Sexual dimorphism and skull size and shape in the highly specialized snake species,Aipysurus eydouxii(Elapidae: Hydrophiinae)

Abstract

Background Snakes exhibit sexual dimorphism in both head size and shape. Such differences are often attributed to different reproductive roles and feeding habits. We aim to investigate how sexual dimorphism is displayed in the highly specialised fish-egg-eating snake, Aipysurus eydouxii, by analysing two complementary features: body size and skull morphology. Methods We used data on body length, weight, and skull shape from 27 measurements of 116 males and females of A. eydouxii. We investigated both sexual dimorphism and allometric (multivariate and bi-variate) properties of skull growth in the analysed data set. Results We found that although there was female-biased sexual size dimorphism in body length, females were not heavier than males, contrary to what is commonly observed pattern among snakes. Moreover, females tend to possess relatively smaller heads than males. However, we only found very subtle differences in skull shape reflected in nasal width, mandibular fossa, quadrate crest and quadrate length. Discussion We suggest that the feeding specialisation in A. eydouxii does not allow for an increase in body thickness and the size of the head above a certain threshold. Our results may be interpreted as support for prey-size divergence as a factor driving skull dimorphism since such species in which the sexes do not differ in prey size also shows very subtle or no differences in skull morphology.

Sexual dimorphism in size and shape of the head in the sea snake Emydocephalus annulatus (Hydrophiinae, Elapidae)

“…Sexual dimorphism in relative head size is labile in snakes, both within and among species 13 . Within the aipysurine lineage of sea snakes, heads of females are larger than those of males in Aipysurus laevis 18 and E. annulatus (present study), but males have larger heads than females in A. eydouxii 19 . The study on A. eydouxii was based on skull lengths (from the rostral point of the premaxilla to the most caudal projection of the cranium) of preserved specimens, rather than measurements of live animals as in the case of the other two species.…”

Sexual dimorphism in skull size and shape of Laticauda colubrina (Serpentes: Elapidae)

“…Because the measurement error was present in both the x- and y-variables the slopes were calculated using Reduced Major Axis Regression ( Sokal & Rohlf, 1995 ). Although the Type II Regression Model (or Major Axis Regression) may seem to be more appropriate in estimating the allometric slopes (see Warton et al, 2006 ), I decided to use RMA Regression as it allows direct comparisons with the similar studies on other species ( Hampton & Moon, 2013 ; Hampton, 2014 ; Hampton & Kalmus, 2014 ; Borczyk et al, 2021 ; Patterson et al, 2022 ). All calculations were done using Statistica and software by Bohonak & van der Linde (2004) .…”

“…Studies on the allometry of snake skulls are restricted to members of a few families. Monteiro (1998) and Jayne et al (2022) studied three Boidae species; Rossman (1980) , Young (1989) , Hampton (2014) , Hampton & Kalmus (2014) and Andjelković, Tomović & Ivanović (2016) studied Natricinae snakes; Jayne et al (2022) studied single Colubrinae species; Murta-Fonseca & Fernandes (2016) and Murta-Fonseca et al (2019) focused on Xenodontinae; Dos Santos & da Costa Prudente (2022) studied malacophagous Dipsadidae; Hampton & Moon (2013) studied a single species of Crotalinae; Borczyk et al. (2021) , Patterson et al (2022) and Ammresh et al (2023) studied three species of Elapidae.…”

“…In addition to the list, Palci, Lee & Hutchinson (2016) compared Anilios bicolor (Scolecophidia), Cylindrophis ruffus (Cylindrophidae), Aspidites ramsai (Pythonidae), Acrochordus arafurae (Acrochordidae) and Notechis scutatus (Elapidae); however, their sample comprises juvenile-adult pairs for each species, not growth series. Some of these studies ignored the potential effect of sexual dimorphism and only Camilleri & Shine (1990) , Murta-Fonseca et al (2019) Borczyk et al (2021) and Dos Santos & da Costa Prudente (2022) addressed this topic directly, confirming some differences between males and females. However, sexual dimorphism in head size and shape has been reported in a variety of snake species ( e.g.…”

Sexual dimorphism in aipysurine sea snakes (Elapidae, Hydrophiinae)

“…Even in snake species in which rival males wrestle each other for access to females, biting is rare [ 20 ]; and thus sex-based divergences in relative head size cannot be attributed to sexual selection for more powerful biting [ 24 ]. Instead, head-size dimorphism in snakes seems to reflect sex-based niche partitioning [ 24 , 41 ]. Longer, wider heads in female than male E. annulatus have been attributed to sex-based differences in foraging sites, driven by seasonal differences in the frequency of feeding and types of available prey [ 42 ].…”