What is the Function of Gastroliths in Crocodiles?
While gastroliths are known to function as a digestive aid in many herbivorous species, their exact function in crocodiles is widely debated. The author of a journal article titled, "A review of gastrolith function with implications for fossil vertebrates and a revised classification," explores an large range of proposed theories for gastrolith function and analyzes a select number of theories. Proposed hypotheses for the function of gastroliths include mixing of foodstuff, mineral supply, stomach cleaning, hydrostatic function, alleviation of hunger, and accidental ingestion. For the majority of these theories the author could provide no evidence to either support or refute the proposed gastrolith function; however, for the hydrostatic function and accidental ingestion theories the author cited the studies of several primary source articles to support these theories.
The hydrostatic function theory contains two sub-theories for the function of gastroliths in crocodiles--buoyancy and ballast--both of which the author supports. The buoyancy theory states that the gastroliths have a function in affecting the animal's ability to float in water. Cott (1961, as cited in Taylor 1993) states that gastroliths increase the specific gravity of the reptile, which aids in prey capture by allowing the predator to draw struggling prey under the surface. In contrast to an alternative method of buoyancy control known as pachyostosis--the thickening of an animal's bone tissue--compensation of positive buoyancy is more easily achieved through the ingestion of gastroliths; this is because rocks have a higher density than bone and can be swallowed and regurgitated rapidly, as opposed to the long-term process of bone growth (Cott 1961, as cited in Taylor 1993). The ballast theory, the other sub-theory of the hydrostatic function theory, states that gastroliths improve the stability and balance of crocodiles. Cott (1961, as cited in Taylor 1993) asserts that the specific location of gastroliths in the gut of the tetrapod elevates the anterior end of crocodiles and stabilizes them against rolling. The hydrostatic function theory of gastrolith function in crocodiles is in actuality a two part theory that suggests two plausible functions of gastroliths within crocodiles: buoyancy, and balance.
A second theory of the function of gastroliths in crocodiles supported by the author is the accidental ingestion theory. This hypothesis states that rocks are eaten accidentally because of sediment attached to prey, or because the prey itself could contain gastroliths. Welman and Worthing (1943, as cited in Taylor 1993) examined the stomach contents of sixty-seven crocodiles and observed feeding behavior and attributed the presence gastroliths in Crocodilus niloticus to accidental swallowing during feeding. Taylor notes, however, that the accidental ingestion theory becomes less plausible as the size and number of stones increase.
In a separate review of gastrolith function titled, "Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods," the author utilizes a reported distribution of gastroliths in the major groups of marine tetrapods to perform a comparative analysis of gastrolith function. The author examines two popular hypotheses for gastrolith function in aquatic tetrapods: buoyancy control and food processing.
The food processing theory of gastrolith function states that the stones aid in the digestion of food, serving to grind up the food into smaller pieces and thus allowing for easier digestion. In examining the reported distribution of gastroliths in the major groups of marine tetrapods, the author observes no correlation between the presence of gastroliths and a particular size or type of prey, thus providing no evidence for the role of gastroliths in food processing (Wings 2007). To further support his claim that gastroliths play no role in food processing, Wings notes that the stones are unnecessary for digestion; aquatic tetrapods typically feed on fish, cephalopods without exoskeletons, and crustaceans, which can be easily digested through gastric juices without the aid of gastroliths (Pollard 1968, as cited in Wings 2007).
The buoyancy control theory of gastrolith function examined by Wings is analogous to the hydrostatic function theory proposed by Taylor; these hypotheses state that gastroliths play a role in buoyancy control, including the adjustment of floating posture and stability of rolling. In examining the reported distribution of gastroliths in the major groups of marine tetrapods, the author observes a correlation between the presence of gastroliths and body form, as well as means of locomotion. Wings (2007) notes a frequent occurrence of the stones among amphibious animals, including the crocodile, that utilize both their tails and their limbs in a rowing action to swim. To further support his claim that gastroliths have a role in buoyancy control, Wings notes their value in predator-prey situations; through enhancing the crocodile's ability to adjust its buoyancy, the animal is enabled to linger just below the water's surface, allowing it to effectively stalk and ambush prey in shallow waters (Cott 1961, as cited in Wings 2007).
Based on the content of the publications of Wings and Taylor, it seems that there is strong support for the buoyancy control/hydrostatic function theory of gastrolith function within crocodiles, which states that the stones play a role in the stability of rolling and adjustment of floating posture. While Taylor did not provide any evidence against the food processing theory, Wings rejected it, noting that there is no correlation between the presence of gastroliths and a particular size or type of prey (Wings 2007). While Wings did not investigate the accidental ingestion theory, Taylor asserts that this hypothesis of gastrolith function is plausible. Based on the large number of uninvestigated hypotheses of gastrolith function, it is clear that more research needs to be performed on this topic before accurate conclusions about the exact function of the stones in crocodiles can be made.
References
Taylor, M. A. (1993). Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 341(1296), 163-175.
Wings, O. (2007). A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica, 52(1), 1.
