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Proboscis MonkeyNasalis larvatus

Every figure, record and mechanism on the Proboscis Monkey profile was written from a source, and this is the working table behind it: 44 claims checked word for word against these sources. The left column is what the page says. The right column is the source's own words, copied, not paraphrased; a ... marks a cut and[brackets] an insertion. Open any source and check it yourself.

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Source
01

New England Primate Conservancy

Proboscis Monkey, Nasalis larvatus. New England Primate Conservancy.

The page sayswebbed fingers and toes that help it swim across rivers
The source saystheir webbed extremities help them to swim, allowing them to make river crossings when necessary.
The page saysa cow-like habit of bringing food back up to chew it again
The source saysthey actively bring food they have already eaten back up from their stomachs in order to rechew it and swallow it again. This is the same eating strategy a cow uses.
The page saysMostly coastal mangrove forests and lowland rainforest along rivers on Borneo, in Malaysia, Indonesia and Brunei
The source saysinhabit the island's tropical forests in each of the three nations that politically divide it: Malaysia, Indonesia, and Brunei. While these monkeys are found mostly on the coast, in mangrove forests, or living along rivers in dipterocarp forests (tropical lowland rainforests
The page saysMostly leaves and fruit, preferring young leaves and unripe fruit, plus flowers, bark and insects
The source saysProboscis monkeys eat mostly leaves and fruit, together making up about 90% of their diet. The remaining 10% is a hodgepodge of foods like flowers, bark, insects, and insect larvae. ... proboscis monkeys prefer young leaves and unripe, non-fleshy fruits.
The page saysAn oversized nose that amplifies a male's calls, making them louder than a female's and useful for warning the group of danger.
The source saysProboscis monkey males have an advantage in vocalizing due to their large noses, which amplify their calls and make them louder than those of females. These louder calls are valuable for alerting the group to dangers.
The page saysIt droops over the mouth and sometimes tops 4 inches (10.1 cm).
The source sayssometimes exceeding 4 inches (10.1 cm) in length! ... bulbous noses that droop down over their mouths. This feature is unique to males.
The page saysMany groups have one adult male, several females and their young, while other males live in bachelor groups.
The source saysMany groups are composed of a single adult male, several adult females, and their offspring. ... Meanwhile, bachelor groups, consisting of multiple adult males
The page saysMales can weigh up to 50 pounds (22 kg) and measure 29 to 30 inches (73 to 76 cm) from head to belly, with a tail nearly as long as the body.
The source saysMales can weigh up to 50 pounds (22 kg) and measure between 29 and 30 inches (73–76 cm), head to belly. Their long tails add an extra 23.6–26.3 inches (60–67 cm).Note: 60-67 cm tail against a 73-76 cm body is 82-88% of it: 'nearly as long'.
The page saysFemales are much smaller, averaging 22 pounds (10 kg) and 24 to 25 inches (61 to 64 cm).
The source saysFemales are quite a bit smaller than their male counterparts, weighing in at 22 pounds (10 kg) on average. Head to belly, they measure between 24 and 25 inches (61–64 cm)
The page saysThe species lives only on Borneo, in Malaysia, Indonesia and Brunei.
The source saysare endemic to Borneo. Here, they inhabit the island's tropical forests in each of the three nations that politically divide it: Malaysia, Indonesia, and Brunei.Note: 'only' renders 'endemic'.
The page saysIt is found mostly in coastal mangrove forests or in lowland rainforest along rivers.
The source saysWhile these monkeys are found mostly on the coast, in mangrove forests, or living along rivers in dipterocarp forests (tropical lowland rainforests
The page saysWhen proboscis monkeys do eat ripe fruit, they usually pick out the seeds and leave the rest, because its sugars can cause severe indigestion.
The source saysWhen chowing down on ripe fruits, they usually pick out the seeds and discard the rest as the sugars they contain can cause them severe indigestion and sometimes even death.Note: 'sometimes even death' rests on this nonprofit page alone and was cut. 'because' renders the quote's 'as'.
The page saysTheir big bellies hold a stomach with several chambers that helps them digest fiber-rich plants.
