Feature

Going in to Bat for Bats

Our only land-based mammals have been neglected far too long.

Colin F.J. O'Donnell

Few people are aware that New Zealand has its own bat population. We have two native bat species, the long-tailed bat and the short-tailed bat, both of which are threatened.

New Zealand bats are little known, but still occur in areas accessible to many people. In general, they are a poorly understood group of animals -- there are many pre-conceived ideas about them and much folklore. Last summer over 150 people joined trips to see bats in the Eglington Valley in Fiordland. People's attitudes change when they see or get to hold bats, and in many a new fascination is born.

After the rodents, bats are the most diverse group of mammals in the world. There are nearly a thousand species, ranging in size from the large fruit bats with wing spans of up to 1.7 metres, right down to the tiny bumblebee bat, or Kitti's hog-nosed bat, from Thailand which weighs in at a barely imaginable 1.8 grams, about the size of your thumb nail. Bats are surprising animals. They live for exceptionally long times (up to 35 years) but breed very slowly.

Nearly 90% of bats live in the tropics, and in these habitats single localities often contain dozens of species and some caves support hundreds of thousands of individuals. Food habits vary as much as size. There are species which feed exclusively on flowers, fruit or insects, and some oddities which specialise in catching and eating fish, frogs, birds, lizards, even other bats, and of course those infamous blood-eating bats, the vampires.

New Zealand has few bat species compared with other countries; the UK has 15 species and Australia has over 60 different types. Before humans arrived in this country at least four species existed. Unfortunately two larger species are now extinct.

Surviving Species

The two remaining species, the long-tailed bat (Chalinolobus tuberculatus) and the lesser short-tailed bat (Mystacina tuberculata) are both classed as endangered. Both species declined considerably when humans arrived in New Zealand and there is very little known about the current status, long-term stability and ecology of their populations. It is not known whether populations are still declining or indeed what the causes of decline were.

Both species are very small, their bodies not being much larger than a thumb, with wing-spans of nearly 30 centimetres. Long-tailed bats are soft with a rich chestnut-coloured fur. Short-tailed bats are grey in colour and their fur has the texture of velvet. Both eat insects from tiny sandflies up to large moths, beetles, craneflies and wetas. Long-tailed bats catch their prey in the air while flying high above the forest canopy. Short-tailed bats also forage on the forest floor and eat nectar, fruit and pollen.

The ancestors of short-tailed bats are believed to have reached New Zealand from South America via Antarctica about 35 million years ago. The long-tailed bat probably arrived much more recently with its ancestors being blown over the Tasman only one million years ago. Short-tailed bats are particularly remarkable in that they have physically evolved for feeding by walking on the ground, in response to the lack of mammalian predators, as have several species of bird in New Zealand.

Once the evening sky may have been filled with these butterfly-like creatures. Long-tailed bats are still widely distributed from the northern North Island to the south of the South Island and on four offshore islands. Last century several bat roosts contained hundreds or thousands of bats. Few large roosts have been reported in the last 30 years.

Short-tailed bats are much more restricted, but fortunately two large populations occur on offshore islands, Little Barrier off Northland and Codfish Island near Stewart Island. Short-tailed bats have fared much worse on the mainland. They are probably extinct in the South Island, and in the North remnant populations occur in Northland kauri forests and across the Volcanic Plateau to North Taranaki.

A number of possible causes of the decline in bat populations have been suggested. These include logging, clearance of lowland forests, predation by rats, cats and mustelids (weasels, ferrets, etc), poisoning from cyanide poison baits and human interference and disturbance at nursery and roost sites. However, the impact of these factors on long term population stability and breeding success has never been assessed.

Bat Detection

Bats are a challenging group to work with. At this stage there is little known about where bats live in New Zealand and what threats face them. How do you begin to study a tiny flying animal that only comes out at night? For most of the time you can't see it or hear it, and during the day it is hidden in its roost in a hollow tree or cave. The challenges of studying bats have confounded many an enthusiast but recent advances in technology are helping us begin this work. The miniaturisation of electronics means that bat detectors and tiny radio-transmitters are now available.

