The Evolutionary History of New Zealand's Short-Tailed Bats

New Zealand is an island nation composed of two main islands along with several smaller surrounding islands. Because of its remarkable diversity of bird species, the country has long been described as a “kingdom of birds.” Equally striking, however, is the scarcity of many other animal groups. Native terrestrial mammals are almost entirely absent from the islands.

Specimen of Mystacina robusta（圖片來源：Auckland War Memorial Museum，採用 CC BY 4.0 授權。）

Illustration of Mystacina tuberculata（圖片來源：Zoological Society of London. ，公有領域。）

Today, the only native land mammals that remain in New Zealand are three endemic bat species. These include the long-tailed bat (Chalinolobus tuberculatus) of the family Vespertilionidae, and two members of the family Mystacinidae: the greater short-tailed bat (Mystacina robusta) and the lesser short-tailed bat (Mystacina tuberculata). The family Mystacinidae itself is endemic to New Zealand and is represented today by only a single genus containing two species.

Chalinolobus tuberculatus（圖片來源：GH Ford，公有領域。）

Historically, both species of short-tailed bats were once widely distributed across much of New Zealand. After humans arrived on the islands, however, the situation changed dramatically. Habitat destruction caused by human activity, along with invasive species that accompanied human settlement—particularly domestic cats and rats—posed severe threats to these bats.

The lesser short-tailed bat has suffered significant population declines and habitat loss due to these introduced predators. One documented incident in 2010 illustrates the scale of the threat: a single domestic cat entering a roosting area killed 102 individuals. Such events highlight how vulnerable these bats are to mammalian predators. In recent years, conservation programs have been implemented, and these measures have helped stabilize the species to some extent.

Colony of roosting Mystacina tuberculata（圖片來源：TheyLookLikeUs，採用 CC BY-SA 4.0 授權。）

The greater short-tailed bat has been far less fortunate. After European colonization, its distribution steadily contracted southward until the species survived only on small islands near Stewart Island, at the southern tip of New Zealand. In 1963, black rats invaded these islands. Their arrival devastated local wildlife, driving two endemic bird species to extinction and delivering a severe blow to the remaining bat population. At the time, the greater short-tailed bat was not yet recognized as a distinct species and therefore did not receive priority conservation attention. After 1965, no confirmed records of the species were obtained. Today, it is classified by the International Union for Conservation of Nature as Critically Endangered, based largely on a possible bat call recorded in 1999 and several unconfirmed sightings after 2000. Nevertheless, many researchers believe the species is most likely extinct.

The last known habitat of Mystacina robusta, the small island shaded in red within the red circle（圖片來源：A proietti，採用 CC BY-SA 4.0 授權。）

Short-tailed bats are extraordinary among bats in their behavior and ecology. Unlike most bat species, they exhibit a strong tendency toward terrestrial activity. They spend substantial amounts of time foraging on the forest floor. When moving on the ground, they fold their wings against their bodies and walk using all four limbs while searching for food with their sense of smell. This unusual lifestyle is thought to have evolved because, prior to human arrival, New Zealand lacked native terrestrial mammalian predators. Under such conditions, moving on the ground posed little danger. In the air, however, short-tailed bats behave much like other small insectivorous bats, using echolocation to locate prey.

Their diet is also unusual. Short-tailed bats are among the relatively few bats that are omnivorous. They feed on a wide range of small terrestrial arthropods as well as plant resources such as flowers, fruit, and nectar. During their foraging activities, they inadvertently carry pollen between plants and thus act as pollinators. Several New Zealand plant species rely on these bats for pollination, making them an important component of the local ecosystem. Protecting short-tailed bats therefore also helps safeguard the plant communities that depend on them.

Molecular evidence indicates that the mystacinid lineage diverged approximately 51 to 41 million years ago. The earliest known members of the family come from Australia. Fossils of the genus Icarops, dating from the Late Oligocene to Early Miocene about 23 million years ago, represent the oldest known mystacinids. These bats already show adaptations associated with terrestrial locomotion, suggesting that the ground-foraging behavior of mystacinids did not originate in New Zealand. Icarops also remains the only mystacinid genus known outside New Zealand.

Lower jaw teeth of Vulcanops jennyworthyae（圖片來源：Hand, S. J et al. (2018). ，採用 CC BY 4.0 授權。）

Upper jaw teeth of Vulcanops jennyworthyae（圖片來源：Hand, S. J et al. (2018). ，採用 CC BY 4.0 授權。）

By around 20 million years ago, a different mystacinid had appeared in New Zealand: Vulcanops jennyworthyae. This species was considerably larger than modern short-tailed bats, which typically weigh only around a dozen grams. Vulcanops may have reached body masses of roughly 40 grams. Dental morphology indicates that this species likely consumed a greater proportion of plant material than modern species.

Fossils of Vulcanops come from the St Bathans fauna, a Miocene fossil assemblage in New Zealand that has yielded multiple mystacinid species. These include Mystacina miocenalis, a member of the same genus as modern short-tailed bats, as well as several other undescribed mystacinid species. The presence of multiple species suggests that the family experienced a radiation around 20 million years ago.

Estimates suggest that Mystacina miocenalis weighed up to 39 grams. Its dental morphology, however, closely resembles that of modern Mystacina. Because plant groups consumed by modern short-tailed bats were already present in the Miocene fossil record, researchers infer that Mystacina miocenalis likely had a diet very similar to that of modern species.

Although the fossil record contains a gap of roughly 16 million years following Mystacina miocenalis, forming what is known as a ghost lineage, scientists propose that ecological interactions between short-tailed bats and certain flowering plants may have persisted throughout this period. The feeding and pollination relationship between these bats and particular plant species may therefore have been evolving for at least 16 million years, eventually producing a deeply rooted mutualistic association within New Zealand's ecosystems.

Author: Bai Leng

Reference:

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