In recent years, mobile data gathering in wireless sensor networks has attracted much interests in the research community. However, despite extensive efforts, many of previous work in this area lies only in theory and evaluates network performance with computer simulations, which leaves a large gap from reality. In this paper, we present the design and implementation of a general purpose, flexible platform for mobile data gathering in wireless sensor networks to evaluate network performance and algorithms in a practical setting. Instead of relying on hand-crafted theoretical models, our platform integrates both mobile data collector and sensor nodes to provide realistic performance evaluations. In addition, the platform adopts a modular design in mobile data collector and sensor nodes, and equips the mobile data collector with advanced computing capability, which makes it versatile for evaluating the performance of a wide-range of applications. Finally, as a case study, we implement a wildlife monitoring system on our platform. Our experimental results demonstrate that real implementations can evaluate many practical performance factors which would have a great impact on the sensing results and are very difficult to fully capture by theoretical models and simulations. We expect that this platform can become a very powerful general tool for more accurate network simulations and facilitate performance optimization in wireless sensor networks.