The Long-eared Bats of Australasia

When most people think of Australian mammals, they imagine the fauna as dominated by marsupials and monotremes, representatives of lineages long isolated from those found elsewhere. But Australia is also home to a remarkable diversity of native placentals. Immigrating from the north as Australia drifted closer to Asia, the rodents and bats underwent their own radiations on the Australian continent and its neighbouring islands. Among these distinctly Australasian assemblages of placentals are the long-eared bats or big-eared bats of the tribe Nyctophilini.

Lesser long-eared bat Nyctophilus geoffroyi, copyright Michael Pennay.

The long-eared bats comprise fifteen or so species found over a range between eastern Indonesia and Australia with outlying species in New Caledonia and Fiji. They are members of the Vespertilionidae, the most diverse recognised family of bats, and share with most other vespertilionids a fairly generalised appearance with dull coloration. They differ from other vespertilionids in having a relatively short muzzle (with a correspondingly reduced number of teeth) with a small nose-leaf at its end (Miller 1907). They also (as the vernacular name indicates) have particularly large ears, as long as or longer than the rest of the head, that are commonly connected medially by a distinct membrane. At rest, the ears may be folded like a concertina along the hind margin to protect them from damage (Hall & Woodside 1989). Historically, the long-eared bats were treated as their own subfamily within the Vespertilionidae that also included a similar North American genus Antrozous. However, the nyctophilins are now regarded as a derived tribe within the larger subfamily Vespertilioninae (albeit one whose exact relationships remain uncertain) and similarities between Nyctophilini and Antrozous are thought to be convergent rather than reflecing a close relationship. The majority of nyctophilins are placed in a single genus Nyctophilus with the exception of the New Guinea big-eared bat Pharotis imogene. This species differs from Nyctophilus in lacking hair at the end of the muzzle.

Nyctophilins are found in a range of habitats but seem to prefer dry woodlands. Vespertilionids as a whole are differentiated from other bats by modifications of the fore arms including a highly developed double joint between scapula and humerus and reduction of the ulna. As a result, they may be less powerful fliers than other bats but they would be more agile. This trend would be particularly pronounced in nyctophilins which have relatively short wings compared to other vespertilionids (Hall & Woodside 1989). The development of a nose-leaf in nyctophilins is associated with their use of signals emitted at a constant frequency through the nose for echolocation whereas other vespertilionids use signals of varying frequency emitted through the mouth. As well as catching insect prey in flight, long-eared bats are able to recognise prey at rest and so glean insects off vegetation or on the ground. This gleaning habit is presumably also associated with long-eared bats having relatively larger eyes than other vespertilionids.

Gould's long-eared bat Nyctophilus gouldi with ears partially reclined, copyright Department of Environment and Primary Industries, Victoria.

Caves in Australia are mostly not very extensive so the formation of colonies by Australian vespertilionids is constrained by the availability of suitable roosting sites such as hollows in trees or crevices in rocks. At least some long-eared bats may be solitary (Hall & Woodside 1989). Their distribution in Australia (as with pretty much all Australian animals) is also largely contingent on the availability of water. Mating happens in autumn but gestation is generally delayed, whether by delaying fertilisation or development of the embryo, and does not kick off until spring. Pregnancy then lasts about six weeks though it may again be slowed down if conditions turn bad. Long-eared bats are unusual among bats in that twins are not uncommon.

Whereas at least some nyctophilin species remain common (the lesser long-eared bat Nyctophilus geoffroyi is found over most of Australia), others are rare or little-known. A species described from Lord Howe Island, N. howensis, is believed to be extinct. The most remarkable case of obscurity is Pharotis imogene which was not recorded between 1890 and 2012, over 120 years. Evidence of extreme rarity? Quite probably, but also possibly evidence of just how few people are paying attention to bats.

REFERENCES

Hall, L. S., & D. P. Woodside. 1989. Vespertilionidae. In: D. W. Walton, & B. J. Richardson (eds) Fauna of Australia vol. 1B. Mammalia pp. 871–886. Australian Government Publishing Service: Canberra.

Miller, G. S., Jr. 1907. The families and genera of bats. Smithsonian Institution, United States National Museum, Bulletin 57: i–xvii, 1–282, pls 281–214.