They're certainly not panting. They are deliberately drawing in air over a curled tongue, which presumably maximises the tongue's surface exposed to air flow. Sticking it all the way out would expose more tongue but not to concentrated air flow. But regardless of the breathing and evaporation effects, another thing they're doing is sitting still, being calm and concentrating without strenuous exertion. Doing that on a hot day, especially in a shady place, should give you a chance to cool down after cycling, harvesting coconuts or whatever.