Rio Beni Titi

Name and Classification

Swedish zoologist Einar Lönnberg described it in 1939.^[4] He named it Callicebus modestus. A. M. Olalla collected the type, an adult male, in December 1937. It came from El Consuelo on the Beni River. The site lies at 196 m (643 ft). Lönnberg named the Olalla brothers' titi in the same paper.

Philip Hershkovitz reviewed the titis in 1990. He placed this species in a group of its own. He noted its long skull and very small braincase.

In 2016, a genetic study split the titis into three genera.^[5] The Rio Beni titi moved to Plecturocebus. No sample of this species was sequenced in that study. It was kept in the donacophilus group on body traits.

The Red List names the other members of that group. They include the white-eared titi and the Chacoan titi monkey. The Rio Mayo titi and the Urubamba brown titi belong there too.

For years, some doubted it was distinct from the Olalla brothers' titi. Field work, new specimens and genetic tests now keep them apart. A 2019 genetic study suggests both arose from one population.^[6] The two are still diverging.

Locally it is called lucachi cenizo, the ashy lucachi. In the Maropa language, it is rhukanrhuka. The name lucachi refers to the titis' distinctive calls.^[7] Other English names include Beni titi monkey and modest titi.

Physical Description

Its coat is brown, but the tone is uneven. Each hair carries alternating light and dark bands. Gray tints run through the fur. Field workers describe the color as orange-brown.^[8]

That banded coat places it among the brown monkeys. Its gray tints also fit the gray monkeys. Its orange-brown tones link it to the orange monkeys.

White fur covers the face and ears. The white ear tufts are conspicuous. A thin blackish fringe runs across the brow. The forehead and crown are reddish brown, and banded. There is no pale stripe on the cheeks.

The tail is grayish and contrasts with the body. It has no conspicuous lighter base. Hershkovitz described it as blackish, darker than the back. In direct sunlight, the tail can look black. Backlit, it looks lighter.

The hands and feet are black, with some light hairs. Bolivia's Red Book describes them as gray, like the tail.

Adult males have a wide head. The area around the mouth forms a modest pentagon shape. That helps tell males from male Olalla brothers' titis. Males and females otherwise look much alike. Like all titis, it cannot grip with its tail.

Size and Weight

Published body measurements come from a single adult male, the type. Its head and body measured 31.5 cm (12.4 in). Its tail measured 40 cm (15.7 in). The tail is longer than the head and body.

Hershkovitz counted it among the smallest titis. No weight from a weighed wild animal appears in these sources. The Handbook of the Mammals of the World gives about 800 g.^[9] That is about 1.8 lb. It applies that figure across the species group. Bolivia's Red Book gives about 1 kg (2.2 lb).

Similar Species Compared

The Olalla brothers' titi is its closest geographic neighbor. It lives along the Yacuma River.^[10] That river runs through the Rio Beni titi's range. Its long coat is a fairly uniform reddish brown.^[11] Each hair has three color bands. The Rio Beni titi looks grayer, with banded agouti hairs. Whitish hair also covers the Olalla brothers' titi's face and ears. Its tail is darker than its body.^[12] A paler band marks its base. The Rio Beni titi's tail is dark gray throughout. The two titis keep to different forests. The Olalla brothers' titi favors wetter forest near the river. It eats mainly fruit, then leaves.^[13] The IUCN lists it as Critically Endangered.^[14]

The white-eared titi lives east of the Maniqui and Mamoré rivers. It has a pale stripe on each cheek. Its chest and belly are orange. Its hands are pale, and its tail has a pale base.^[15] The Rio Beni titi lacks all four of those marks. Studied white-eared titi groups held four to six animals.^[16] The IUCN lists that species as Least Concern.^[17]

Range and Distribution

It lives only in Bolivia, in the Beni Department. That makes it one of the Bolivian monkeys. Its range lies in southwestern Beni, between the Beni and Maniqui rivers. It spans three municipalities.^[18] They are Reyes, Santa Rosa del Yacuma and San Borja.

The Beni River marks its western edge. West of it lives the Madidi titi. San Borja marks the known eastern limit. Farther east, the white-eared titi takes its place. The southern limit lies near the Yucumo to Rurrenabaque road. The northern limit is still unclear.

The range is split into two zones. They lie on both sides of the Yacuma River. The Olalla brothers' titi lives along that river. Seasonally flooded grassland there holds no titis. It seems to keep the two species apart.

