The mystery of Hanuman tails

In the past I’ve often seen tribes of hanuman (Semnopithecus entellus, northern plains grey langur) sitting in a line at the sides of highways. As I sped past, I would notice they had their backs to the road, and tails spread behind straight behind them on the tarmac. I always wondered what makes them do it. But when you are on a highway, going at speed, you can’t possibly stop in time to check.

I solved this mystery quite by chance one day when I was walking down a road near a village and saw exactly this sight. But now I could stop and watch. They were foraging among the dense weeds on the sides of the road. “Of course,” I mentally kicked myself. They are primarily herbivores, just occasionally adding a caterpillar or some other insect to their food as supplement. Unusually for monkeys, they eat mature leaves too. That is exactly what they were doing as I watched. And then I realized that there was little choice in where and how their stiff tails would be placed. It must be convenient to just let it rest on the road behind them, exactly as I’d seen.

Like all mammals which have adapted to a leaf-heavy diet, the hanuman has a very complex digestive system. It turns out that they have a four chambered stomach, with a first chamber hosting a specialized microbiome which helps it to digest, and perhaps detoxify, the leaves that it eats. Interestingly, there is a clade of species, called the colobine monkeys, distributed across Africa and Asia which all have such high-fibre diets. This first chamber has evolved separately in the two continents. Other colobines have three chambered stomachs. They also eat leaves, but they generally eat more fruits than leaves. The solution to the mystery of the hanuman tails seems to be in their stomachs!


Uttarakhand is heavily forested, and forests in this region have monkeys. One of the largest is the species of the hanuman langur called the Nepal gray langur (Semnopithecus schistaceus). I saw the individual in the featured photo during a walk at an altitude of about 2400 meters, inside the Binsar National Park. The seven species of hanuman, genus Semnopithecus, which are found in India separate into distinct geographical ranges, with little overlap. At this altitude, and this far north, the Nepal gray langur is the only one that is found. As I concentrated on taking photos of this troupe of leaf eaters, I missed a photo op which will probably never recur: two yellow throated martens (Martes flavigula) pulled themselves up the cliff next to the road I was on, sat on the edge and stared at me for a long time. They were gone by the time I mentally kicked myself into swinging my camera round to photograph them. They are shy and swift, and because of that are hard to photograph, in spite of being fairly common in these forests. The longer you live the more regrets you have.

Watching the troupe I was reminded of the graffiti I’d seen in Haridwar a couple of days earlier, when I visited the abandoned ashram of Maharishi Mahesh Yogi. I found a photo on my phone (above). It is a lovely piece of art, but it does not show the Nepal gray langur. This species holds its tail above, and parallel to, their bodies when they walk. The tail is long, and the tip can project forward ahead of the head. The drooping tail that the artwork shows belongs to the southern plains gray langur (Semnopithecus johnii) whose range is far to the south.

Monkey’s business

If you observe monkeys, or read accounts by those who do, you’ll find that most of the social life of monkeys centres around grooming. It seems to be as important to monkeys as chatting is to us. I watched the two langurs you can see in the featured photo. The smaller one was initially sitting by itself, when the larger one sauntered by. The smaller was startled, and was ready to run in fright, when the bigger one reached out, pulled it down and began to groom it.

These gray langurs, sometimes called hanuman langurs, have been studied extensively. Much is known of their genetics and social structure. Still, interesting new material comes to light from amateur observation of these herbivorous monkeys. Many interesting new observations are of deception. I wasn’t lucky enough to see anything as interesting as this.

Monkeys hugging each other in Ranthambore

If you spend even a little time in an Indian forest you cannot fail to see an association of Cheetal and langurs. Of course, if amateurs and tourists can spot this, professionals must be writing papers about it. The bottom line seems to be that Cheetal gain from staying around langurs.

We stopped to watch the deer, but my attention turned to the monkeys. After the first incident, which I described above, I saw many more pairs grooming each other. Another encounter was also startling. A smaller monkey ran towards a larger one and hugged it (as you see in the photo). The larger one then started grooming the smaller one. What was it? Mother and older child?

Monkey behaviour is complex enough to be endlessly fascinating.