Geoscience Reference
In-Depth Information
that later somehow make their way inside while that animal is still alive. Most of
these rocks are composed of silica-rich minerals, such as quartz and feldspars, and
originallywerepartsofmuchlargerrocksthatwerebrokendownandperhapsroun-
ded by streams over many years before residing in a gut or two.
Sizes of these rocks can vary considerably from sand-grain-sized to as big as
baseballs, and even in the same animal. Size is roughly correlated with the body
size of the host animal, meaning small gastroliths are in smaller animals and big
gastrolithsareinbiggercritters.However,thewidthofagastrolithseeminglynever
exceeds 3% the length of the animal. In terms of numbers, an animal with gastro-
liths could have as few as two or three, or more than a hundred. Many gastroliths
areroundedandhavesmooth,polishedsurfaces,butsomearedullerandirregularly
shaped, with angular outlines. In short, gastroliths have a few traits in common, but
each can have its own personality, and a “population” of these in any given animal
can be quite diverse.
How these rocks get in the belly of an animal is one of the most important
questions surrounding them, but answers to that inquiry can be reduced to just two:
accidental and purposeful. In the first instance, an animal could be going about
its daily business and accidentally swallow a few rocks. For example, an alligator
might use its snout to help dig a den, and a few rocks then get in its mouth and go
down its throat while it is shoveling. Similarly, nest-building birds that are picking
up ground debris, especially soil, also might inadvertently take in small stones. Al-
ternatively, an alligator or bird might see some rocks in its environment, and this
triggers a behavioral response along the lines of “must eat rocks.” They then ac-
cordingly scoop these up in the mouth and swallow.
Birds certainly do this, intentionally selecting sand-, pebble-, or gravel-sized
particles and ingesting them. Anyone who has owned or otherwise watched chick-
ens knows about this behavior, in which these backyard or barnyard fowl grab
grains of sediment with their beaks. I have watched videos of crows doing the same
thing, deliberately walking along and selecting pea-sized gravel to eat. Most telling
