Adaptations in the Nasal Passages for Sand Burrowing in the Saurian Genus Uma

Members of the iguanid genus Uma possess looped, U-shaped nasal passages which in structure and functioning prevent the inspiration of sand particles during subsurface respiration. The following factors are involved in the exclusion and elimination of sand particles: (1) The sand particles interfere with one another as they are drawn toward the external naris during inspiration. There is a tendency toward the formation of a grill of interlocking fragments which obstruct other loose grains. (2) When Uma lies buried the vestibule is situated at an angle to the horizontal. The sand particles must be drawn up the approach tube against the pull of gravity before reaching the principal nasal cavity. (3) Sand grains which adhere to the walls of the vestibule tend to impede the movement of free grains. (4) Under conditions of subsurface respiration the valve at the external naris reduces the nasal aperture to a narrow crescentic slit, limiting the inspiration of particles. (5) The postulated reduction in the diameter of the nasal tubes concomitant with the valvular action of the floor of the vestibule adjacent to the external naris, may tend to obstruct sand particles. (6) Gradual inspiration coupled with explosive expiration aids in avoiding the accumulation of sand in the nasal tubes. The configuration of the olfactory channels accentuates the explosive effect of expiration. (7) The small number of sand grains reaching the principal cavity are conveyed in a mucous film moved by ciliary action into the mouth, ultimately passing into the digestive tract. The genus Uma is not unique with respect to nasal structure. A number of other genera of the Iguanidae were found to possess a nasal configuration closely similar to that occurring in this genus. Callisaurus, Phrynosoma, Uta, Sceloporus are such genera. A tendency toward soil submergence occurs among these groups as based on field and laboratory observation. Other nonburrowing genera of the Iguanidae exhibited a nasal configuration quite different from the Uma-like types. The occurrence of the looped nasal structure within the Iguanidae doubtless was of considerable importance in the development of the almost completely psammophilous habit of the genus Uma.