The source saysTheir large pot bellies encase a complex, multi-chambered stomach that helps them digest a cellulose-rich diet
The page saysProboscis monkeys rarely come down from the trees. When they do, it's usually to escape a predator or cross a river.
The source saysthese arboreal monkeys rarely descend to the forest floor. When they do leave the safety of the trees, it's usually to escape a predator or to cross a river.
The page saysWebbed fingers and toes, rare among primates, help them swim.
The source saysa close-up on their hands and feet reveals webbed digits, an adaptation rarely found in primates! ... their webbed extremities help them to swim
The page saysOften they leap far out from high in the canopy to keep the swim short, splaying their arms and legs, then tucking them in just before they hit the water.
The source saysVery often, however, they leap! ... they know to leap as far out as possible in order to minimize the distance they will have to swim. In order to ensure optimal distance, they leap from high up in the canopy, often staggering heights. Falling, they splay their arms and legs out wide before tucking them back in just before hitting the water.
The page saysits numbers are falling.
The source saysThe proboscis monkey is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species, and populations are currently in decline.Note: current page: it shows an 'IUCN 2024' range map, so the decline statement postdates 2024.
The page saysHabitat loss is a major cause. Across Borneo, forests are being cleared for timber and farms, palm oil and rubber plantations have replaced large areas, and mangroves are being cut down for shrimp farms.
The source sayshabitat loss is a major contributor to this species' decline. All across Borneo, forests are being degraded and destroyed, cleared for timber or to make space for farms and human settlements. Palm oil and rubber plantations have replaced swaths of what were once some of the world's most biologically diverse forests with carefully managed monocultures. The island's mangrove forests are being cut down to make way for shrimp farms.
The page saysTheir stomach. Proboscis monkeys are one of the only primates able to digest the plants of Borneo's mangrove forests, which are rich in tannins, bitter chemicals that can be toxic. Their chambered stomach breaks down the tannins along with the huge amount of fiber they eat. Some researchers think this is why the species first lived in coastal mangroves, and the same ability let it spread to forests further inland.
The source saysSome researchers speculate that proboscis monkeys originally evolved to live in mangrove forests along Borneo's coast as they are one of the only species of primates capable of digesting the tannin-rich plants found in these ecosystems. This is made possible by their chambered stomachs, which break down these toxic tannins and the extreme amounts of fiber they ingest. This amazing adaptation made it possible for proboscis monkeys to spread to other habitats further inland.Note: 'Some researchers think this is why' renders 'Some researchers speculate ... as they are one of the only species'; 'tannins, bitter chemicals' adds a definition for young readers, and 'toxic' is the source's word, softened to 'can be toxic'.
The page saysBy dropping into the water. Groups resting in trees tend to pick branches directly above the water. The slender branches shake when a predator such as a clouded leopard comes too close, which works as an alarm. If the monkeys have to flee, they can drop into the water and swim to safety.
The source saysgroups tend to choose branches directly above the water. Here, the slender boughs are more sensitive to movement, acting as a natural alarm system the moment a predator, like a hungry clouded leopard, comes too close. If the monkeys suddenly have to flee, they can drop down into the water and swim to safety.Note: the resting context is the source's own: 'When at rest, groups tend to choose branches directly above the water.'
The page saystags: rainforest
The source saysliving along rivers in dipterocarp forests (tropical lowland rainforestsNote: #454

Source
02

AnAge: The Animal Ageing and Longevity Database, Human Ageing Genomic Resources

Nasalis larvatus. AnAge: The Animal Ageing and Longevity Database, Human Ageing Genomic Resources.

The page saysIn captivity, the record lifespan is 25.1 years.
The source saysMaximum longevity 25.1 years (captivity)

Source
03

Boonratana et al. 2021

Boonratana et al. (2021). Nasalis larvatus. The IUCN Red List of Threatened Species 2021: e.T14352A195372486.

