Langurs belong to the colobine group, whose stomachs are divided into chambers with a near-neutral forestomach where symbiotic bacteria ferment plant material. That fermentation does two jobs: it releases energy from cellulose that primate enzymes cannot touch, and it degrades many of the alkaloids and tannins that plants use as deterrents. Langurs consequently exploit mature and toxic leaves that fruit-eating monkeys sharing the same forest cannot.
The same anatomy imposes limits. The forestomach depends on a stable microbial community, so sudden diet changes can be dangerous, and easily fermented sugars can acidify the chamber. Captive colobines have historically been difficult to keep for this reason.
Grey langurs are also the species behind one of the longest disputes in primatology: infants are sometimes killed when a new male takes over a group. Whether this is a reproductive strategy that shortens the mother's return to fertility, or a pathological response to crowding, was argued for decades, with the strategic interpretation now better supported but not universally accepted.