly, and the female’s slightly longer tail.

Photo credit: ©Jhoin Bard/iNaturalist/Creative Commons

As true omnivores, most of the San Martin titi’s diet is composed of fruit and insects and is supplemented with seeds, flowers, leaves, and shoots. Their fruit diet is not especially varied, as only one or two species make up the majority of the fruit they eat in a given month, depending on what’s available during that season. A particularly important fruit for San Martin titis are those from the mistletoe family. As for insects, San Martin titis mostly consume immobile insects and spiders, although occasionally they may snag a fast-moving insect or feed from a congregation of them. During food scarcities, they switch their diet to mainly insects and flowers, supplemented with fruit that’s available in disturbed habitat or for an extended period of time, such as mistletoe fruit. Females have a more insect-heavy diet than males, particularly while lactating.

Behavior and Lifestyle

San Martin titis are diurnal, like most New World monkeys. They wake with the sun and stay active until sunset. They have two main feeding periods throughout the day with a rest in the middle, and spend the majority of their waking hours foraging. They are arboreal (tree-dwelling) and quadrupedal (travel on all fours) and typically get around with short leaps. They are almost never seen on the forest floor. San Martin titis have been described as somewhat skittish and hesitant, and they do not tend to be particularly curious or explorative.

Fun Facts

​The German word for titi monkeys, Springaffen, means “jumping monkeys,” a reference to their leaping method of locomotion.

Daily Life and Group Dynamics 

​San Martin titis live in small family groups composed of a mated pair and their offspring, typically between three and seven members in total. One study found that one group used an area of 6 acres (2.5 ha), although this group lived in an isolated fragment, so this may be a smaller territory than a group under natural, undisturbed habitat conditions. There are small areas of range with a high population density of San Martin titis, upwards of 3.5 individuals per acre (1.4 individuals per ha), with the rest of the range supporting lower population densities, around 0.82–0.89 individuals per acre (0.33–0.36 individuals per ha), based on two surveys. It is likely that this is a result of the species’ desperate situation as they are forced into smaller and smaller fragments of forest, surrounded entirely by pastures or fields. This leads to unsustainable crowding found in the last remaining pieces of habitat.


Titi monkeys have a wide array of vocalizations with which they communicate. They are split into two types: higher-pitched squeaks, trills, and chirps, and lower-pitched chirrups, moans, honks, bellows, and screams. The higher-pitched sounds are often used when they are anxious or in stressful situations, or when they encounter violence. Their lower-pitched calls are most typically used in intra-group signaling among family members. “Chirrup” calls are believed to communicate information about the age and sex of the vocalizing monkey.

San Martin titis also have a specific sequence of calls that they perform around the outer edge of their home range. First thing in the morning, a male of the group emits a series of grunts, moans, and other loud calls. If another group comes close, they “duet” by performing these calls at the same time. This reinforces each group’s boundaries. If the groups come in close contact, the calls get more intense and females join in. If the groups directly confront each other, the members lash at each other with their tails, exhibit piloerection (hair-bristling to make themselves appear larger), and chase each other.

Reproduction and Family

Like all titi monkeys, San Martin titis are monogamous, and often mate for life. Mated pairs are very closely attached—often literally! They engage in a behavior known as “twinning,” in which they rest together with their hands clasped and their tails intertwined. A closely related species, the Bolivian titi (C. donacophilus) has been known to show anxiety and distress when separated from their mate, and the male will mount and tightly grasp his mate when in the presence of another adult male.

Based on the closely related Bolivian titi, San Martin titis likely give birth most frequently in the months preceding the rainy season. Their gestation is about 18 weeks long, after which they give birth to a single offspring. The male plays the dominant role in childcare. They carry their baby except when the female nurses, which adds up to about 80% of the time. Babies experience more stress and higher heart rates when separated from their fathers than when separated from their mothers. In fact, after the first month of life, a baby San Martin titi will have very limited contact with its mother. The offspring reach sexual maturity at about two years of age, and give birth to their first baby, on average, at about 4 years of age.

Ecological Role

​Predators of San Martin titis likely include eagles, jaguars, and arboreal snakes. They live sympatrically in parts of their range with other primates, including Saddleback tamarins (Saguinus fuscicollis)White-tailed titis (Plecturocebus discolor) and possibly yellow-tailed woolly monkeys (Lagothrix flavicauda), among others. As avid fruit eaters, they likely play a role as seed dispersers.

Conservation Status and Threats

The International Union for Conservation of Nature lists San Martin titis as Critically Endangered (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species. Critically Endangered is the last step before extinction in the wild. It is believed that they have suffered a population loss of more than 80% in the last 25 years, and their population continues to dwindle.

The region of Peru where San Martin titis live has seen an intense push of human immigration over the last few decades. Many of these people are coming to the region to farm, resulting in even more habitat loss as acres upon acres of forest are removed to make room for the cultivation of rice and coffee, cattle ranching, and timber harvesting. The region is estimated to experience forest loss of between 99,000 and 247,000 acres (40,000 and 100,000 ha) per year and has lost 40% of its forest cover over the last 20 years.

A new road running through the region was constructed in 2003, and has expedited human settlement and expansion. In addition to outright habitat loss, San Martin titis are also significantly threatened by habitat fragmentation. Most groups live in small, highly degraded, fragmented pieces of forest, cut off from other groups. This has severe implications for the species, as it potentially limits their ability to find sufficient food and resources, limits gene flow leading to inbreeding depression, and puts them more at risk from the impacts of climate change, as they cannot as easily move around as the ecosystem changes.

Other threats to the species include hunting pressure and collection for the pet trade. San Martin titis are not a particularly good source of protein, but they are expected to become more popular targets for hunters as other game becomes more scarce, and their fragmented habitat makes them less elusive.

​Conservation Efforts

​San Martin titis are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES). Overall, very little conservation work has been done in the San Martin region. San Martin titis tend to only be found on the edges of the few protected areas in the region. One non-profit organization, Proyecto Mono Tocón, operates in the region to promote the conservation of San Martin titis. This organization provided some of the information that resulted in the species being reclassified from “Threatened” to “Critically Endangered” by the IUCN.


  • DeLuycker, A. M. 2006. Preliminary Report and Conservation Status of the Río Mayo Titi Monkey, Callicebus oenanthe Thomas, 1924, in the Alto Mayo Valley, Northeastern Peru. Primate Conservation 2006(21):33-29.
  • DeLuycker, A. M. 2007. The Ecology and Behavior of the Rio Mayo Titi Monkey (Callicebus Oenanthe) in the Alto Mayo, Northern Peru. Retrospective Theses and Dissertations 10. Washington University.
  • Van Kuijk, S. M. 2013. Living on the Edge: Critically endangered San Matin titi monkeys (Callicebus oenanthe) show preference for forest edges in C.C. Ojos de Agua, Peru. Dissertation. Oxford Brookes University. 

Written by K. Clare Quinlan, September 2021