While gastroliths are known to function as a digestive aid in many herbivorous species, their exact function in crocodiles is widely debated. The author of a journal article titled, "A review of gastrolith function with implications for fossil vertebrates and a revised classification," explores an large range of proposed theories for gastrolith function and analyzes a select number of theories. Proposed hypotheses for the function of gastroliths include mixing of foodstuff, mineral supply, stomach cleaning, hydrostatic function, alleviation of hunger, and accidental ingestion. For the majority of these theories the author could provide no evidence to either support or refute the proposed gastrolith function; however, for the hydrostatic function and accidental ingestion theories the author cited the studies of several primary source articles to support these theories.
The hydrostatic function theory contains two sub-theories for the function of gastroliths in crocodiles--buoyancy and ballast--both of which the author supports. The buoyancy theory states that the gastroliths have a function in affecting the animal's ability to float in water. Cott (1961, as cited in Taylor 1993) states that gastroliths increase the specific gravity of the reptile, which aids in prey capture by allowing the predator to draw struggling prey under the surface. In contrast to an alternative method of buoyancy control known as pachyostosis--the thickening of an animal's bone tissue--compensation of positive buoyancy is more easily achieved through the ingestion of gastroliths; this is because rocks have a higher density than bone and can be swallowed and regurgitated rapidly, as opposed to the long-term process of bone growth (Cott 1961, as cited in Taylor 1993). The ballast theory, the other sub-theory of the hydrostatic function theory, states that gastroliths improve the stability and balance of crocodiles. Cott (1961, as cited in Taylor 1993) asserts that the specific location of gastroliths in the gut of the tetrapod elevates the anterior end of crocodiles and stabilizes them against rolling. The hydrostatic function theory of gastrolith function in crocodiles is in actuality a two part theory that suggests two plausible functions of gastroliths within crocodiles: buoyancy, and balance.
A second theory of the function of gastroliths in crocodiles supported by the author is the accidental ingestion theory. This hypothesis states that rocks are eaten accidentally because of sediment attached to prey, or because the prey itself could contain gastroliths. Welman and Worthing (1943, as cited in Taylor 1993) examined the stomach contents of sixty-seven crocodiles and observed feeding behavior and attributed the presence gastroliths in Crocodilus niloticus to accidental swallowing during feeding. Taylor notes, however, that the accidental ingestion theory becomes less plausible as the size and number of stones increase.
In a separate review of gastrolith function titled, "Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods," the author utilizes a reported distribution of gastroliths in the major groups of marine tetrapods to perform a comparative analysis of gastrolith function. The author examines two popular hypotheses for gastrolith function in aquatic tetrapods: buoyancy control and food processing.
The food processing theory of gastrolith function states that the stones aid in the digestion of food, serving to grind up the food into smaller pieces and thus allowing for easier digestion. In examining the reported distribution of gastroliths in the major groups of marine tetrapods, the author observes no correlation between the presence of gastroliths and a particular size or type of prey, thus providing no evidence for the role of gastroliths in food processing (Wings 2007). To further support his claim that gastroliths play no role in food processing, Wings notes that the stones are unnecessary for digestion; aquatic tetrapods typically feed on fish, cephalopods without exoskeletons, and crustaceans, which can be easily digested through gastric juices without the aid of gastroliths (Pollard 1968, as cited in Wings 2007).
The buoyancy control theory of gastrolith function examined by Wings is analogous to the hydrostatic function theory proposed by Taylor; these hypotheses state that gastroliths play a role in buoyancy control, including the adjustment of floating posture and stability of rolling. In examining the reported distribution of gastroliths in the major groups of marine tetrapods, the author observes a correlation between the presence of gastroliths and body form, as well as means of locomotion. Wings (2007) notes a frequent occurrence of the stones among amphibious animals, including the crocodile, that utilize both their tails and their limbs in a rowing action to swim. To further support his claim that gastroliths have a role in buoyancy control, Wings notes their value in predator-prey situations; through enhancing the crocodile's ability to adjust its buoyancy, the animal is enabled to linger just below the water's surface, allowing it to effectively stalk and ambush prey in shallow waters (Cott 1961, as cited in Wings 2007).
Based on the content of the publications of Wings and Taylor, it seems that there is strong support for the buoyancy control/hydrostatic function theory of gastrolith function within crocodiles, which states that the stones play a role in the stability of rolling and adjustment of floating posture. While Taylor did not provide any evidence against the food processing theory, Wings rejected it, noting that there is no correlation between the presence of gastroliths and a particular size or type of prey (Wings 2007). While Wings did not investigate the accidental ingestion theory, Taylor asserts that this hypothesis of gastrolith function is plausible. Based on the large number of uninvestigated hypotheses of gastrolith function, it is clear that more research needs to be performed on this topic before accurate conclusions about the exact function of the stones in crocodiles can be made.
References
Taylor, M. A. (1993). Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetrapods. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 341(1296), 163-175.
Wings, O. (2007). A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica, 52(1), 1.