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

posted on 2022-06-10, 02:37 authored by Rick ShineRick Shine, Claire Goiran
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).


Annual mark-recapture surveys in small bays beside Noumea, New Caledonia. Snakes measured and released soon after capture. Every January from 2004 to 2021, we snorkelled through the study sites at least 20 times in total (30 to 60 minutes per session). All snakes seen were captured by hand, and processed at a nearby laboratory (measured, weighed, assessed for sex, palpated for pregnancy and litter size; implanted with a microchip, and released at the site of capture). We determined sex based on tail shape and scale rugosity. We used calipers to measure the length of the head (tip of snout to rear end of quadrate-articular projection of the lower jaw) and maximum width of the head. We did not include the rostral spine of males in our measurement of head length. We only took data from a snake once per year, at its first capture in that period, but many snakes were recaptured in subsequent years. In total, we have data on 1,293 individual snakes (662 males, 631 females) captured a total of 3,012 times over the period January 2004 to January 2021.

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