Crocodile bulks of swallowed prey flesh and bone. Many large crocodilians swallow stones (called gastroliths or stomach stones), which may act as ballast to balance their bodies or assist in crushing food, similar to grit ingested by birds. Herodotus claimed that Nile crocodiles had a symbiotic relationship with certain birds, such as the Egyptian plover, which enter the crocodile's mouth and pick leeches feeding on the crocodile's blood; with no evidence of this interaction actually occurring in any crocodile species, it is most likely mythical or allegorical fiction. Since they feed by grabbing and holding onto their prey, they have evolved
Symbiotic bacteria endonuclear symbiotic bacterium Holospora, it has been discovered that Holospora species can maintain their infectivity for a limited time and form a symbiotic relationship with Paramecium species. It is well accepted and understood that there is a mutualistic relationship between plants and rhizobial bacteria and mycorrhizal fungi enabling the plants to survive in an otherwise nitrogen-poor soil environment. Co-evolution is described as a situation where two organisms evolve in response to one another. In a study reported in "Functional Ecology", these scientists investigated whether such a mutualistic relationship conferred an evolutionary advantage to either plant or symbiont. They did not