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Recent study reveals that great benefit can be achieved for data gathering in wireless sensor networks by employing mobile collectors that gather the data via short-range communications. To pursue maximum energy saving at sensor nodes, intuitively, a mobile collector should traverse the transmission range of each sensor in the field such that the transmission of each packet can be constrained to a single hop. However, this approach may lead to significantly increased data collection latency due to the low moving velocity of the mobile collector. On the other hand, data collection latency can be effectively shortened by performing local aggregation via multi-hop transmissions and then uploading the packets from relay sensors to the mobile collector. However, local transmission hops should not be arbitrarily increased since it may incur too much energy consumption on packet relays, which would adversely affect the overall efficiency of mobile data collection. Based on these observations, in this paper, we study the tradeoff between energy saving and data collection latency in mobile data gathering by exploring a balance between the relay hop count of local data aggregation and the moving tour length of the mobile collector. We first propose a polling-based mobile collection approach and formulate it into an optimization problem, named bounded relay hop mobile data collection (BRH-MDC). Specifically, a subset of sensors will be selected as polling points that buffer locally aggregated data and upload the data to the mobile collector when it arrives. In the meanwhile, when sensors are affiliated with these polling points, it is guaranteed that any packet relay is bounded within a given number of hops. We then give two efficient algorithms to select polling points among sensors. The effectiveness of our approach is validated through extensive simulations.