Why It Is NOT True That The Veileds Eat Leaves Neither For The Purpose Of Additional Hydration Nor For Nutrition?
The primary reason often cited of increasing water intake through eating leaves (and other plant material) is doubtful, because the massive eating leaves happens in the rainy season, when there is enough rain water to drink. So, it makes no sense for them to eat leaves if they are anyway richly hydrated...
In the dry season it is pointless to eat greens for additional water intake, as there are either no green leaves at all or, if there are any, they are completely dry, so no water can be extracted from them.
Why the purposely eating green leaves is NOT the nutrition?
The most of the plant material is indigestible for chameleons and it passes through the digestive tract with almost no modification than partly dehydration and partly degrading through the poo-forming process.
So, why they eat green leaves at all?
The reason is simple but very spectacular. In the rainy period, which is very limited in time in Yemen, and lasts 5months maximum, they eat what is available widely: nymphs of orthopterans and caterpillars which are specifically abundant in the grass and lower bush level, to which in the period of their growth their presence is confined. Both food items have one in common: they are not rich in chitin, means in indigestible particles. Chameleon's peristaltic is due to the relatively short digestive tract not well adapted for passing amorphous gut content (which you get out of caterpillars), thus they use indigestible plant material to better pass the food through their digestive tract. I have seen many faeces of them, which looked like the actual dropping is wrapped in a rolled-together leave.