The largest of all the Asian big cats, tigers rely primarily on sight and sound rather than smell for hunting. Tigers generally gain independence at around two years of age and attain sexual maturity at age three or four for females and four or five years for males. Juvenile mortality is high, about half of all cubs do not survive more than two years. Tigers have been known to reach up to 20 years of age in the wild. Across their range, tigers face unrelenting pressures from poaching, retaliatory killings, and habitat loss. They are forced to compete for space with dense and often growing human populations. Since 2017, IUCN has recognized two tiger subspecies, commonly referred to as the continental tiger and the Sunda island tiger. All remaining island tigers are found only in Sumatra, with tigers in Java and Bali now extinct. These are popularly known as Sumatran tigers. The continental tigers currently include the Bengal, Malayan, Indochinese and Amur (Siberian) tiger populations, while the Caspian tiger is extinct in the wild. The South China tiger is believed to be functionally extinct.