Honk! These monkeys have truly legendary noses – now we better understand why they evolved

Slavianin/Shutterstock Katharine Balolia, Australian National University

Of all the monkey species around the world, one stands out with its large, bizarre nose. In male proboscis monkeys, their bulbous noses will often hang past their mouths.

But why evolve such a strange feature? Are they a visual sign of health and status to potential female mates, and to other males? Or did they evolve to help the monkeys make honks and other loud sounds?

In our new study, published in Scientific Reports, we have deepened our understanding of these enlarged nasal structures by investigating what lies beneath: the structures in the skull.

Our findings help to explain how these noses function as visual and acoustic signals of health and status. They also add to a growing body of evidence that shows researchers can use close examinations of skulls to glean information about primate social behaviour.

A battle of noses

One of the largest monkey species in Asia, proboscis monkeys (Nasalis larvatus) are endemic to the island of Borneo. They live in coastal mangroves, peat swamps and riverine forests, and have an unusual diet made up mostly of leaves.

They can swim quite well and have webbed fingers and toes. They typically live in harem groups, made up of a single adult male (who tends to have a large, bulbous nose), some adult females and their offspring.

Males don’t often get the opportunity to attract a harem until they reach middle age. These older, dominant and large-nosed males don’t easily tolerate other large-nosed males, often trying to ward them off aggressively with deep honks and “nasal roars” – loud calls they make using their noses.

Young adult males with smaller noses often live in all-male bachelor groups, and don’t tend to fight aggressively with each other. When these bachelor males get older and become large (and large-nosed) enough to compete with males that are part of a breeding group, they are in a position to overthrow the tenured male. Females then often choose to form a harem group with this new, high-status male.

The nose is considerably smaller in female proboscis monkeys. Milan Zygmunt/Shutterstock

What’s behind the nose?

We investigated the size and shape of the proboscis monkey nasal cavity. That’s the bony chamber of the skull that sits behind the fleshy nose. Our goal was to find out if the size and shape of the nasal aperture – the front part of the cavity, where the fleshy nose tissue attaches – can tell us more about why these peculiar appendages evolved.

Previous research that looked at the bulbous nose in males suggests it evolved to advertise status. In our new research, we wanted to better understand how this could be the case, this time using data taken from the skull.

We used 3D surface models, downloaded from a public repository, to take size and shape measurements from 33 adult proboscis monkey skulls. We compared these with the adult skulls of king colobus monkeys, blue monkeys and crab-eating macaques, three old world monkey species.

Crab-eating macaques have tiny noses by comparison. Erik Klietsch/Shutterstock

We chose some measurements to quantify the nasal cavity, and others to quantify the nasal aperture in all the species.  We also looked at tooth wear, since older adult monkeys have more worn teeth than younger adults. That would allow us to find out if older adult males had a larger nasal aperture than younger adult males.

Better honks

If male proboscis monkeys have a different nasal cavity shape to females, and a unique shape compared to the other monkey species, it would support the idea these enhanced nasal structures – both the fleshy nose and the cavity behind it – evolved to allow for more effective honks and nasal roars.

That was indeed what we found. The shape of the male nasal cavity was low and long compared to females. This allows males to build up resonance (sound vibration) in their nasal cavities, allowing them to emit deeper and louder calls through their noses.

The nasal aperture shape was also different between the sexes. In males, it looks a bit like an eggplant, while in females it looks more like an upside-down pear. This unique opening shape in males allows for higher intensity sounds to be emitted through the nose.

3D model screenshots of a male proboscis monkey (left) and a female proboscis monkey (right). Male nasal aperture size is 29% larger than that of females, and males and females differ in their nasal aperture shape. Katharine Balolia/Morphosource Media (USNM521841; ID 000345556 and USNM142224; ID 000345144)

The sex differences in cavity shape were also larger than what we found in other old world monkey species. This further supports the idea that the nasal cavity of male proboscis monkeys underwent an evolutionary change for the purpose of making certain sounds.

Lastly, the age. Older proboscis monkey males really do have larger nasal apertures than younger adult males, but the cavity itself didn’t increase with age. This supports the idea that the large noses act as a visual signal. It’s also consistent with the fleshy nose size increasing in middle-aged or older adult males, which we know from behavioural studies in the wild.

Making honks and nasal roars really does seem to be the evolutionary purpose of these fleshy noses. Nokuro/Shutterstock

Our evidence from the skull allows us to better understand how nasal structures in male proboscis monkeys evolved for both acoustic and visual signalling.

The more we know about how regions of the skull function as social signals, the better chance we have of reconstructing extinct primate social behaviour using fossilised skull remains.

