Many mammals including dogs, cats, deer, and dolphins, but not humans, have a reflective membrane at the back of the eye. This tapetum lucidum affords the light-sensitive retinal cells a second chance for photon-photoreceptor stimulation, enhancing low light visual sensitivity. Dissection of preserved specimens and computed tomography (CT) scans of live dolphins have shown prominent air cavities around the ear, especially medial and posterior. Histologic evaluation of peri-otic tissue revealed epithelial lined air sinuses and erectile tissue containing nerves, ganglia, smooth muscle bands, vascular spaces, and periarterial venous networks. Two dolphins were trained to periodically recognize and respond to a 1.5-s, 70-kHz target tone. In three experiments, 20 target tones were presented through a jaw phone after injection of 18 Fluro-deoxy-d-glucose (FDG). Subsequent positron emission tomography (PET) scans revealed that the tone, to which the dolphin had responded and been rewarded numerous times in the past, elicited a marked increase in metabolism in the erectile tissue medial to the stimulated ear but not the contralateral ear. Tissue around reflective air sinuses medial to the ear become active with acoustic stimulation, suggesting that sinus air may be shaped so as to provide an ‘‘acoustic tapetum’’ for dolphins.