Patricia L. R. Brennan

The Family Tinamidae comprises 45 species with broad Neotropical distributions (Remsen et al. 2026). Despite their widespread occurrence in a variety of habitats, ranging from tropical rainforest t...

While the vocalisations of anurans (frogs and toads) have been the subject of in-depth investigations for decades (Bogert 1960; Blair 1963; Gans 1973; Schiotz 1973; Littlejohn 1977), the sounds pro...

Synopsis: Genital spines are present in many groups of animals, including insects, squamates, fish and mammals, and their diverse functions have been established in some species. However, we have yet to characterize the morphology of genital spines in depth. Genital spines are known to influence mating success through several natural and sexual selection mechanisms and examining their morphology may help us to understand their function. Snakes have extraordinary genital spine diversity that has been used in taxonomy for decades. In this study, we characterize the morphological attributes of hemipenial spines in 14 snake species, based on μCT scan segmentations of 39 hemipenes with over 10,000 spines and describe continuous variation in spine morphology, including number, volume, sphericity (shape), and distribution that does not serve to categorize spines into types as has been done in some taxonomic studies, with the exception of the basal hook. We identify areas of the volume/sphericity morphospace that may impose a constraint in spine functional morphology, and find significant variation in ontogenetic changes of the spines that include spine size and number, mineralization rate during ontogeny, and relationship with hemipene shape development. Our work is clearly only the first step to understanding the complex spine morphology in this group, and further work must consider puncture interactions with vaginal tissue as well as hemipene eversion mechanics. This exploratory work gives us a baseline with which to compare the morphology of genital spines in other groups of animals and make better inferences about copulatory function.

Genital spines are widespread in animals with internal fertilization, and their morphology is diverse. Many hypotheses have been proposed to explain their presence, yet their function has not been tested in most systems. We do not yet have a framework to characterize their morphology and potential function, even to determine if they puncture the vaginal tract (as is often assumed). Here, we examine the morphospace and puncturing ability of genital spines by testing different 3D-printed spines from a diverse sample of μCT-scanned snake hemipenes, which are among the most morphologically variable intromittent genitalia in vertebrates. We performed serial compression tests into polydimethylsiloxane silicone polymer at increments of 10° until puncture was impossible. We found that the range of successful puncture angles differed based on spine curvature and tip sharpness, suggesting that some spines are built to puncture at almost every angle of approach, while others are extremely unlikely to puncture vaginal tissues at all. These results provide a framework for continued study and characterization of the shape and performance of genital spines in other animal groups and help to refine functional hypotheses in the context of copulatory interactions.

Despite the remarkable morphological diversity found in vertebrate genitalia, it has historically been difficult to quantify shape variation of soft tissue structures due to limitations of 3D landmarking methods. New techniques such as automatic landmarking now allow us to examine such structures in detail, and with these methods we quantify the intraspecific variation in the genitalia of Burmese pythons ( Python bivittatus ). Despite previous assertions that a vaginal pouch is not present in pythons, we find that P. bivittatus have well developed vaginal pouches, that are morphologically diverse, and change shape over ontogeny. Vaginal pouches and hemipenes are isometric. Hemipenes also vary in shape ontogenetically, but we find no evidence of directional asymmetry in shape or size between adult right and left hemipenes suggesting a lack of laterality. We identify a potentially intersex neonate with hemipenes, testes, and a vaginal pouch. We discuss our results in the context of snake genital evolution and suggest other mechanisms for selection beyond the standard “lock and key” hypothesis. Future work examining genital shape variation of other snake families will provide more insight into the coevolutionary patterns shaping the genitalia diversity across snakes and vertebrates more broadly.

Alpacas (Vicugna pacos) are reported to be the rare mammal in which the penis enters the uterus in mating. To date, however, only circumstantial evidence supports this assertion. Using female alpacas culled for meat, we determined that the alpaca penis penetrates to the very tips of the uterine horns, abrading the tract and breaking fine blood vessels. All female alpacas sacrificed one hour or 24 hours after mating showed conspicuous bleeding in the epithelium of some region of their reproductive tract, including the hymen, cervix and the tips of each uterine horn, but typically not in the vagina. Unmated females showed no evidence of conspicuous bleeding. Histological examination of mated females revealed widespread abrasion of the cervical and endometrial epithelium, injuries absent in unmated females. Within one hour of mating, sperm were already present in the oviduct. The male alpaca’s cartilaginous penis tip with a hardened urethral process is likely responsible for the copulatory abrasion. The entire female reproductive tract interacts with the penis, functioning like a vagina. Alpacas are induced ovulators, and wounding may hasten delivery of the seminal ovulation-inducing factor beta-NGF into the female’s blood stream. There is no evidence of sexual conflict in copulation in alpaca, and thus wounding may also be one of a variety of mechanisms devised by mammals to induce a beneficial, short-term inflammatory response that stimulates blastocyst implantation, the uterine remodeling associated with placental development, and thus the success of early pregnancy.

Alpacas ( Vicugna pacos ) are reported to be the rare mammal in which the penis enters the uterus in mating. To date, however, only circumstantial evidence supports this assertion. Using female alpacas culled for meat, we determined that the alpaca penis penetrates to the very tips of the uterine horns, abrading the tract and breaking fine blood vessels. All female alpacas sacrificed one hour or 24 hours after mating showed conspicuous bleeding in the epithelium of some region of their reproductive tract, including the hymen, cervix and the tips of each uterine horn, but typically not in the vagina. Unmated females showed no evidence of conspicuous bleeding. Histological examination of mated females revealed widespread abrasion of the cervical and endometrial epithelium, injuries absent in unmated females. Within one hour of mating, sperm were already present in the oviduct. The male alpaca’s cartilaginous penis tip with a hardened urethral process is likely responsible for the copulatory abrasion. The entire female reproductive tract interacts with the penis, functioning like a vagina. Alpacas are induced ovulators, and wounding may hasten delivery of the seminal ovulation-inducing factor beta-NGF into the female’s blood stream. There is no evidence of sexual conflict in copulation in alpaca, and thus wounding may also be one of a variety of mechanisms devised by mammals to induce a beneficial, short-term inflammatory response that stimulates blastocyst implantation, the uterine remodeling associated with placental development, and thus the success of early pregnancy.

Female genitalia are conspicuously overlooked in comparison to their male counterparts, limiting our understanding of sexual reproduction across vertebrate lineages. This study is the first complete description of the clitoris (hemiclitores) in female snakes. We describe morphological variation in size and shape ( n = 9 species, 4 families) that is potentially comparable to the male intromittent organs in squamate reptiles (hemipenes). Dissection, diffusible iodine contrast-enhanced micro-CT and histology revealed that, unlike lizard hemiclitores, the snake hemiclitores are non-eversible structures. The two individual hemiclitores are separated medially by connective tissue, forming a triangular structure that extends posteriorly. Histology of the hemiclitores in Australian death adders ( Acanthophis antarcticus ) showed erectile tissue and strands/bundles of nerves, but no spines (as is found in male hemipenes). These histological features suggest the snake hemiclitores have functional significance in mating and definitively show that the hemiclitores are not underdeveloped hemipenes or scent glands, which have been erroneously indicated in other studies. Our discovery supports that hemiclitores have been retained across squamates and provides preliminary evidence of differences in this structure among snake species, which can be used to further understand systematics, reproductive evolution and ecology across squamate reptiles.