Abstract:
The male crocodylian phallus, an intromittent organ, transfers sperm to the female cloaca during reproduction. During
copulation, the distal phallic glans inflates via blood-filled spongiform tissues; it enlarges into an elaborate shape that directly interacts with
the female urodeum—the cloacal chamber that contains the female reproductive tract openings. Alas, the specific mechanics of crocodylian
insemination and gamete transfer remain unclear. To that end, we investigated the gross and cellular morphology of the Nile crocodile
(Crocodylus niloticus) glans characterizing tissues types and structural morphologies to better predict how these male tissues may interact
with those of the female. We tracked blood flow from the descending aorta to the phallic glans by way of sulcus spermaticus-adjacent blood
vessels. Utilizing an artificial inflation technique, we documented how the glans tissue shape changes with increased hydrostatic pressure in
spongiform tissues including increases in height and width and the enlargement of a cup-like distal lumen. Sectioning the glans, we traced
the decrease in dense collective tissues and the proliferation of inflatable tissues moving from proximal to distal. Concomitant with the
development of the inflatable glans, we identified elastin-rich tissues around the inflatable glans regions and the deep sulcus spermaticus
semen conduit. Together, these observations demonstrated the dynamic nature of the tissues, where collagen fibers supply mechanical
strength and elastin fibers provide resilience and recoil. We hypothesize how these glans characteristics may interact with female tissues
during copulation to increase the chance of successful gamete transfer.
