Morphological characteristics regulating phallic glans engorgement in the American alligator.

Abstract

The distal part of the crocodilian phallus consists of a bulbous glans containing well-developed vascular tissues that can inflate before or during sexual activity, enlarging and elaborating the glans into a complex, though still functionally undefined, copulatory structure. An enlarged glans putatively interacts with the female cloaca and may change the shape of her reproductive tract to facilitate insemination and increase the probability of fertilization. Here, we investigated the cellular-level properties of the glans and other inflatable phallic tissues associated with the sperm-conducting sulcus spermaticus in the American alligator (Alligator mississippiensis). Using histochemical staining, we visualized and defined collagen and elastin fiber densities and orientations in these tissues. Extracellular matrix architectures provided insights about phallic glans material properties and how they may affect tissue strength and flexibility during inflation and in response to copulatory forces. We also investigated the potential sources of fluids that induce inflation in alligator phalli. Combining serial sectioning and three-dimensional reconstruction, we identified a pair of supracrucal plexus vascular bodies at the proximal end of the alligator phallus that extend distally adjacent to ventro-medial sulcus tissues. Together, our gross and histological examination of the American alligator phallic glans suggests that its tissues are arranged in a manner that would allow vascular inflation to expand the glans to a specific and repeatable shape, and potentially release secretory products into the female reproductive tract. Both elements could play roles in postcopulatory sexual selection, by mechanically and/or chemically affecting female reproductive physiology.