Patterns of cetacean vaginal folds yield insights into functionality

Abstract

Complex foldings of the vaginal wall are unique to some cetaceans and artiodactyls and are of unknown function(s). The patterns of vaginal length and cumulative vaginal fold length were assessed in relation to body length and to each other in a phylogenetic context to derive insights into functionality. The reproductive tracts of 59 female cetaceans (20 species, 6 families) were dissected. Phylogenetically-controlled reduced major axis regressions were used to establish a scaling trend for the female genitalia of cetaceans. An unparalleled level of vaginal diversity within a mammalian order was found. Vaginal folds varied in number and size across species, and vaginal fold length was positively allometric with body length. Vaginal length was not a significant predictor of vaginal fold length. Functional hypotheses regarding the role of vaginal folds and the potential selection pressures that could lead to evolution of these structures are discussed. Vaginal folds may present physical barriers, which obscure the pathway of seawater and/or sperm travelling through the vagina. This study contributes broad insights to the evolution of reproductive morphology and aquatic adaptations and lays the foundation for future functional morphology analyses.

The evolution of genital shape variation in female cetaceans*

“…; Orbach et al. ). The vaginal folds of cetaceans vary in number, thickness, positioning, shape, and size across species and appear to be under strong selection pressures because fold lengths are positively allometric with body size (Orbach et al.…”

“…; cetaceans, Orbach et al. ). Because female genital adaptations can be subtle, careful quantification of morphological attributes is critical to identify patterns and the underlying evolutionary drivers of variation (Brennan and Prum ).…”

“…Recent work on cetaceans has documented widespread variation in female genital structures called vaginal folds, which appears to be unparalleled in other mammalian taxa (Orbach et al. ). With the exception of artiodactyls, the closest relatives to cetaceans, other mammalian groups do not have vaginal folds (Pabst et al., ).…”

“…The vaginal folds of cetaceans vary in number, thickness, positioning, shape, and size across species and appear to be under strong selection pressures because fold lengths are positively allometric with body size (Orbach et al. ). However, quantifying vaginal fold attributes is difficult, as morphological characteristics may vary within a species (Orbach et al.…”

The evolution of female genitalia

“…Many studies of genital evolution have found it difficult to separate the effects of cryptic female choice and sexual conflict, and therefore, the relative importance of one compared to the other is difficult to gauge. For example, an analysis of vaginal morphology of 20 species of cetaceans revealed extensive diversification in vaginal fold size, number, positioning and shape (Orbach et al., ), although the functional role of these vaginal folds in sexual selection is not immediately clear (Figure ). Further studies have found evidence for both congruent (genitalia are complementary) and antagonistic (genitalia are combative) coevolution between male and female genitalia in marine mammals, hinting at roles for either Fisherian cryptic female choice or sexual conflict mechanisms (Orbach et al., ).…”