22 February 2007

Evolution of jaw asymmetry in snail-eating snakes

Southeast Asian snakes in the subfamily Pareatinae prey mostly on terrestrial snails and slugs. But their jaws are not strong enough to crush snail shells. So, instead, they grab the snail's body and pull it out of its shell. A new study by Hoso et al.1 demonstrates that snail-eating pareatine snakes have more teeth on their right jaws than on the left and attribute this difference to the prevalence of dextrally coiled (right-handed) shells in snails.

The snake Pareas iwasakii, from Japan, comes from behind following the snail's mucus track, grabs the snail's foot and then pulls the snail's body out of its shell with alternate retractions of its left and right jaws. The snail used in the experiments was Bradybaena similaris. Photographs from Hoso et al.1.

The study found that the snail predator Pareas iwasakii had an average of 17.5 teeth on its left jaw and 24.9 teeth on its right jaw (n = 28). A similar asymmetry was demonstrated in the jaws of one unhatched snake, implying that the trait has a genetic basis. Furthermore, the teeth numbers on the jaws were asymmetric in 12 out of 14 pareatine species. One of the remaining 2 species feeds on lizards in addition to snails, while the other one feeds exclusively on slugs.

The jaws (stained with alizarin red) of one specimen of P. iwasakii with 16 left and 24 right teeth. Photograph from Hoso et al.1.

In the feeding tests Hoso et al. conducted using dextral and sinistral (left-handed) individuals of the snail Bradybaena similaris, P. iwasakii required significantly longer handling time with sinistral snails than with dextrals and failed to extract sinistral snails more frequently than dextrals.

According to the phylogeny of these snakes (cited in the paper), the ancestral snakes had symmetric dentition, which suggests that the unequal teeth numbers on the snake's jaws is an adaptation to preying on dextral snails. The snails, on the other hand, may not have been marching in place. Hoso et al. speculate that the wide diversity of sinistral land snails in Southeast Asia may have resulted from selective pressure the snakes have been putting on dextral snails; if dextral snails are more likely to get eaten, then natural selection will naturally select for sinistral variants.

Appreciations are due to the good folks at CONCH-L for bringing this paper to my attention.

1Masaki Hoso, Takahiro Asami, Michio Hori. Right-handed snakes: convergent evolution of asymmetry for functional specialization. Biology Letters. DOI: 10.1098/rsbl.2006.0600

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