I originally started this post with warm weather in mind; not too long ago, it was 95 and sunny. But, as I sit here editing and putting the finishing touches on this post, it's a gloomy 55 and rainy, and I wish I had a second sweater with me. Summer seems like a myth right now, but hopefully it shows itself soon.
I've always been intrigued by taxonomy, or the classification of organisms. Seeing how different species are related to each other can be oddly satisfying and aesthetically pleasing. Looking at not only the species, but the big picture, reveals a literal circle of life (conveniently shown in the top left of this page). That circle is pretty detailed, so we've gotta tone it down a bit to make it blog-appropriate. With that, today's post is guest starring a one-of-a-kind species: the proboscis monkey, Nasalis larvatus.
Proboscis monkeys are an Old World monkey within the Colobinae subfamily. Colobines are a folivorous group of arboreal monkeys in Africa and Asia, with one exception living in India (I'm looking at you, Hanuman langur).
Asian colobines are often split into two groups with 3 and 4 genera, respectively: the langurs (Semnopithecus, Trachypithecus, and Presbytis) and the odd-nosed monkeys (Nasalis, Pygathrix, Rhinopithecus, and Simias). Below is a photo, Figure 2 from Wang et al. 2012, showing phylogeny for colobines following genetic analysis. You'll find Nasalis on its own within the Odd-nosed section (and humans at the bottom of the Hominoidae section!).
Nasalis is a monotypic genus, with Nasalis larvatus being the sole living species. They are only found on Borneo, but are found in all three countries (Brunei, Indonesia, and Malaysia) that split the island. No one knows the exact range or distribution of the species on the island. Unfortunately, due to hunting and habitat destruction, these guys are endangered and their populations are continuing to decrease.
Nasalis larvatus lives sympatrically with several other primates, most notably, orangutans (Pongo pygmaeus), the silvery lutung (Trachypithecus cristatus), and the long-tailed macaque (Macaca fascicularis) with whom proboscis monkeys may compete with for food resources.
At night, proboscis monkeys sleep in trees near rivers, but move inland during the day, never being more than a kilometer away from water. Possibly as a result of proximity, these are the most aquatic of any primate--they swim, wade, and easily cross rivers, which often serve as a barrier for other species. Diving has also been observed in the species. They even have partially webbed toes to help them in their aquatic endeavors!
Pictured below are a female and male proboscis monkey, respectively. The males are big, weighing about 45 lbs, and have long, pendulous noses that often hang over the mouth, reminiscent of Squidward Tentacles. On the flip side, the females are about half the size of males (about 22 lbs) and have large pointed noses, a la Cindy Lou Who. The National Primate Research Center at University of Wisconsin Madison describes them as looking "perpetually pregnant, due to oversized, protruding stomachs", a little rude, perhaps, but they're not wrong.
Like other colobines, proboscis monkeys are folivorous, eating mostly leaves. They have a much larger GI tract than other primates, due to a multi-chambered stomach that allows for forestomach fermentation and digestion of more hard-to-digest structural carbohydrates. Interestingly, rumination (chewing again what has already been chewed and swallowed) has been observed in the species. These monkeys do not have digestive adaptations for it, like other animals that ruminate regularly. This behavior appears to not be a physiological requirement for digestion, but rather may be a pathological response to something else.
Nasalis larvatus have a fission-fusion social structure. They live in one male groups, made up of an adult male, his monopolized females, and their offspring (Mitani et al. 2012). These groups combine at the end of the day to lower predation risk and maintain their sleeping sites, and may interact with individuals outside of their family unit at this time. During the day, units may combine, but individuals will not interact socially with each other (let's be real, who wants to talk to strangers?). Typically, adolescent males will leave their family groups and either roam on their own, or more likely, will join a bachelor group of many single males until they can overtake their own harem of females.
As shown in the photo to the left, infants have black and bluish faces, and lack the namesake nose. The noses develop as they mature, with male noses continuing to grow as they age.
So what's the deal? Why do Proboscis monkeys wield this astonishing schnoz?
Not much is known for sure; some suggest that swelling and a subsequent increase in size of the nose helps to project male vocalizations and alarm calls. Consensus generally suggests that the unique nose is a result of sexual selection--the idea being that males with bigger noses have more potential for reproductive success. Even so, male facial adornment isn't unique to Nasalis. Mandrills have bold blue stripes and red noses; bornean orangutans have broad, disk-like faces; you can even consider beards as a form of adornment for our species! Male facial adornment isn't unique to primates, either; many species of birds utilize ornamentation to attract females.
What can I say? The heart wants what the heart wants--sometimes, you just need the right nose.
Matsuda, I., Sha, J. C. M., Ortmann, S., Schwarm, A., Grandl, F., Caton, J., Jens, W., ... Clauss, M. (October 01, 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, 149, 45-52.
Mitani, J. C., Call, J., Kappeler, P. M., Palombit, R. A., & Silk, J. B. (2012). The evolution of primate societies.
Wang, X. P., Yu, L., Roos, C., Ting, N., Chen, C. P., Wang, J., & Zhang, Y. P. (2012). Phylogenetic relationships among the colobine monkeys revisited: new insights from analyses of complete mt genomes and 44 nuclear non-coding markers. PLoS One, 7(4), e36274.
By Charlesjsharp (Own work, from Sharp Photography, sharpphotography) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
Proboscis Monkey - Male
Proboscis Monkey - Mother with young
By Bernard DuPont, https://www.flickr.com/photos/berniedup/albums/72157632525562027