Using GPS To Monitor Trekking Behavior In Crabs
November 15, 2012

Using GPS To Monitor Trekking Behavior In Crabs

Alan McStravick for - Your Universe Online

We all know that crabs walk sideways--it´s because that´s the way their legs bend. This side-walking trait allows them to move quickly into and out of small holes and crevasses to escape potential threats. So, we know how and why they walk. We just don´t necessarily know where they walk. That is, until now.

Scientists this week from the Zoological Institute at the University of Greifswald, in cooperation with their colleagues from the Max Planck Institute for Chemical Ecology in Jena, Germany, have embarked on the first large-scale study of any arthropod utilizing GPS technology to monitor their behavior. The GPS-based telemetric system is meant to analyze the movements of the freely roaming robber crabs during their land migration on Christmas Island. Their findings have been published in the current issue of the open journal PLoS ONE.

The Jena and Greifswald scientists focused their analysis on the coconut crab Birgus latro. B. latro is a terrestrial species of crab that is native to the tropical islands of the Indo-Pacific region. B. latro, also known as the giant robber crab, can grow to a weight of almost nine pounds. At this weight, it is the world´s largest land-living arthropod. These creatures can often live as long as 60 years. The scientists carried out their study on Christmas Island, because it has a largely undisturbed population of several hundreds of thousands of these coconut crabs.

The research, conducted over three years between 2008 and 2011, focused on 55 male robber crabs that were equipped with GPS tags. The tags were able to record over 1,500 crab days of activity. Some individual crabs were monitored for as long as 3 months at a time. The team knew their data would reflect several short-distance movements. What they also learned was that the robber crab ventured on much longer-distance trips from the coastline to the interior, rainforest-covered inland. They believe these longer sojourns are most likely related to mating, saltwater drinking and foraging.

What the GPS tracking pattern data showed was that path following was a primary navigation strategy for B. latro. Other experiments focusing on translocation indicated that these crabs are also able to home over large distances. When the robber crab is engaged in search behavior that was induced in these experiments, the team found that path integration was another important navigation strategy in the arthropods arsenal.

Produced in a limited-lot production, the custom-made GPS tags used in this study were made by e-obs GmbH (digital telemetry) in Munich. These specially designed tags were able to be programmed so that they would record GPS positions at intervals of one hour over a period of several months. Additionally, an accelerometer was integrated into the device that allowed the recording of crab movement in different directions. This allowed the researchers to monitor the animals general activities.

Once a week, data were downloaded by the researchers via a radio link within a range of about 600 feet. This allowed the crabs to act normally without being disturbed. Much like a previous study reported here on redOrbit regarding bird migration, the devices send out radio impulses that enable localization of the crabs via wireless connection using a high-sensitivity directional antenna. As development of these miniaturized electronic devices and batteries continues, this method will become the accepted means for study of even smaller animal species in the future.