A small, established population of wallabies lives on the Hawaiian island of Oahu. These marsupials, native only to Australia and New Guinea, are a remnant of an early 20th-century introduction. They have since carved out a niche for themselves in the rugged landscape of the Koolau Mountains.
The Species Found in Hawaii
The wallaby species that has naturalized in Hawaii is the brush-tailed rock-wallaby (Petrogale penicillata). Adult males typically weigh between 5.5 and 8 kilograms and average about 55 centimeters in head and body length. They are characterized by a grayish-brown coat, a pale cheek stripe, and a long, dark tail that ends in a prominent brush of fur.
The entire Hawaiian population is confined to the island of Oahu, with the main colony residing in the steep, rocky cliffs of the upper Kalihi Valley. This species is highly agile, a trait that allows it to navigate its native habitat of rocky escarpments and boulder piles. Their preference for dry, rocky ledges and crevices allows them to avoid the moist forest floor.
Tracing the Wallaby’s Origin
The establishment of this population dates back to 1916, originating from a single breeding pair that was part of a private collection. The wallabies were initially purchased by a Honolulu resident from an Australian zoologist. Historical accounts indicate that three wallabies were brought over, but one joey was killed by dogs while the animals were temporarily housed.
The surviving adult pair subsequently escaped captivity into the surrounding wilderness, which offered the rocky terrain necessary for their survival. Genetic analysis suggests the Hawaiian wallabies were likely derived from the northern populations of Petrogale penicillata, most likely from southeast Queensland in Australia. This small founding group successfully bred and adapted to the new environment, establishing a feral population from such limited stock.
Current Status and Ecological Impact
The brush-tailed rock-wallaby population on Oahu is considered a non-native, introduced species, but it is not classified as an invasive threat. The population, which was estimated in the range of 250 to 400 individuals in the 1990s, is protected by state law and is designated as a game mammal. The population’s growth is naturally limited by the availability of their preferred steep, rocky cliff habitat and predation from dogs.
Because they primarily inhabit the rugged, steep cliffs of the Koolau Mountains, their interaction with native Hawaiian flora and fauna is limited compared to other introduced species. The wallabies’ diet in Hawaii consists mainly of non-native plants, such as Christmas-berry and Vlei berry, which reduces their direct impact on the native ecosystem. The Department of Land and Natural Resources does not actively monitor the population, and management is generally limited to returning any stray animals captured in residential areas back to the forest.