The crocodylian phallic glans is the distal inflatable structure that makes the most direct contact with the female cloacal and associated reproductive tract openings during copulation. Therefore, its form and function directly impact female tissue sensory interactions and insemination mechanics. Compared to mammals, less is known about glans functional anatomy among other amniotes, including crocodylians. Therefore, we paired an ex vivo inflation technique with magnetic resonance imaging 3D-reconstructions and corresponding histological analyses to better characterize the morphological glans restructuring occurring in the Nile crocodile (Crocodylus niloticus) at copulation. The expansion of contiguous inflatable spongiform glans tissues is variably constrained by adjacent regions of dense irregular collagen-rich tissues. Therefore, expansion shows regional differences with greater lateral inflation than dorsal and ventral. Furthermore, this enlargement elaborates the cup-like glans lumen, dorsally reorients the glans ridge, stiffens the blunt and bifid glans tip, and putatively works to seal the ventral sulcus spermaticus semen conduit groove. We suggest how these dynamic male structures may interact with structures of the female cloacal urodeum and how these morphological changes, in concert with the varying material properties of the structural tissue compartments visualized in this study, aid copulatory gamete transfer and resulting fecundity.
RESEARCH HIGHLIGHTS :
Nile crocodile glans inflation produces a reproductively relevant copulatory structure directing insemination and female tissue interactions.
Pairing magnetic resonance imaging 3D reconstruction with corresponding histology effectively studies functional anatomy.
DATA AVAILABILITY STATEMENT : The MRI data that support the findings of this study are openly available in Morphosource at https://www.morphosource.org/reference number S30335.
SUPPORTING INFORMATION : Figure S1. Correspondence between proximal glans tissue compartment histological staining and MRI. (a) Milligan's trichrome section: dense collagen tissue (DC), spongiform tissue (Sp), elastin-rich tissues (E), sulcus spermaticus (SS) note the groove opening is a histological processing artifact, smooth muscle fiber bundles (SM). Scale bar = 2 mm. (b) MRI dotted line traces: epithelia (pink), dense collage tissues (blue), spongiform tissues (purple), elastin-rich tissues (green), sulcus spermaticus (red), and smooth muscle fiber bundles (yellow).