MYERS, R.P.*; MEYERS, R.A.; Weber State University: Taking the plunge: Pouch morphology and function in Brown Pelicans

Brown Pelicans (Pelecanus occidentalis) plunge-dive for food, and can attain speeds of 18 m/s before impact with the water. After immersion, their mandibles bow from a resting position of about 5 cm apart to over 15 cm apart and their pouches distend with water (and fish) to hold a volume of about 11 liters. In order to determine how this is accomplished we used both gross dissection and histological techniques. We found that the pouch is composed of several layers: an oral epithelium, a muscular layer, a layer of elastin bundles, and skin. The oral epithelium and the skin both have longitudinal folds that provide extra material for pouch expansion. The fibers of the M. mylohyoideus and the elastin layer are arranged perpendicularly to each other, providing the necessary support and elastic recoil for the pouch to function. Bending occurs along the rostral 2/3 of the mandible, which contains a flexible zone near the symphysis and is made up of several bones joined by connective tissue. We believe that these syndesmotic joints act as leaf springs and combine with the flexible zones to allow the otherwise rigid mandible to bow. The pterygoid muscles function in mandibular bowing by rotating the mandible outwards on its articulation with the quadrate. Due to the attachment at the symphysis and the presence of the flexion zones, the mandible bows. It has been suggested that pouch filling, and possibly even mandibular bowing, could be accomplished passively. We propose that when a Brown Pelican dives into the water it sets up a case of Bernoulli’s Principle in which the water outside of the pelican moves faster than that within the pelican’s mouth. Therefore, the pressure within the mouth would exceed that on the outside. This pressure difference could account for passive pouch filling.