Feature

Wired Wildlife

Tiny radio transmitters are proving useful in learning more about the behaviour of a large range of animals.

By Cathryn Crane, NZSM

Creatures as diverse as wetas, water buffalo, stoats, and crocodiles are being wired for sound in studies which combine electronics and ecology to learn more about how the animals behave in their native environments.

Tiny radio transmitters, produced by a commercial offshoot of DSIR Land Resources, are used to track the animals, note their movements and home ranges, and monitor how they adapt to new environments.

"In 1985, we became frustrated with the poor quality of the radio transmitters that we were buying for our own research, and we resolved to manufacture our own," recalls Dave Ward. Demand for efficient, reliable equipment led to the establishment of Sirtrack Electronics to take advantage of a burgeoning market and to bring in some money.

Since then, over 100 species have been tracked and monitored using Sirtrack equipment. Equipment is custom built for each species, taking into account the animal itself and the environment in which it lives. In some cases, the researchers hadn't even heard of the animals they were asked to design equipment for.

"We do at times run to the books to find out what it is," laughs Ward. Quolls and mala had the team stumped. The spotted quoll and its southern and northern counterparts are Australian marsupials related to the Tasmanian devil, but more aboreal, they found out. Mala are a type of small wallaby, about the size of a rabbit.

Radio transmitters are glued, tagged and strapped to animals. One of the most awkward animals to fit, according to Ward, was the cassowary, a large flightless bird something like an emu. It has no wings for transmitter attachment, and the bird's habit of swallowing large things meant that any restriction around the neck was ruled out. The team finally managed to attach a transmitter to the primary feathers on the wing stubs, and that has appeared to work successfully.

Sirtrack also produces a range of temperature-monitoring equipment, and these are designed to be swallowed by the animal for tracking. Researchers in Australia are following a pair of crocodiles with the monitors, and a number of cane toads are hopping around with the temperature monitors inside them, as well as echnidas and Tasmanian devils. Ward hopes that successful results will help boost sales to Australian wildlife managers.

Stoat Studies

It hasn't all been plain sailing for the equipment's users. Elaine Murphy, of the Science and Research Division of the Department of Conservation, recalls some frustrations. She has been studying the movement of stoats in Eglinton Valley, in Fiordland.

Murphy and her research partner, John Dowding, captured stoats in the area, anaesthetised them and soldered the radio-transmitter collars into place. In the initial study, it took the stoats only a short time to chew through the resin used to protect the electronics.

"We can make something for a crocodile and it stays on, but when it comes to stoats they're in a class of their own," remarks Ward.

To try and counter the chewing, Sirtrack coated the next run of transmitters in dental acrylic, reasoning that this would be more difficult for the stoats to chew. It was, but it has tended to wear away -- a low-slung animal like the stoat puts a great deal of wear and tear on a radio-transmitter, as it tends to catch on undergrowth as the stoat slinks along.

"Fiordland was particularly bad for this because it's gravelly," observes Murphy. The gravel in the long, wide river valleys where the stoats roamed was the major cause of the wear. Murphy is hoping that a forthcoming North Island study will be less hard on the equipment.

Female stoats presented the most problems, as their light weight of around 200 grams required the lightest possible equipment configuration. Even the weight of the battery became a significant limiting factor. Initially, the females were fitted with a single-stage transmitter, weighing just under six grams, and transmitting for 53 days -- if they lasted that long. Later in the study, they got the same two-stage transmitters used on the males, with a lifetime of 3.6 months. The lifetime of the equipment was something of a moot point -- the longest any transmitter remained working was nine days.

The single-stage transmitters were impractical in the beech forest areas, as the range for that equipment was to a maximum of 150 metres. The two-stage devices had a range of 150-300 meters, making it much easier to find and track the stoats. It was a significant difference, as stoats spend about 80% of their time in the forest.

The forested ranges did not allow accurate fixes by triangulation because of interference from trees, so it was necessary for Murphy to get within 20 metres of the moving animal.

Despite the problems, Murphy managed to fit 16 stoats with radio-transmitters in her first study, and followed them through the Fiordland valleys, checking their positions and activities.

Some of the results were surprising. Female stoats, once thought to be relatively sedentary animals, had a much bigger range than expected and moved about a great deal.

"One juvenile female circled her entire home range of 56 hectares in 2.5 hours," notes Murphy. Another stoat was found 65 kilometres away from the area in which she had been initially tagged, having traversed a mountain range in between. Murphy believes that stoats may undertake "excursions". She observed one male which had a home range of 29 hectares; he went on a five kilometre excursion in one day, including crossing the Eglinton River, and returned home two days later.

Such data has significance in evaluating the effects of stoat population and behaviour on other forest populations, particularly the native bird species. Birds and mice are the most common prey for stoats. The study has important implications for control programmes, according to Murphy.

"Juvenile females are almost always pregnant before they disperse," she notes. "Even clearing stoats from large buffer zones around protected areas may be ineffective, as immigration can occur from very great distances."

Wired Wetas

Dr Mary McIntyre, of the School of Biological Sciences, has had an easier time with her subjects. She's just come back from two weeks of using radio equipment to track giant ground wetas on Mercury Island.

"It's a rather weird weta. It's not related to the other true giant wetas," says McIntyre. Not much has been known about the Mercury Island weta, and the radio study has given McIntyre a chance to study the wetas and their behaviour without unduly disturbing them.

"Monitoring's going to be the key to figuring out what's happening," she says. McIntyre and the Department of Conservation are hoping to learn enough about the wetas to develop appropriate conservation strategies for them.

Sirtrack developed a tiny transmitter weighing less than two grams for the study, and redesigned the aerial to make it shorter and sturdier. The transmitters are attached to the wetas with adhesive. The actual transmitter weighed only 0.8 grams, with the rest of the 1.4 grams total weight coming from the battery and protective covering. The battery was made as small as possible, enabling only two weeks of monitoring.

"These guys are really big," notes McIntyre. "When you see them on the forest floor standing there, you're really bowled over. They're invertebrate mice."

Unlike most other weta species, it is the male which is the largest, weighing in at around 30 grams. This make it almost ten times the size of its nearest relatives.

The radio study has produced some surprising results. McIntyre has found that wetas like to dig tunnels near the entrance of petrel burrows, taking advantage of the soft soil there. The telemetry has also shown that the male ground wetas head up into the trees at night.

"We've never seen a [giant ground] weta in a tree and no-one's seen them climbing," says McIntyre. "But the radio signals show that they're whizzing up there.

Cathryn Crane is a freelance journalist with an interest in environmental issues.