Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Video: Proboscis monkeys chew their food twice

Mmm, cud (Image: Ikki Matsuda)

Species: Nasalis larvatus

Habitat: swamps, mangroves and forests of Borneo, sticking their noses into each other’s business

Pinocchio has nothing on the proboscis monkey. The fabled wooden boy’s nose grew to extreme lengths only when he told lies, but this real-life primate has a big nose no matter what it does.


Combined with their pot bellies – the result of the complicated digestive system necessary to stomach their diet of leaves and fruit – their appearance could charitably be described as peculiar. But it now seems the proboscis monkey has a truly peculiar foible: it is the only primate known to chew the cud – it regurgitates its food and chews it a second time.

Chewing things over and over

Ikki Matsuda of Kyoto University in Inuyama, Japan, and colleagues made two trips to the monkeys’ habitat in Borneo in recent years. Watching from a boat while the animals perched in riverside trees, they were surprised to see them apparently regurgitating food and chewing it again.

Matsuda saw monkeys of both sexes and all ages doing this, chewing the regurgitated food for an average of 5 minutes. Monkeys spent longer feeding on days when they regurgitated.

He thinks the regurgitation helps clear large food particles from the monkeys’ foreguts, allowing them to eat more. Large tree-living animals like proboscis monkeys are often forced to eat poorer-quality foliage because they are confined to big branches, so being able to eat more could be crucial.

The evidence is not totally convincing, says Murray Logan of Monash University in Melbourne, Australia, because Matsuda did not examine the monkeys’ mouths to confirm that they contained food – although Logan knows from experience how difficult it is to do that. He studies rumination in koalas, and says the behaviour is difficult to catch because it lasts only minutes. If you do successfully capture an animal in the act, they tend to swallow the food before you can examine it. “I sympathise completely,” he says.

Hey, big guy

So much for chewing the cud: what is it with the enormous noses? The first thing to bear in mind is that, although females do have large noses, the males’ are significantly bigger and sometimes hang down below the mouth.

That suggests there’s more than a whiff of sexual selection in the noses: it may be that they are the proboscis monkey equivalent of peacocks’ tails, making males more attractive to females – although there is as yet no direct evidence of that.

Adult males are larger than females and some control groups of females with whom they regularly mate. Males without a harem live in all-male groups.

The males may seem to be in charge, but females wield a lot of power. They are often the ones who initiate mating, by pursing their lips into a pout and shaking their heads from side to side. They sometimes swap groups, and may even swing by an all-male group for a bit of extra mating.

Nevertheless, it’s clearly better to be a male in charge of a harem – and it might be that having a big nose helps out.

Hooter

Big noses might boost a male’s ability to vocalise. A longer nose means a longer nasal tract, changing the nature of the sound produced and effectively making the male sound larger than he actually is. In particular, males warn off intruders with aggressive honks, and their noses seem to be critical for making these sounds: they stiffen slightly with each honk.

Long noses are not without disadvantages, however. Copulating pairs often get harassed by juvenile members of the group, and a common tactic is to pull the mating male’s nose.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2011.0197

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