Australasian Science: Australia's authority on science since 1938

Extinct Burrowing Bats Betray Biodiversity Loss

Credit: Gavin Mouldey

Credit: Gavin Mouldey

By Sue Hand

The fossilised remains of burrowing bats that lived millions of years ago shed light on this remarkable group of mammals while signalling loss of biodiversity in New Zealand.

Some 150 km from New Zealand’s adrenaline-pumping adventure capital Queenstown, the historic village of St Bathans nestles quietly in the golden foothills of the St Bathans Range. Here, life moves at a gentler pace, but in its heyday St Bathans was also a boom town, riding high on Central Otago’s gold rush of the late 1800s to early 1900s. Today, its heritage-listed buildings of sun-dried mud bricks, stone and kauri pine – and laidback New Zealand style – make this village a popular stop on Central Otago’s rail trail.

At the town’s heart is the Vulcan Hotel, built in 1882 and once patronised by the district’s 2000 miners. Among the Vulcan’s claims to fame is its infamous ghost, The Rose, a lady of the night strangled in Room 1 in days long gone. Another drawcard for the village is the man-made Blue Lake, once a conspicuous hill but transformed by goldmining sluicing into a 68-metre pit (in its day the deepest mining shaft in the Southern Hemisphere) and now filled by astonishingly blue water.

Not far from the village, evidence of a much older, natural lake is revealing a far deeper history for the St Bathans area. Some 16–19 million years ago (Ma), a vast lake covered more than 5600 km2 of the South Island’s Maniototo region. At that time, temperatures in New Zealand were warmer than today, and semitropical to warm temperate forests and ferns edged palaeolake Manuherikia. A diverse fauna lived in and around the lake, and over time thousands of fossil bones and teeth accumulated in the lake sediments.

Today this fossil treasure trove is being excavated by New Zealand and Australian palaeontologists, and a detailed picture of ancient New Zealand is emerging as we raise the long-dead of St Bathans. The ancient remains belong to many new bird species, including geese, ducks, herons, gulls, waders, rails, owlet-nightjars, parrots, pigeons, moas, kiwi, adzebills and wrens, as well as tuatara, geckos, skinks, frogs, galaxiid and other endemic fish, and freshwater and terrestrial snails.

These fossils fill an otherwise enormous gap in the land vertebrate record of New Zealand between 5 and 65 Ma, and for many of New Zealand’s distinctive lineages they provide their oldest and sometimes first deep-time records. Several unexpected past residents have also been revealed – among them crocodiles, land turtles, flamingo-like birds and a small ground-dwelling mammal of unknown affinities.

Also recovered from the St Bathans deposits are the remains of some extraordinary burrowing bats. Today these bats only survive in New Zealand, but they once also occurred in Australia.

Fossils indicate that this radiation numbered at least ten species: six from New Zealand and four from Australia. The only member of the family definitely surviving is New Zealand’s Mystacina tuberculata. A second New Zealand species, Mystacina robusta, has not been sighted since 1967, and is critically endangered or extinct.

St Bathans was once home to four of New Zealand’s burrowing bat species. One of these is the biggest burrowing bat yet known. It may only have weighed about 40 grams yet it is about three times the size of an average bat today. We have recently named this bat Vulcanops after Vulcan, the mythological Roman god of fire and volcanoes, in reference to New Zealand’s tectonic nature – but also after the historic Vulcan Hotel in the nearby mining town of St Bathans.

Today, bats make up more than 22% of the world’s mammals and are distributed from the tropics to cool temperate regions on all continents except frozen Antarctica. Three bat species are New Zealand’s only living native land mammals; all other modern land mammals in New Zealand were introduced by people within the last 800 years.

Bats first appear in the global fossil record following a sharp spike in global temperatures that occurred around 56 Ma at the Paleocene–Eocene boundary. After the spike, global temperatures dropped sharply, and then steadily climbed. During this time, plants and insects rapidly diversified, and in response many groups of mammals radiated – including bats. As soon as they achieved powered flight, at least 55 Ma, bats appear to have dispersed rapidly and widely across the globe.

Of the 1400 bat species alive today, all fly but a few also have the ability to walk. These rare bats include New Zealand’s burrowing bats, the three blood-feeding vampires of South America and Central America, and the two naked bulldog bats of South-East Asia. Because these groups are only distantly related to each other, it is likely that each independently re-developed the capacity to walk after losing this ability very early in the evolution of bats.

