All
extant cephalopods have a two-part
beak, or
rostrum, situated in the
buccal mass and surrounded by the muscular
head appendages. The
dorsal (upper)
mandible fits into the
ventral (lower) mandible and together they function in a scissor-like fashion. Composed primarily of
chitin and cross-linked
proteins, beaks are more-or-less indigestible and are often the only identifiable cephalopod remains found in the stomachs of predatory species such as
sperm whales. They can be used to estimate the
mantle length and total body weight of the original animal as well as the total ingested
biomass of the species. Cephalopod beaks gradually become less stiff as one moves from the tip to the base, a gradient that results from differing chemical composition. In hydrated beaks of the
Humboldt squid (
Dosidicus gigas) this stiffness gradient spans two
orders of magnitude.