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!