The page saysThe IUCN Red List classifies the proboscis monkey as Endangered
The source saysIUCN Red List category: Endangered (EN).

Source
04

Balolia & Fitzgerald 2024

Balolia & Fitzgerald (2024). Male proboscis monkey cranionasal size and shape is associated with visual and acoustic signalling. Scientific Reports.

The page saysAn oversized nose that works as both a visual badge and a vocal amplifier
The source saysOur findings provide robust support for the male proboscis monkey nasal complex serving both visual and acoustic functions.
The page saysMale proboscis monkeys (Nasalis larvatus) grow a large, bulbous nose unlike any other primate's.
The source saysAmong primates, the large, bulbous nose of male adult proboscis monkeys (Nasalis larvatus) is unique
The page saysEarlier studies found males in breeding groups have bigger noses than bachelors, and bigger-nosed males tend to have more adult females in their group.
The source saysmale nasal enlargement is associated with the number of adult females per proboscis monkey breeding group, and the noses of males who are in breeding groups are significantly larger than those of males in non-breeding, all male groups
The page saysThe nose may let males make louder or deeper honks and roars.
The source saysenlarged nasal structures in male proboscis monkeys may allow them to emit louder or deeper nasal vocalisations, namely brays, honks and nasal roars
The page saysA 2024 study of skulls strongly backed both roles, display and sound.
The source saysHere, we use cranionasal data to assess whether large noses found in male proboscis monkeys serve visual and/or acoustic signalling functions. ... Our findings provide robust support for the male proboscis monkey nasal complex serving both visual and acoustic functions.Note: 2024 is the reference's publication year; 'skulls' renders 'cranionasal data' (keyword: Cranium).
The page saysA male's bony nose opening keeps growing after his adult teeth are in, which fits a display.
The source saysWe additionally find nasal aperture size increases beyond dental maturity among male proboscis monkeys, consistent with the visual signalling hypothesis.Note: 'bony nose opening' = nasal aperture, 'the bony region to which nasal soft tissue attaches'; 'after his adult teeth are in' = beyond dental maturity. 'after' renders 'beyond'.
The page saysHis nasal cavity is shaped to make loud, deep-sounding calls.
The source saysWe show that the cranionasal region has an acoustic signalling role through pronounced nasal cavity sexual shape dimorphism, wherein male nasal cavity shape allows the expression of loud, low-formant nasalised calls.Note: 'deep-sounding' renders 'low-formant' (low resonances) for an 11-14 reader.
The page saysThe nose seems to do two jobs. As a visual signal, males in breeding groups have bigger noses than males in all-male groups, and bigger-nosed males tend to have more adult females in their group. As a sound signal, the nose may let males make louder or deeper honks and roars. A 2024 study of skulls found strong support for both jobs.
The source saysOur findings provide robust support for the male proboscis monkey nasal complex serving both visual and acoustic functions. ... It serves as a visual signal, where male nasal enlargement is associated with the number of adult females per proboscis monkey breeding group, and the noses of males who are in breeding groups are significantly larger than those of males in non-breeding, all male groups ... enlarged nasal structures in male proboscis monkeys may allow them to emit louder or deeper nasal vocalisations, namely brays, honks and nasal roarsNote: 2024 is the reference's publication year.
The page saysnasal cavity is shaped to make loud, deep-sounding calls
The source saysmale nasal cavity shape allows the expression of loud, low-formant nasalised callsNote: vibe.reason (#454); 'deep-sounding' renders 'low-formant'.

Source
05

Matsuda et al. 2011

Matsuda et al. (2011). Regurgitation and remastication in the foregut-fermenting proboscis monkey (Nasalis larvatus). Biology Letters.

