Intromittent Organ Morphology and Biomechanics: Defining the Physical Challenges of Copulation

Abstract

Intromittent organs-structures that place gametes into a mate for internal fertilization-evolved many times within the animal kingdom, and are remarkable for their extravagant morphological diversity. Some taxa build intromittent organs from tissues with reproductive system antecedents, but others copulate with modified fins, tentacles, or legs: anatomically, these structures can include combinations of stiff tissues, extensible tissues, and muscle. Their mechanical behavior during copulation is also diverse: males in some taxa reorient or protrude genital tissues, others inflate them and change their shape, while still other taxa combine these strategies. For these animals, the ability to ready an intromittent organ for copulation and physically interact with a mate's genital tissues is critical to reproductive success, and may be tied to aspects of postcopulatory selection such as sperm competition and sexual conflict. But we know little about their mechanical behavior during copulation. This review surveys mechanical strategies that animals may use for intromittent organ function during intromission and copulation, and discusses how they may perform when their tissues experience stresses in tension, compression, bending, torsion, or shear.

Testing hypotheses for the function of the carnivoran baculum using finite-element analysis

“…Likewise, the baculum is modelled as experiencing axial compression during initial intromission. Yet this overlooks the complex interactions of soft tissues possibly experiencing shear [ 21 ]. This will particularly be the case in taxa characterized by the presence of keratinous spines (for example, felids and the fossa) on the surface of the glans, although the extent of shear will vary with the direction of spines.…”

“…By taking this novel ‘whole bone’ approach to hypothesis testing, this study is an important contribution to our understanding of the functionality of this common component of mammalian anatomy. More broadly, the field of reproductive biomechanics has lagged behind that of locomotor, feeding and respiratory biomechanics, despite its obvious evolutionary importance [ 21 ]. We hope the present application of computational simulation techniques to animal genitalia represents an important step towards a rigorous biomechanical approach to reproductive anatomy.…”

Genital morphology associated with mating strategy in the polymorphic lizard, Uta stansburiana

“…Although differences in genital length across alternative mating morphs has been documented in snails and fish, evidence of subtle differences in genital ornamentation is less well‐documented (Doums et al, ; Greven, ; Oliveira et al, ). In species with intromittent copulation, complex male genital ornamentation is widespread in vertebrates (Kelly, ) including lizards and snakes (Böhme & Ziegler, ; Dowling & Savage, ). In U. stansburiana , the enlarged apical lobe of the hemipenis spines is covered in calyces.…”

Non-pharmacological and non-surgical strategies to promote sexual recovery for men with erectile dysfunction

“…Neither the biomechanics of coitus nor the psychological distress that can emerge when penile kinematics is affected by hinging has been adequately investigated ( 99 ). From the limited information at hand, any partial solution to ED that produces an erection operationally “hard enough for penetration” (i.e., yielding a high International Index of Erectile Function, IIEF, score, ( 100 ) yet hinged, may lead to a less than fully satisfying experience.…”