Kautilya's Arthashastra 9.1.45

9.1.45

atyuṣṇopakṣīṇe kāle hastibalaprāyo yāyāt //

When the weather is free from heat, one should march with an army mostly composed of elephants. Elephants with profuse sweat in hot weather are attacked by leprosy; and when they have no water for bathing and drinking, they lose their quickness and become obstinate. Hence, against a country containing plenty of water and during the rainy season, one should march with an army mostly composed of elephants. Against a country of the reverse description, i.e., which as little rain and muddy water, one should march with an army mostly composed of asses, camels, and horses.