Of the five native mammals, two have become extinct since the arrival of humans. Maclears rat (Rattus macleari) and the bulldog rat (Rattus nativitatus) apparently became extinct within a few years of the introduction of exotic rodents by early human colonisers.
The Christmas Island shrew (Crocidura attenuata trichura) was thought to be extinct before two specimens were found in 1984 and 1985, and it is now listed as Critically Endangered. There have been no confirmed sightings since, although there has been some research to ascertain its current status.
The Christmas Island pipistrelle (Murray's pipistrelle bat, Pipistrellus murrayi), which is an endemic small insectivorous bat, was previously common and widespread on the island. However, in the last decade it has declined markedly in distribution and abundance, and is now classified as critically endangered. It is not fully understood what has caused this rapid decline.
The Christmas Island flying-fox (Pteropus melanotus natalis) is an endemic subspecies which was formerly widespread but has experienced significant population declines, the causes of which are still unknown.
Four exotic wild mammals have been introduced, the black rat (Rattus rattus), the house mouse (Mus musculus) the feral cat (Felis catus) and wild dog (Canis familiaris).