If you have ever tried to take a ‘freeze-frame’ photograph your pet leaping in a trick move, a bird flicking past your feeder, or your kid at a sporting event, you have probably looked at more than a few blurry photographs. Getting a crisp photograph of a body in motion requires a faster shutter speed to provide a sharp image of what is happening at the instant the photo is snapped.
Using GPS tags to study animal movement can be compared to taking snapshots of behaviors we can’t observe with our own eys – scientists connect a series of locations taken by an animal’s GPS collar to create a picture of how the animal uses its environment. The schedule of the GPS unit sets the resolution of the picture we get about the animal movement. More frequent fixes (e.g. every few minutes) give a high resolution image of where the animal goes while less frequent fixes (e.g. every few hours) are analogous to a blurry photograph. But as anyone who has ever used a handheld GPS unit knows, they chew through batteries like nobody’s business if they collect fixes constantly. Unfortunately, wild animals won’t change their collar’s batteries when they run out, so scientists have to make the most out of one battery, and therefore face a dilemma in how to program the collars they will use on animals: infrequent GPS sampling will make the collar last longer, but give a fuzzy picture of movement paths; more frequent GPS sampling gives sharp paths but dramatically shortens battery life. This is an especially difficult problem when animals spend long stretches of down time in tree cavities, burrows, or thick vegetation where a GPS collar has little hope of successfully connecting with satellites and wastes even more battery power in futile attempts to record locations on schedule.
What is needed is a flexible GPS schedule tied to the behavior of the animal wearing the collar. A more active animal would trigger the GPS unit to record locations more often, while a resting animal would signal the GPS unit to record locations less often. What kind of sensor monitors the moment-by-moment behavior of wild free-living animals in any type of habitat? An accelerometer—a matchbook-sized wireless device that measures the change in speed of an animal’s body over time as it moves through its environment. Accelerometers are in everything from vehicle airbags to Nintendo Wii handsets and over the past few years they have exploded onto the scene in the world of animal movement research. They are an ideal sensor for linking animal behavior to the location recording schedule of a GPS collar because they are low-cost, can easily be incorporated into a standard GPS collar and they sample movement behavior every few seconds without using much battery power. So in theory a researcher can have the best of both worlds: a long-lasting GPS collar that gives sharply focused pictures of the paths animals take as they move around their habitats.
Testing the performance of just such a collar is the subject of a paper we recently published in Wildlife Society Bulletin (Brown et al 2012). We worked both with fisher in upstate New York and Tamandua anteaters on Barro Colorado Island in Panama, testing two types of GPS: one that recorded locations on a fixed schedule every 15 minutes and the other with a flexible schedule that recorded locations every 2, 15 or 60 minutes based on accelerometer-measured movement behavior. The accelerometer-informed collars performed considerably better than the traditional GPS collars: they attempted 74% more locations per day and had 62% higher location success rates, which means that on days animals were more active, GPS collars recorded more locations thus providing more detailed movement paths. At the same time they spent 28% less time searching for satellites and recorded 67% fewer locations when animals were at rest, reducing the overall amount of battery power used for each unique location and lengthening the lifespan of the collar. Ultimately the accelerometer-informed GPS collars produced more information about animal movement for a given battery size and study period when compared to traditional fixed-schedule collars. This technological development is a boon for researchers and potential study animals alike: ecologists get higher quality data with little additional cost per collar and can instrument fewer study animals for shorter periods of time than they would using traditional collars because the snapshots of daily movements are so much clearer. Currently only two companies (e-obs (used by our study) and Telemetry Solutions) produce accelerometer-informed GPS collars, but as word gets around, those scientists studying animal movement ecology are sure to appreciate the value of this novel tool.
Reference: Brown, D.D., LaPoint, S., Kays, R., Heidrich, W., Kuemmeth, F. and M. Wikelski. 2012. “Accelerometer-Informed GPS Telemetry: Reducing the Trade-Off Between Resolution and Longevity” Wildlife Society Bulletin 36(1):139-146.
Written by Danielle Brown