New Zealand’s burrowing bats today spend about 30% of their foraging time on the ground, and may completely disappear under leaf-litter and even snow while searching for arthropods. They use a true quadrupedal walking gait when moving on the ground, scurrying about on their wrists and backward-facing feet, keeping their wings tightly furled in a protective sheath-like portion of the flight membrane. Their wing and tail membranes are reduced, enabling free movement of the fore and hind limbs. Large secondary talons on the thumb and toe claws increase their grip on surfaces, as does a system of adhesive grooves in the soles of their feet. These behaviours are reflected in many specialisations in their skeleton, including in the arm, foot, leg, spine, and pectoral and pelvic girdles, which can in turn be detected in their fossilised bones.

Lack of snakes and ground-based predatory mammals in pre-modern New Zealand are assumed to have facilitated the evolution of walking in the modern species, but our research shows that Australian members of the family also used terrestrial locomotion at least 12–26 Ma. This was before the isolation of the members of this group in New Zealand and despite the existence of numerous ground-based reptilian and mammalian predators.

As a group, burrowing bats are more closely related to bats in the Americas than to other bats found in the south-west Pacific today. Their closest living relatives are South America’s ghost-faced bats, fishing and frog-eating bats, nectar-feeding bats and vampire bats. All of these belong to a bat superfamily that once spanned the southern landmasses of Australia, New Zealand, South America, Africa and probably Antarctica.

Exactly when burrowing bats first arrived in New Zealand is not yet known. St Bathans provides their oldest record in New Zealand, but some Australian fossils are at least 26 million years old, and molecular data suggest that the lineage separated from other bats as much as 51 Ma. At that time, the land masses of Australia, Antarctica and South America were still connected as the last vestiges of the southern supercontinent Gondwana. Separation of New Zealand from the rest of Gondwana began earlier, about 81 Ma, but its last connection to Australia, via the Lord Howe Rise, was not severed until about 52 Ma.

Global temperatures were up to 12°C warmer than today, and Antarctica was forested and frost-free. With subsequent fragmentation of Gondwana (50–35 Ma), redirected ocean currents, cooling climates and the growth of ice-sheets in Antarctica, Australasia’s burrowing bats became isolated from their South American relatives.

Possibly associated with their Gondwanan forest origins and their semi-terrestrial habits, New Zealand’s living burrowing bats are renowned for their extremely broad diet. They eat many insects and other arthropods such as weta and spiders, which they catch on the wing or chase by foot, and they also regularly consume fruit, flowers and nectar. This wide range of foods makes them the most unfussy eaters of any bats known. They are also among the few temperate zone bat species known to pollinate plants (flower-visiting bats are otherwise found mainly in the tropics and subtropics).

At least one of the extinct St Bathans burrowing bats, Mystacina miocenalis, was probably closely related to the living New Zealand species. It is noteworthy that a diverse array of plant and invertebrate fossils found in the St Bathans fossil deposits, and other deposits of similar age nearby, are remarkably similar to modern kinds. These include plant and animal species used by Mystacina as their colonial roost trees, arthropod prey, and plants they pollinate and some whose seeds they disperse. This suggests long-term ecological associations between New Zealand’s semi-terrestrial bats and the forest ecosystems they have evolved in for millions of years.

Vulcanops jennyworthyae, on the other hand, had conspicuously more specialised teeth than other burrowing bats. This suggests it had a slightly different diet and lifestyle. It was probably capable of eating more diverse plant food as well as a range of small vertebrates such as frogs, lizards and birds – a diet more like some of its South American cousins rather than any Australasian bat today.

At least two additional bat species, identified from among the St Bathans fossils but not yet formally described, don’t appear to be related to modern New Zealand bats, and these hint of even greater bat diversity in New Zealand’s past. They add to the number of groups that evidently became extinct in New Zealand after the early to middle Miocene (12–23 Ma), including crocodiles, terrestrial turtles, flamingo-like palaelodids, swiftlets, several pigeon, parrot and shorebird lineages, some bat groups and non-flying mammals. Most of these were probably warm-adapted species.

After the middle Miocene, global climate change brought colder and drier conditions to New Zealand, with significant changes to vegetation and environments. It is likely that this general cooling and drying trend triggered overall loss in bat diversity in New Zealand, and probably for many other groups as well.

Sue Hand is Director of the Palaeontology, Geobiology and Earth Archives Research Centre at UNSW Sydney.