The wolf's comeback in Europe has preoccupied people all across the continent over the last years. Where is it? What is its range? What does it live on? The only way to get solid answers to these questions is through GPS tracking. In December, it was a cause for celebration when a GPS collar was fitted onto a wolf for the first time in Denmark. Only three months later, the signal stopped.

GPS trackers that stop working or run out of power prematurely are a frequent problem and source of frustration among researchers who want to track mammals for longer periods, says biologist and postdoc Rasmus W. Havmøller of the University of Copenhagen. Typically, batteries are the problem:

"When studying wildlife with GPS technology, the biggest limitation is always going to be the battery. It's enormously frustrating. It is not uncommon that one gets to track an animal for a few months at most before the GPS device goes dead. But tracking an animal for a longer period of time is often important, as in the case with wolves here in Denmark. Therefore, we need a more reliable power source," says Rasmus W. Havmøller and continues:

"Solar cells work fine for birds, but solar cells are so fragile that mammals tend to crush them. Moreover, many mammals are nocturnal. So we needed to come up with an alternative. I had long thought about the cleverness of the automatic wristwatches that many of us wear, which harvest energy from our own body's movements."