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Features of network protocols, originally designed for good reasons when energy considerations were not paramount and multimedia communications were not common, are sometimes discovered to have high energy costs in new application domains involving multimedia. In this paper, we examine the energy cost of an old, simple and pervasive feature in wireless medium access control protocols: the mandatory acknowledgment at the link layer in response to a data frame, imposing the requirement that if node A can reach node B with its transmissions, then B should be able to reach A as well. We show that this mandated bidirectionality has a significant energy cost in ad hoc networks, multiplying the energy cost of a transmission at a node by as much as three. We also show that this negative impact increases further with increasing non-uniformity of the radio environment. Many multimedia applications are characterized by two features: a larger volume of traffic in one direction than in the reverse direction and by less of a need for immediate acknowledgement in the reverse direction. Mandated bidirectionality, therefore, carries an especially higher and unnecessary energy cost for multimedia communications. These findings suggest a new set of research problems on how best to design new features in protocols for medium access control for multimedia communications.