Wireless sensor networks are getting more and more attention from researchers and industrial communities. They consist of distributed event-based sensors and a base station for data processing. These sensor networks have severe energy constraints. Data aggregation mechanisms are used to decrease the power consumption in these networks by combining several messages. In this paper, we present an energy efficient data aggregation mechanism. In this mechanism, the node which is aggregating and transmitting the message to the base station is chosen based on its energy and proximity to the base station. The nodes decide to participate in the aggregation process based on a participation relevance function. The efficiency of this aggregation mechanism is shown through simulation results in terms of network lifetime and reception rate.