Abstract only In 1940 Scholander proposed that marine mammals have stiffened upper airways that would stay open and receive air from highly compressible alveoli during diving. However, this idealized alveoli–up system would tend to underestimate the importance of tracheal collapse or the role of the trachea at pressure. There are little data available on the structural and functional adaptations of the marine mammal respiratory system. Although biomechanical and pressure‐associated changes have been measured, the aim of this research is to investigate the microscopic tracheal characteristics of five different species of diving mammals, specifically focusing on elastic fibers and distances of cartilage overlap. Our histological measurements have revealed the presence of “slip features” in both pinniped and cetacean tracheas. These “slip features” are characterized by partial or incomplete cartilaginous rings, which, at pressure, will overlap maintaining constricted airways. This finding lends evidence for pressure‐induced collapse and re‐inflation of the trachea. In some cases, more than one “slip feature” is present per tracheal ring, perhaps allowing for more rapid and effective compression. There is great variation in the anatomy of the “slip feature” between cetacean and pinniped, as some truly overlay one another and others are separated by elastic fibers, connective tissue and smooth muscle. There also seems to be a significant difference between the quantity of elastic fiber in pinniped species versus cetacean species, for example cetaceans tended to have more elastic fibers, perhaps correlating to deeper dive depths or higher ventilatory rates.