The page saysA first for primates
The source saysto our knowledge we report, for the first time, regurgitation and remastication in 23 free-ranging individuals of a primate species
The page saysIn 2011, scientists reported 23 wild proboscis monkeys bringing food back up from the stomach to chew it again, the first time this had been seen in a primate.
The source saysto our knowledge we report, for the first time, regurgitation and remastication in 23 free-ranging individuals of a primate species, the foregut-fermenting proboscis monkey (Nasalis larvatus).Note: 2011 is the reference's publication year.
The page saysRuminants such as cattle do it far more consistently.
The source saysthis so-called 'merycism' apparently does not occur as consistently as in ruminants.Note: cattle are the ruminant the 2015 study compared them with ('compared the marker excretion patterns to those in domestic cattle').
The page saysScientists watched one wild male for 169 days and saw him do it on 11 of them, mostly in the morning.
The source saysIn one male that was observed continuously during 169 days, the behaviour was observed on 11 different days occurring mostly in the morning

Source
06

Matsuda et al. 2015

Matsuda et al. (2015). Excretion patterns of solute and different-sized particle passage markers in foregut-fermenting proboscis monkey (Nasalis larvatus) do not indicate an adaptation for rumination. Physiology & Behavior.

The page saysResearchers call it a "facultative" strategy, meaning the monkeys use it only some of the time.
The source saysfree-ranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism(rumination) ... which may explain why merycism is only a facultative strategy in this species.
The page saysA 2015 study of four captive monkeys found their digestion lacks the typical features of a ruminant's, which may explain why.
The source saysmeasured mean retention times (MRTs) in four captive proboscis monkeys ... the digestive physiology of proboscis monkeys does not show typical characteristics of ruminants, which may explain why merycism is only a facultative strategy in this species.Note: 2015 is the reference's publication year.
The page saysTheir droppings hold smaller food particles than other primates', one clue that they rechew.
The source saysBehavioral observations and small fecal particles compared to other primates indicate that free-ranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism(rumination).

Source
07

Matsuda, Tuuga & Higashi 2009

Matsuda, Tuuga & Higashi (2009). The feeding ecology and activity budget of proboscis monkeys. American Journal of Primatology.

The page saysOne group studied along a river in Sabah, Malaysia, spent about three-quarters of its time resting and a fifth feeding.
The source saysthe Menanggul River, Sabah, Malaysia. ... The activity budget of members of our study group was 76.5% resting, 19.5% feeding, and 3.5% moving.Note: 76.5% is about three-quarters; 19.5% is a fifth.
The page saysYoung leaves took nearly two-thirds of that feeding time and fruit about a quarter, and over 90% of the fruit feeding was on unripe fruit.
The source saysYoung leaves (65.9%) and fruits (25.9%) accounted for the majority of feeding time. Over 90% of fruit feeding involved the consumption of unripe fruits

Source
08

Matsuda, Tuuga & Higashi 2008

Matsuda, Tuuga & Higashi (2008). Clouded leopard (Neofelis diardi) predation on proboscis monkeys (Nasalis larvatus) in Sabah, Malaysia. Primates.

The page saysClouded leopards and crocodiles may be important predators.
The source sayswe suggest that clouded leopard and crocodile might be significant potential predators of proboscis monkeys of any age or sex
The page saysA 2008 study in Sabah recorded clouded leopards killing two young females from one group, an infant and a juvenile.
The source sayswe have reported two direct observations of individuals from a one-male group of proboscis monkeys (Nasalis larvatus) being killed by clouded leopards (Neofelis diardi) in the riverine forest along the Menanggul river, a tributary of the Kinabatangan river in Sabah, Malaysia. One of the two individuals was an infant female and the other was a juvenile female.Note: 2008 is the reference's publication year.
The page saysIt suggested adult males in one-male groups act as protectors.
The source saysthe observations of the monkeys' behaviour when the group is attacked by a predator suggest that the adult males in one-male groups play an important role as protectors.

How the table was made: a writer drafted the profile from these sources and logged each claim with its quote, a reviewer with nothing but this table checked every sentence said no more than its quote, and a script confirmed each quote appears word for word in the source. The method is in the site'sAbout page.