Sexual dimorphism in size and shape of the head in the sea snake Emydocephalus annulatus

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).