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2008 |
BatsThe endangered lesser short-tailed bat has been found in considerable numbers on the slopes of Ruapehu. Brian Lloyd and Shirley McQueen studied the bats behaviour using radios, mist-nets and video cameras. Just after dusk, thousands of lesser short-tailed bats leave their hideaway in the remnant Rangataua Forest on Ruapehu in search of food. The tiny animals, endangered and known till recently from only three places in New Zealand, have a population here in excess of 7000. Up to 5500 may roost by day in the hollow bole of a single tree. The remaining animals may occupy three or more other trees, probably because the colony is too large to fit into one. The bats emerge to feed, after nightfall, in an impressive display of high speed flight. Quite randomly, often in clusters, the bats peel off in all directions, to hunt. Once they were the main prey of the now-extinct laughing owl. The speed of their nightly exodus still helps them avoid the moreporks which also prey on them. The short-tailed bats of Rangataua Forest are a relatively recent rediscovery: the initial clue was a dead bat picked up by a DoC worker in 1993; then a cat caught one in a raspberry patch at Rangataua settlement, at the edge of the forest. Brian Lloyd and Shirley McQueen have been studying the bats in Rangataua Forest since 1995. With video cameras and radio tracking equipment they have followed the shifting colonies from hollow tree to hollow tree and studied their life cycle. One spectacular roost in an old red beech has been nick-named Notre Dame. The entrance is a split in the trunk some four metres high. Several thousand bats can shelter here. The nick-name refers both to the Gothic arch-like entrance and to the importance of this roost to the population. At the base of the tree there is a cascade of bat guano some two metres high. When the bats are in residence they add 10kg of guano to the pile every day. Using low-light video cameras, it is possible to spy on the darkened colony. Infra-red light is used to illuminate the site. Unlike the long-tailed bat which hunts in the dusk, short-tailed bats don’t go hunting till after dark. It can take 40 minutes for them all to leave the roost. Playing back the video at slow speed facilitates a count of how many thousand bats are using a roost. Through Rangataua there are some 30 large roosting trees used by the shifting and changing colony of bats. All are in red beeches, hollowed through time. During summer the animals move roosts, usually every few weeks, though sometimes every few days. The bats’ progress from tree to tree follows a seasonal pattern through the forest. The colonial roosts are ‘traditional’; some have probably been used for several hundred years. The roosts are probably begun as solitary roosts in young trees. Pin-hole borer and consequent fungal attack softens the wood into which the bats burrow by actually eating wood. A huge chimney develops within the main trunk as the tree grows larger. Short-tailed bats are belligerent creatures. Their colonies are quite noisy places. The bats squabble for better positions in the seething mass of tightly packed animals. Yet as colonial animals they are remarkably cooperative with each other. Female bats from all the Rangatau colonies share just one ‘maternity’ tree. At the altitudinal limit of red beech is a hollow tree where all the young are sheltered. For the past two years, several thousand females have brought their young to this creche in a tree. At birth a young bat weighs about a third of the weight of its mother and is carried hanging from a nipple by the animal in flight. There is only one young, born around Christmas. While there is up to a year between mating and birth, implantation of the egg is delayed till six weeks before birth. The young is suckled once or twice a night in the maternity roost after the mother’s hunting flights. Some baby bats fail to thrive, dropping out of the maternity tree, to die. After living in the creche for about five weeks it learns to hunt. If a bat survives its first year then it will have a life expectancy of 20-30 years. It is relatively easy to survey for bats using a ‘bat detector’ which picks up the bats’ echo-guidance signals. While these mostly can’t be heard by the human ear (they are emitted as ultrasounds of 20-200kHz) the signals are loud and frequent — easily heard by the bat detector. Bats navigate by bouncing their ultrasound calls off nearby surfaces, thus picturing their environment. Such a bat’s-ear view of the world is remarkably detailed. They can read everything within a distance of 20-50m, seeing items as fine as 2mm from several metres away. The signals bounce variously off different surfaces, suggesting