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title: "What the sources say about the Proboscis Monkey | AnimalTracks.wiki"
description: "44 claims from the Proboscis Monkey profile, each beside the words of the source that backs it."
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# What the sources say about the Proboscis Monkey (*Nasalis larvatus*)

> 44 claims from the Proboscis Monkey profile, each beside the words of the source that backs it. A "..." marks a cut in a quote and [brackets] an insertion.

Profile: [Proboscis Monkey](https://animaltracks.wiki/animals/proboscis-monkey/)

## Proboscis Monkey, Nasalis larvatus. New England Primate Conservancy.

https://www.neprimateconservancy.org/proboscis-monkey

| The page says | The source says |
| :--- | :--- |
| webbed fingers and toes that help it swim across rivers | their webbed extremities help them to swim, allowing them to make river crossings when necessary. |
| a cow-like habit of bringing food back up to chew it again | they 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. |
| Mostly coastal mangrove forests and lowland rainforest along rivers on Borneo, in Malaysia, Indonesia and Brunei | inhabit 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 |
| Mostly leaves and fruit, preferring young leaves and unripe fruit, plus flowers, bark and insects | Proboscis 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. |
| An oversized nose that amplifies a male's calls, making them louder than a female's and useful for warning the group of danger. | Proboscis 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. |
| It droops over the mouth and sometimes tops 4 inches (10.1 cm). | sometimes exceeding 4 inches (10.1 cm) in length! ... bulbous noses that droop down over their mouths. This feature is unique to males. |
| Many groups have one adult male, several females and their young, while other males live in bachelor groups. | Many groups are composed of a single adult male, several adult females, and their offspring. ... Meanwhile, bachelor groups, consisting of multiple adult males |
| Males 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. | Males 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'.) |
| Females are much smaller, averaging 22 pounds (10 kg) and 24 to 25 inches (61 to 64 cm). | Females 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 species lives only on Borneo, in Malaysia, Indonesia and Brunei. | are 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'.) |
| It is found mostly in coastal mangrove forests or in lowland rainforest along rivers. | While these monkeys are found mostly on the coast, in mangrove forests, or living along rivers in dipterocarp forests (tropical lowland rainforests |
| When proboscis monkeys do eat ripe fruit, they usually pick out the seeds and leave the rest, because its sugars can cause severe indigestion. | When 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'.) |
| Their big bellies hold a stomach with several chambers that helps them digest fiber-rich plants. | Their large pot bellies encase a complex, multi-chambered stomach that helps them digest a cellulose-rich diet |
| Proboscis monkeys rarely come down from the trees. When they do, it's usually to escape a predator or cross a river. | these 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. |
| Webbed fingers and toes, rare among primates, help them swim. | a close-up on their hands and feet reveals webbed digits, an adaptation rarely found in primates! ... their webbed extremities help them to swim |
| Often 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. | Very 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. |
| its numbers are falling. | The 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.) |
| Habitat 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. | habitat 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. |
| Their 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. | Some 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'.) |
| By 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. | groups 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.') |
| tags: rainforest | living along rivers in dipterocarp forests (tropical lowland rainforests (note: #454) |

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

https://genomics.senescence.info/species/entry.php?species=Nasalis_larvatus

| The page says | The source says |
| :--- | :--- |
| In captivity, the record lifespan is 25.1 years. | Maximum longevity 25.1 years (captivity) |

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

https://doi.org/10.2305/iucn.uk.2021-1.rlts.t14352a195372486.en

| The page says | The source says |
| :--- | :--- |
| The IUCN Red List classifies the proboscis monkey as Endangered | IUCN Red List category: Endangered (EN). |

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

https://doi.org/10.1038/s41598-024-60665-8

| The page says | The source says |
| :--- | :--- |
| An oversized nose that works as both a visual badge and a vocal amplifier | Our findings provide robust support for the male proboscis monkey nasal complex serving both visual and acoustic functions. |
| Male proboscis monkeys (Nasalis larvatus) grow a large, bulbous nose unlike any other primate's. | Among primates, the large, bulbous nose of male adult proboscis monkeys (Nasalis larvatus) is unique |
| Earlier studies found males in breeding groups have bigger noses than bachelors, and bigger-nosed males tend to have more adult females in their group. | 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 |
| The nose may let males make louder or deeper honks and roars. | enlarged nasal structures in male proboscis monkeys may allow them to emit louder or deeper nasal vocalisations, namely brays, honks and nasal roars |
| A 2024 study of skulls strongly backed both roles, display and sound. | Here, 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).) |
| A male's bony nose opening keeps growing after his adult teeth are in, which fits a display. | We 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'.) |
| His nasal cavity is shaped to make loud, deep-sounding calls. | We 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 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. | Our 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 roars (note: 2024 is the reference's publication year.) |
| nasal cavity is shaped to make loud, deep-sounding calls | male nasal cavity shape allows the expression of loud, low-formant nasalised calls (note: vibe.reason (#454); 'deep-sounding' renders 'low-formant'.) |

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

https://doi.org/10.1098/rsbl.2011.0197

| The page says | The source says |
| :--- | :--- |
| A first for primates | to our knowledge we report, for the first time, regurgitation and remastication in 23 free-ranging individuals of a primate species |
| In 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. | to 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.) |
| Ruminants such as cattle do it far more consistently. | this 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').) |
| Scientists watched one wild male for 169 days and saw him do it on 11 of them, mostly in the morning. | In one male that was observed continuously during 169 days, the behaviour was observed on 11 different days occurring mostly in the morning |

## 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.

https://doi.org/10.1016/j.physbeh.2015.05.020

| The page says | The source says |
| :--- | :--- |
| Researchers call it a "facultative" strategy, meaning the monkeys use it only some of the time. | free-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. |
| A 2015 study of four captive monkeys found their digestion lacks the typical features of a ruminant's, which may explain why. | measured 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.) |
| Their droppings hold smaller food particles than other primates', one clue that they rechew. | Behavioral observations and small fecal particles compared to other primates indicate that free-ranging proboscis monkeys (Nasalis larvatus) have a strategy of facultative merycism(rumination). |

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

https://doi.org/10.1002/ajp.20677

| The page says | The source says |
| :--- | :--- |
| One group studied along a river in Sabah, Malaysia, spent about three-quarters of its time resting and a fifth feeding. | the 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.) |
| Young 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. | Young 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 |

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

https://doi.org/10.1007/s10329-008-0085-2

| The page says | The source says |
| :--- | :--- |
| Clouded leopards and crocodiles may be important predators. | we suggest that clouded leopard and crocodile might be significant potential predators of proboscis monkeys of any age or sex |
| A 2008 study in Sabah recorded clouded leopards killing two young females from one group, an infant and a juvenile. | we 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.) |
| It suggested adult males in one-male groups act as protectors. | the 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. |