The recent acquisition of hand-held ultrasonic bat detectors from overseas has meant that extensive, systematic bat survey work is now possible in New Zealand. Bats navigate by echolocation. They emit high frequency (ultrasonic) sounds, usually through their mouths, in rapid pulses at frequencies which are usually undetectable by humans. These sounds are reflected back to bats in flight as echoes, enabling them to discern the texture of the environment and the location of flying insects in the dark. Bat detectors convert these high frequency pulses to audible "clicks" through the detector's speaker.

Each bat species has a distinctive echolocation signature and often a specific frequency at which the signal strength is loudest. In countries where there are many species there can be considerable overlap in the frequencies of echolocation calls, but in New Zealand the two species have very different call structures and bat detectors provide a good, species-specific tool for monitoring bat activity.

A simple, low-cost, automatic bat detection system has recently been developed. The system involves putting a bat detector, a small voice activated tape recorder, a talking clock and an optional long-life battery into a waterproof container which is placed out in the field all night. When a long-tailed bat flies within 50 metres of the detector the "clicking" noise activates the recorder and the sounds are recorded on tape. The clock speaks the time once an hour and this too is recorded on tape. An index of bat activity can be recorded by listening to the tape during the following day.

For a long time now radio-transmitters have helped biologists track down rare and endangered animals. They have mostly been used with large birds such as kiwi and kakapo, but until recently weight restrictions have meant they couldn't be used effectively on tiny animals.

Long-tailed bats which weigh only nine or ten grams are now being tagged with the latest transmitters which weigh as little as 0.64 grams. The batteries will last up to a month, and in good conditions the signal can be picked up four kilometres away. The transmitters are glued to fur on the back of the bat, the glue only lasting as long as the battery, and then falling off.

Fiordland Study

During last summer, most nights were spent catching, tagging and following bats around the Eglington Valley in Fiordland. While this work is just beginning, already we have found some exciting information. Almost-invisible mist-nets are hoisted into the forest to catch bats for radio-tracking. These tiny bats often fly over 16 kilometres to their favourite feeding grounds and perhaps 30 or 40 kilometres in total each night. We can now track them back to their day-time roosts to find where they are living, then count how many bats fly out to feed each night.

Already we have found that they do not live in caves very much but prefer the abundant hollow trees in the forest. They move roosts almost on a daily basis. In Fiordland the number of bats in a roost varies each night. Often they sleep alone during the day but sometimes join roosts of 60 to 120 bats.

Bats in New Zealand don't seem to hibernate all through the winter. For example, long-tailed bats in Fiordland appear to be active throughout the year but activity is greatly decreased as temperatures get colder in the autumn. Some bats are still active in August and May at least to temperatures of -1.5^oC. They are feeding in much colder conditions than many species overseas. There was a strong correlation between the nightly abundance of insects, temperature and the number of bats recorded on counts. Bats flew in heavy rain, but activity was reduced.

Bats in Fiordland use a wide range of habitats, but peak activity was at the forest edge and above the forest canopy. Roads running through forest were also important, particularly during the colder hours of the night but the forest interior and grasslands were seldom used. Long-tailed bats are active throughout the night, but peak activity is within two hours of sunset.

Bat Action Plan

The Department of Conservation has an action plan for the recovery of our threatened bats. One of the primary aims over the next five years is to describe the distribution and population trends. DOC needs to identify where declines are still occurring and to monitor rates of population change before any effective management can be undertaken.

Developing the ability to manage healthy bat populations in mainland forests will assist in the conservation and management of other forest wildlife.

The main objectives of the plan will be to:

• undertake or promote research programmes on bats which will assist in their management
• evaluate the status of both short and long-tailed bats
• establish a population of the rarest subspecies, the Volcanic Plateau short-tailed bat, on a predator free island
• protect and monitor representative populations of both species of bat throughout their geographic range
• raise public awareness of bats and the threats they face.

Although New Zealand bats probably have a slow natural rate of increase they have a good potential for slow recovery if the factors that have caused their decline can be eliminated or reduced significantly.

Colin O'Donnell is in DOC's Science and Research Division.