textures, identifying prey by species, judging speed. Tape recorders can be set up so they are activated whenever these ultrasonic noises are detected. Tiny radio transmitters can also be attached to bats which are captured in mist nets set high in the trees. By this method, bats can be tracked as they move through the darkened forest. The creatures are very manoeuvrable, capable of hawking insects on the wing. Fine fingers carry the webs of the ‘wings’ which begin at the foreleg and are anchored back to the ankles of the back legs. The tough elastic membranes adjust shape and camber, so that bats can ‘outperform’ birds. Though birds have higher top speeds, bats are capable of carrying heavier loads (their young) and can fly more slowly without stalling. Although some food is taken on the wing, most is obtained by gleaning and hunting on the ground and among epiphytes in the trees. The bats shuffle through the leaf litter like mice, in search of insects. Generally insectivorous, they also drink nectar and can even play a part in the fertilisation of the rare wood rose, Dactylanthus taylori, carrying pollen while moving from plant to plant. (See Forest & Bird Journal, February 1993). Bats need plenty of food to stay active. When supplies fall off they drop into a torpor, even hibernate. Daily torpor, which may take place at any time, reduces their energy consumption by 15 per cent. In winter when it snows on the mountain they may hibernate from six to 10 days. Then their metabolism slows to the point when at four degrees Celsius they may be using less than one per cent of their normal resting energy. Yet even when it snows, short-tailed bats may occasionally emerge to hunt. In winter up to a metre of snow can fall in the red beech forest and then the bats congregate in a few hundred hectares of warmer lowland forest around Lake Rotokura. Once the scene of a conservation campaign against logging, protection of this area has probably been crucial to the survival of the Rangataua bats. Indeed, logging rather than introduced predators seems to be their worst enemy.
The short-tailed bat separated from other species of bats between 50 and 60 million years ago, early in the evolutionary history of bats. Its nearest relatives are the bats of South America, including the vampire. The connexion may be through the ancient continent of Gondwanaland, now broken up into the continents of South America and Australia, and islands such as New Zealand. Southern beech Nothofagus species are common to these places and the bats of Rangataua show a preference for red beech when making their roosts. There are more than 900 bat species world wide, comprising a quarter of all mammals. Bats are the second biggest group of mammals, dividing into 17 families. Ninety percent are tropical and subtropical species. The various species vary in size from 2.5 gram ‘bumble-bee’ bats to 1.5kg megabats, like the ‘flying fox’. Australia has 85 species of bats. In some forest areas of northern New South Wales and southern Queensland as many as 35 species can occur together. The lesser short-tailed bat is one of three species of bat which comprise New Zealand’s only land mammals. The long-tailed bat is more frequently found, though still rare, and the third species, the greater short-tailed bat, is believed extinct. For survival, the short-tailed bat needs large areas of mature forests; at least 5000 hectares growing on fertile soils. The availability of large, old trees, suitable for burrowing, seems to be the key factor determining distribution. Rangataua extends for 10,000 hectares and contains many red beech trees up to 40 metres high and more than two-metres in diameter— trees which are probably more than 400 years old. Omahuta in Northland, where short-tailed bats roost in hollow kauri, is also part of a large forest. The other habitats include two islands, Little Barrier and Codfish/Whenua Hou. In recent years, often by using ‘bat detectors’, short-tailed bats have also been found in areas of Pureora, Whirinaki and Kaimanawa forests, in Warawara (Northland), Waitotara (south Taranaki), Waitaanga (north Taranaki), Waimarino, Fiordland and north-west Northland. The primary threat to these bats has been, and remains, forest destruction. Selective logging is likely to be as devastating for them as forest clearance. The longer-term threat to their survival is the degradation of the forests, by pests, and the subsequent failure to produce mature old-growth trees. Brian Lloyd is a scientist with the Science, Technology and Information Services of the Department of Conservation. He is based at Ohakune. Shirley McQueen is now conservancy advisory scientist for the Department of Conservation in Otago.
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