The Red List puts its range at 9,073 km² (3,503 sq mi). Only about 40% of that area is forest. A 2007 field study used a different method. It estimated an extent of occurrence near 1,800 km² (695 sq mi).

The Red List places it below 200 m (656 ft). The Handbook gives elevations up to 400 m (1,312 ft).

Habitat Needs

It lives in a natural mosaic of grassland and forest islands. This is the Llanos de Moxos, a seasonally flooded savanna. Its forest patches are relatively dry. Unlike the Olalla brothers' titi's forests, they lack permanent rivers.

The forest is low, with dense lianas. Motacú palms and spiny garabatá bromeliads are common. Mean canopy height at its sites was 10.2 m (33 ft). Groups were seen at 8.8 m (29 ft) on average. Some patches even hold cacti, a sign of the dry season.

Other primates share these forests. They include the black-capped squirrel monkey and the Bolivian red howler. The black howler monkey occurs along the Yacuma River.

Diet and Behavior

It eats mainly fruit. It also takes flowers and a large share of invertebrates. That diet places it among the frugivorous monkeys. Its insect eating also fits the omnivorous monkeys.

A year-long study followed two groups.^[19] In the dry season, fruit grew scarce. The titis then ate more flowers and moved less. Their home ranges and daily paths shrank.

It lives in small family groups. Each holds a breeding pair and its young. Groups averaged three animals in a 2004 to 2006 survey. A later study found an average of 2.64. Infants were seen in August and September. The adult male usually carried the infant. Females may give birth only once a year.

One studied group used about 9.7 ha (24 acres).^[20] Its range overlapped its neighbors' by about 10%. Neighboring groups met 64 times in one year. Twenty-eight meetings turned aggressive, and three became fights. Most aggression came in the dry season. In two fights, adult males fell about 7 m (23 ft).

Pairs sing loud duets to mark their territory. Its duet is lower and slower than its relatives'.^[21] Its dominant pitch sits below 1 kHz. That finding comes from one recorded family group. Groups called on 52% of days.^[22] Olalla brothers' titi groups called on only 33%.

The titis also rub plants into their fur.^[23] They used a Piper shrub and a forest vine. Most rubbing happened in the wet season.

Population and Status

For over 60 years after its description, little was known of it. WCS researchers rediscovered it in the wild in 2002.

No full census exists. A 2015 study counted groups by their calls. Heard groups gave 4.93 groups per km². Groups actually seen gave 2.65 per km². The two methods gave about 10,800 or 20,100 animals. Both totals carry very wide ranges. They assumed titis fill all forest in the range. The Red List notes that may not hold.

The IUCN lists it as Endangered, assessed in 2020. Assessors suspect a decline above 50% over 26.4 years. That span covers three generations. The population is decreasing and severely fragmented. It was listed as Endangered in 2008 and Vulnerable in 2003. Bolivia's Red Book has listed it as Endangered since 2009.

Major Threats

Cattle ranching is the main economy in its range. Ranchers burn grasslands each year to renew pasture. These fires can escape into the forest islands. Fire, flooding and clearing have been mapped as risks across its range.

A major road, the Northern Corridor, crosses its range. Paving it may bring new settlers and more forest clearing. Beni's new land use plan also promotes mechanized farming.

Assessors project forest loss of at least 13% over 27 years. Human numbers may rise by about 38% over that time. That could bring more hunting.

Local people report some hunting. It is said to be rare, since the titi is small. Still, one was kept as a pet in a village. Another was confiscated from a wildlife trader.

Its groups show more inbreeding than the Olalla brothers' titi. That hints at isolation in fragmented forest. Groups in small patches held only pairs, without young.

Conservation Efforts

About 85% of its range overlaps two municipal protected areas. Pampas del Yacuma lies in Santa Rosa del Yacuma. Rhukanrhuka lies in Reyes.^[24] Its name means titi monkey in the Maropa language. Both areas treat the endemic titis as a priority.

Only a small part of the range lies in a national reserve. That is the Beni Biological Station Biosphere Reserve. Pampas del Yacuma has developed a management plan.

WCS has studied both endemic titis since 2002. Outreach in Santa Rosa and Reyes began in 2009. A titi now appears on both protected areas' logos. It also features on Santa Rosa del Yacuma's municipal shield. Local students gave it the name lucachi cenizo. It is listed on CITES Appendix II.

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