Glans inflation morphology and female cloaca copulatory interactions of the male American alligator phallus†.


Moore BC(1)(2), Brennan PLR(3), Francis R(2), Penland S(2), Shiavone K(2), Wayne K(2), Woodward AR(4), Does MD(5), Kim DK(5), Kelly DA(6).
Author information:
(1)College of Veterinary Medicine, Department of Biomedical Science, University of Missouri, Columbia, MO, USA.
(2)Biology Department, Sewanee: The University of the South, Sewanee, TN, USA.
(3)Department of Biological Sciences, Mount Holyoke College, South Hadley, MA, USA.
(4)Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Gainesville, FL, USA.
(5)Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, USA.
(6)Department of Psychological and Brain Sciences, University of Massachusetts, Amherst MA, USA.


The phallic glans of the American alligator (Alligator mississippiensis) is the distal termination of the semen-conducting sulcus spermaticus and during copulation has the closest, most intimate mechanical interactions with the female urodeum, the middle cloacal chamber that contains the opening to the vaginal passages and oviducts. However, the details of this interface leading to insemination and gamete uptake are unclear. Here, we: (1) histologically characterize the underlying tissue types and morphologically quantify the shape changes associated with glans inflation into the copulatory conformation, (2) digitally reconstruct from MRI the 3D shape of functional tissue compartments, and (3) diffusible iodine-based contrast-enhanced computed tomography image the copulatory fit between male phallus and female cloaca. We discuss these results in relation to tissue type material properties, the transfer on intromittent forces, establishing potential copulatory lock, inflated glans volume scaling with body mass/length, the mechanics of semen targeting and insemination, and potential female cryptic choice impacting multiple clutch paternity. In part, this study further clarifies the phallic morphological variation observed among crocodylians and begins to investigate the role(s) these divergent male forms play during copulation interacting with female cloacal forms to increase reproductive success.