Geoscience Reference
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on purpose? Perhaps surprisingly, the reasons are many and quite sensible from an
evolutionary perspective. Yet the best-understood one is to use these geologic ma-
terials as substitutes for teeth: letting the rocks do the “chewing” of food so that its
surface area is increased and more easily digestible. This process, also known as
trituration ,islikehavingafoodprocessorintheupperpartofananimal'sdigestive
system.
In birds that use gastroliths for just this function, their alimentary canal, from
top to bottom, goes like this: mouth, esophagus, crop, proventriculus, ventriculus,
small intestine, large intestine, and rectum, the latter also known as the cloaca.
(Recall that in females, this is also where eggs exit.) Let's say a chicken has just
eatensomegrassseedsmixedinwithdelicious, nutritiousinsectlarvae.Thischick-
en may briefly chop its food with its beak, but more grabbing and swallowing hap-
pensthanchewing.Herfood,someofitstillsquirming,travelsdownheresophagus
through a series of muscular contractions, or peristalsis. It may then reside briefly
in her crop, which is an enlarged storage area at the end of the esophagus that acts
as a holding room. Once ready for digesting, this mixture of grains and insects
is passed down to her proventriculus, sometimes nicknamed “soft stomach.” The
proventriculus has low pH (acidic) fluids which start dissolving the food and other-
wise regretfully informing any insects still holding on to their lives that resistance
is futile.
Still, this chemical attack might not be enough for proper digestion, especially
if the seeds had tough outer coatings and the insects had some hard parts, such
as mandibles. This is where the chicken's ventriculus (more popularly known as a
gizzard) comes to the rescue, aided by its super geo-friends, gastroliths. The food
is then moved down to her gizzard, a strong, muscular organ hosting the gastro-
liths. The gizzard squeezes and releases, squeezes and releases, over and over, re-
petitively and redundantly. The hard stones inside the gizzard act like a mortar and
pestle,mechanicallybreakingdowntheseseedsandinsectpartsintotinierparticles,
andcorrespondinglyincreasetheirsurfaceareas.Thisfoodthenmaybepassedback
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