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Energy efficiency is a major concern in the design of wireless sensor networks (WSNs) and their communication protocols. As the radio transceiver typically accounts for a major portion of a WSN node's power consumption, researchers have proposed energy-efficient medium access (E2-MAC) protocols that switch the radio transceiver off for a major part of the time. Such protocols typically trade off energy-efficiency versus classical quality of service parameters (e.g. throughput, latency, reliability). Today's E2-MAC protocols are able to deliver little amounts of data with a low energy footprint, but introduce severe restrictions with respect to throughput and latency. Regrettably, they yet fail to adapt to varying traffic loads and changing requirements of the imposed traffic load. This paper evaluates the energy-throughput and energy-latency tradeoff of today's most prominent E2-MAC protocols for WSNs, and motivates the need for more flexible and traffic-adaptive E2-MAC protocols. It proposes an intuitive definition for the ability of a protocol to adapt to varying traffic load at run-time, and introduces a tri-partite metric to measure and quantify this ability, further called traffic-adaptivity, taking into account the protocol energy-efficiency, throughput and latency. The paper concludes with a comparative analysis of the traffic-adaptivity of today's E2-MAC protocols.