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The author would like to acknowledge the paper’s co-author, former ANU Masters student Pippa Fitzgerald.

Katharine Balolia, Senior Lecturer in Biological Anthropology, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Read More........→May 31, 2024

Ape Treating His Wound Using Medicinal Plant is a World First for a Wild Animal

Facial wound on adult male orangutan – Max Planck Institute of Animal Behavior via SWNS

Even though there is evidence of certain self-medication behaviors in animals, so far it has never been known that animals treat their wounds with healing plants. Now, biologists in Indonesia have observed this in a male Sumatran orangutan.

After sustaining a facial wound, he ate and repeatedly applied sap from a climbing plant with anti-inflammatory and pain-relieving properties commonly used in traditional medicine. He also covered the entire wound with the green plant mesh.

The closest relatives to humans, the great apes, are known to ingest specific plants to treat parasite infection and to rub plant material on their skin to treat sore muscles.

A chimpanzee group in Gabon was recently observed applying insects to wounds, although the efficiency of the behavior is still unknown. Wound treatment with a biologically active substance, however, has never been documented before.

Cognitive and evolutionary biologists from the Max Planck Institute of Animal Behavior in Konstanz, Germany—Caroline Schuppli and Isabelle Laumer—conducted the study at the Suaq Balimbing research site in Indonesia, which is a protected rainforest area home to around 150 critically endangered Sumatran orangutans.

“During daily observations of the orangutans, we noticed that a male named Rakus had sustained a facial wound, most likely during a fight with a neighboring male,” says Laumer, the first author of the study.

Three days after the injury Rakus selectively ripped off leaves from a vine with the common name Akar Kuning (Fibraurea tinctoria). He chewed on them, and then repeatedly applied the resulting juice precisely onto the facial wound for several minutes. As a last step, he fully covered the wound with the chewed leaves.

“This and related liana species that can be found in tropical forests of Southeast Asia are known for their analgesic and antipyretic effects and are used in traditional medicine to treat various diseases, such as malaria.

“Analyses of plant chemical compounds show the presence of furanoditerpenoids and protoberberine alkaloids, which are known to have antibacterial, anti-inflammatory, anti-fungal, antioxidant, and other biological activities of relevance to wound healing.”

Observations over the following days did not show any signs of the wound becoming infected and after five days the wound was already closed.Rakus, 47 days after first treating the wound using the medicinal plant – Max Planck Institute of Animal Behavior via SWNS

“Interestingly, Rakus also rested more than usual when being wounded. Sleep positively affects wound healing as growth hormone release, protein synthesis and cell division are increased during sleep,” she explained.

Like all self-medication behavior in non-human animals, the case reported in this study raises questions about how intentional these behaviors are and how they emerge.

“The behavior of Rakus appeared to be intentional as he selectively treated his facial wound on his right flange, and no other body parts, with the plant juice. The behavior was also repeated several times, not only with the plant juice but also later with more solid plant material until the wound was fully covered. The entire process took a considerable amount of time,” says Laumer.

“It is possible, that wound treatment with Fibraurea tinctoria by the orangutans at Suaq emerges through individual innovation,” said Schuppli, a senior author of the study published in Nature. “Orangutans at the site rarely eat the plant. However, individuals may accidentally touch their wounds while feeding on this plant and thus unintentionally apply the plant’s juice to their wounds. As Fibraurea tinctoria has potent analgesic effects, individuals may feel an immediate pain release, causing them to repeat the behavior several times.”

Since the behavior has not been observed before, it may be that wound treatment with Fibraurea tinctoria has so far been absent in the behavioral repertoire of the Suaq orangutan population. Like all adult males in the area, Rakus was not born in Suaq, and his origin is unknown.

“Orangutan males disperse from their natal area during or after puberty over long distances to either establish a new home range in another area or are moving between other’s home ranges,” explains Schuppli. “Therefore, it is possible that the behavior is shown by more individuals in his natal population outside the Suaq research area.”

This possibly innovative behavior presents the first report of active wound management with a biological active substance in a great ape species and provides new insights into the existence of self-medication in our closest relatives and in the evolutionary origins of wound medication more broadly.

“The treatment of human wounds was most likely first mentioned in a medical manuscript that dates back to 2200 BC, which included cleaning, plastering, and bandaging of wounds with certain wound care substances,” said Schuppli. Source: https://www.goodnewsnetwork.org/ape-treating-his-wound-using-medicinal-plant-is-a-world-first-for-a-wild-animal/

Read More........→May 08, 2024