Rolling Stones in the stomach
Several organs of the body can contain or develop objects normally referred to as “stones”: the gall bladder choleliths, the urinary bladder uroliths, the kidney nephroliths, the testicles testicular microliths and the salivary glands sialoliths.
Even in the prostate some mini stones can develop and are then known as prostatic calculi. None of these “-liths” are wanted, but when we turn our attention to otoliths (= ear stones, and a topic for another blog) or to gastroliths (= stones in the stomach: the topic of this blog) the story is a different one. I once saw on Japanese Television an animated film about plant-eating dinosaurs. Many of these extinct behemoths apparently swallowed stones the size of footballs, which then remained in their stomachs. Birds, especially seed-eaters, also still swallow stones regularly and gastroliths recovered from ostriches may be the size of a walnut or a chicken egg.
What could be the reason for animals to collect stones and swallow them? It is certainly not the mineral content present in these indigestive objects; there would be other reasons. One obvious function of the stones in the muscular stomach of the birds is that they could help crack open ingested seeds or nuts and to grind up insect cuticles as in the stone-swallowing African aardwolf (Proteles cristata). Penguins, crocodiles, and sea-lions, however, are neither seed-eating nor insectivorous animals and yet, their stomachs always contain gastroliths. Incidentally, the fish-eating extinct plesiosaurs also had them, but in their case, they might have used them to break down fish bones. Anyway, isn’t there something these stomach stone-containing animals have in common other than a diet of fish?
According to the Leicester scientist Dr M.A. Taylor, all of these animals swallow stones not accidentally and not primarily to grind up fish food, but for the purpose of buoyancy regulation. He surmises that originally accidentally swallowed, the stones were found to be useful as an extra-weight and stabilizer, preventing rolling about the long axis and tail-heaviness. They were then deliberately picked up and swallowed for that purpose. Human divers, of course, also use weight-increasing devices, but they don’t swallow stones and use belts with lead weights instead.
Stones are unquestionably more efficient than skeletal bone when it comes to the question of weight adjustments and ways to control sinking forces. But elephant seals fit neither explanation: they swallow stones just prior to clambering ashore for breeding purposes and vomit them out later when the job is done and they can return to the water. Since these enormous seals do not feed while on land for several weeks, could it be that they enjoy the feeling of the stones in their empty stomachs so that their hunger does not bother them too much? That would be an explanation, wouldn’t it?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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