Morphological changes associated with Nile crocodile (Crocodylus niloticus) phallic glans inflation

Abstract

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 3Dreconstructions 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.

Genital anatomy and copulatory interactions in the broad snouted Caiman (Caiman latirostris)

“…Male and female cloacal anatomy is similar, and both sexes have an unpaired genital structure on the ventral wall of the cloacal proctodeum (Grigg & Kirshner, 2015). The male phallus and female clitoris develop from the same antecedent tissues and share many morphological features but are size dimorphic; male phalli continue to enlarge through the juvenile years (Ciocan, Leiva, & Simoncini, 2020; Otaño et al, 2010), especially after puberty (Moore et al, 2020; Moore, Fitri, & Augustine, 2020; Tavalieri et al, 2019).…”

“…Phallus anatomy has been characterized in detail for the saltwater crocodile ( Crocodylus porosus ) (Johnston et al, 2014), Nile crocodile ( Crocodylus niloticus ) (Moore, Francis, et al, 2020), American alligator ( Alligator mississippiensis ) (Kelly, 2013; Moore, Spears, Mascari, & Kelly, 2016) and false gharial ( Tomistoma schlegelii ) (Moore, Fitri, & Augustine, 2020), spanning species from all three families of modern Crocodylia (Oaks, 2011; Pan et al, 2020). In all of these taxa, the phallus can be divided into three regions: a pair of proximal crura, a cylindrical shaft, and the distal glans.…”

“…The shaft is anchored to a thick tendon originating on the ischium that acts as a fixed point around which the male phallus rotates when it is everted for copulation via cloacal muscle contraction (Kelly, 2013). Both the sulcus spermaticus and the distal end of the phallus contain spongiform erectile tissue that inflates with blood to close the sulcal groove (Moore & Kelly, 2015) and produce species‐specific glans morphology (Moore et al, 2016; Moore et al 2020,b; Moore et al, 2021). The shaft contains a core of dense, collagenous tissue that does not change shape during vascular inflation (Kelly, 2013; Moore et al, 2021); this tissue maintains a high resistance to bending that is likely to facilitate intromission through the female vent (Kelly, 2016).…”