Recent technological advances like satellite-based global positioning systems allow ecologists and population biologists to track their study animals around the globe, from polar bears prowling across Arctic tundra to lions basking on the Serengeti plains. But there's one drawback when you're using satellite tracking - the devices are so big that only large animals can carry them. We simply cannot track most of the world's smaller species at the moment, and many of these animals are of great scientific interest,conservation concern or global economic importance. So there's a strong incentive to design transmitters that even a hummingbird or an insect can tote. Enter Beat Naef-Daenzer and his team at the Swiss Ornithological Institute, who have created a transmitter so small that including the battery it only weighs 0.2 grams (p. 4063).
Realising that none of the radio transmitters currently on the market were suitable to study dainty barn swallows, Naef-Daenzer teamed up with Daniel Früh, Martin Stalder and Patrick Wetli to design small custom-made transmitters. Their first challenge was to find a design that draws a maximal output from a minimal number of electronic parts. Then Edgar Weise, a physicist specializing in the miniaturization of electronics, assembled the smallest components available into the most compact package, while avoiding interferences between components.
To test the miniature transmitter in the lab, the team attached it to a dummy animal. They filled a finger of a latex glove with physiological sodium chloride solution and attached little stick legs so they could mount the transmitter with the same small leg-loop harnesses that are used for real birds. They tested two common attachment techniques - attaching the harness to the transmitter's battery, and attaching the harness to the transmitter's electronic circuit. To their surprise, they found that attaching the harness to the battery greatly improved the radiated power of the transmitter, which means that the animal can be detected at a larger range. They suspect that this is because the transmitter becomes coupled to the animal's electrically conductive body, which acts as a `ground plane' that improves the transmitter's radiated power. `But this effect is not seen with conventional,larger transmitters', Naef-Daenzer says. Clearly, with such miniaturized designs, the details of the attachment technique become more important than with larger transmitters.
With the smallest power cell, the miniature tag weighs in at 0.2 g and can transmit for three weeks. In free air it has an operational range of roughly 2 km, but the team has yet to test the tag's range when it's attached to a free-living animal. It's so light that researchers can now tag a 4 g mammal or bird or a 2 g insect, providing huge scope for future work on diminutive species.
Although the team initially designed the transmitters with small birds in mind, requests for the miniature tags are already coming in from enthusiastic colleagues working on butterflies, tree frogs and bats. Naef-Daenzer is now carrying out the first field trials of the tiny transmitters, tagging 2 g owl butterflies in a Swiss butterfly zoo to see if the tags affect the insects'behaviour.