Asian elephants ("Elephas maximus") were once widespread throughout the country with numbers as high as 10,000 animals, but in the numbers have dwindled, partly due to transferring the animals to logging camps. Other species reported to be present include the leopard, Bengal tiger, crab-eating macaque, wild dog, and otters ("Panthera pardus, P. tigris, Cuon alpinus", and "Lutra" species). The saltwater crocodile ("Crocodylus porosus") can be found in the southern part of the delta. The species was formerly abundant in coastal regions, but population numbers have decreased because of a combination of commercial skin hunting, habitat loss, drowning in fishing nets and over-collection of living animals to supply crocodile farms. It was at this river that a battle between a saltwater crocodile and a tiger was observed that ended with the reptile devouring the tiger. Despite recent declines in the sea turtle populations, five species are known to nest in Myanmar at well known island and mainland beaches known as turtle-banks. These are the olive ridley sea turtle ("Lepidochelys olivacea"), the loggerhead sea turtle ("Caretta caretta"), the green sea turtle ("Chelonia mydas"), the hawksbill sea turtle ("Eretmochelys imbricata"), and the leatherback sea turtle ("Dermochelys coriacea"). The Irrawaddy River has five major tributaries. As they flow through the northern tip of Myanmar – the Kachin State – they cut long north-south alluvial plains and relatively narrow upland valleys between the mountain ridges. The rivers joining the Irrawaddy are, from north to south:   As early as the sixth century, the Bamars were using the Irrawaddy to gain power in the region through trade and transport on the China – India route. By the twelfth century, a well-developed network of canals made for flourishing rice cultivation. Later, the river became a key economic tool of Imperial British interests, who set up trading ports along its shores. Today, the Irrawaddy is still the country's most important commercial waterway. Despite Mandalay's position as the chief rail and highway focus in northern Myanmar, a considerable amount of passenger and goods traffic moves by river. As the Irrawaddy Delta is one of the world's major rice-growing areas, one of the most important goods transported is rice. Teak logs – Myanmar is one of the world's top exporters – are floated down the river as large rafts.