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

Sexual dimorphism in aipysurine sea snakes (Elapidae, Hydrophiinae)

The transition from terrestrial to aquatic life by hydrophiine elapid snakes modified targets of natural selection and likely affected sexual selection also. Thus, the shift to marine life also might have affected sexual dimorphism. Our measurements of 419 preserved specimens of six species of aipysurine snakes (genera Emydocephalus and Aipysurus) revealed sexual dimorphism in mean adult snout-vent length (SVL), body width relative to SVL, lengths and widths of heads and tails relative to SVL, and eye diameter relative to head length. Females averaged larger than males in all taxa, and generally were wider-bodied with shorter and wider tails and smaller eyes. For other traits, sexual dimorphism varied among species: for example, relative head length ranged from male-biased to female-biased, and head shape (width relative to length) was highly dimorphic only in A. laevis. The transition to marine life may have eliminated male-male combat (reducing selection for large males) and favoured visual rather than pheromone-based mate-searching (favouring larger eyes in males). Variation in head-size dimorphism may reflect intersexual niche partitioning, with different taxa following different trajectories. Repeated evolutionary transitions from terrestrial to aquatic life in snakes provide a powerful opportunity to explore selective forces on sexually dimorphic traits.

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

Background

Sexual dimorphism in size and shape is widespread among squamate reptiles. Sex differences in snake skull size and shape are often accompanied by intersexual feeding niche separation. However, allometric trajectories underlying these differences remain largely unstudied in several lineages. The sea krait Laticauda colubrina (Serpentes: Elapidae) exhibits very clear sexual dimorphism in body size, with previous studies having reported females to be larger and to have a relatively longer and wider head. The two sexes also differ in feeding habits: males tend to prey in shallow water on muraenid eels, whereas females prey in deeper water on congerid eels.

Methods

I investigated sexual dimorphism in skull shape and size as well as the pattern of skull growth, to determine whether males and females follow the same ontogenetic trajectories. I studied skull characteristics and body length in 61 male and female sea kraits.

Results

The sexes differ in skull shape. Males and females follow distinct allometric trajectories. Structures associated with feeding performance are female-biased, whereas rostral and orbital regions are male-biased. The two sexes differ in allometric trajectories of feeding-related structures (female biased) that correspond to dietary divergence between the sexes.

Conclusions

Sea kraits exhibit clear sexual dimorphism in the skull form that may be explained by intersexual differences in the feeding habits as well as reproductive roles. The overall skull growth pattern resembles the typical pattern observed in other tetrapods.

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

In snakes, divergence in head size between the sexes has been interpreted as an adaptation to intersexual niche divergence. By overcoming gape-limitation, a larger head enables snakes of one sex to ingest larger prey items. Under this hypothesis, we do not expect a species that consumes only tiny prey items to exhibit sex differences in relative head size, or to show empirical links between relative head size and fitness-relevant traits such as growth and fecundity. Our field studies on the sea snake Emydocephalus annulatus falsify these predictions. Although these snakes feed exclusively on fish eggs, the heads of female snakes are longer and wider than those of males at the same body length. Individuals with wider heads grew more rapidly, reproduced more often, and produced larger litters. Thus, head shape can affect fitness and can diverge between the sexes even without gape-limitation. Head size and shape may facilitate other aspects of feeding (such as the ability to scrape eggs off coral) and locomotion (hydrodynamics); and a smaller head may advantage the sex that is more mobile, and that obtains its prey in narrow crevices rather than in more exposed situations (i.e., males).