Long-term association between the commensal Florida Mouse (Podomys floridanus) and the Gopher Tortoise (Gopherus polyphemus) in the fossil record of Florida

Abstract

The Florida Mouse Podomys floridanus and the Gopher Tortoise Gopherus polyphemus are linked in modern ecosystems by a commensal relationship in which Podomys uses the burrows of Gopherus. However, previous paleoecological research demonstrated that species interactions, including commensalisms, are not necessarily stable through geologic time. Given that the longevity of species interactions cannot be assumed, we asked: is the current association between Podomys and Gopherus a coincidental feature of the modern biota or is it a long-term phenomenon detectable in the fossil record? We explored that question using literature and collection records of Podomys and Gopherus from throughout the published geographic and temporal extent of the fossil record of Podomys (Florida, ~1.35 Ma – 0.01 Ma). We expected that a long-term commensal relationship would result in the presence of Gopherus in sites preserving Podomys significantly more often than expected by chance. Given the asymmetric nature of the relationship, a complementary expectation is that a search for sites containing Gopherus within the same spatiotemporal extent should not result more sites containing both species than would be expected by chance. After accounting for potential collection biases, fourteen Irvingtonian and Rancholabrean sites preserving Podomys were searched for occurrences of Gopherus as well as other vertebrates whose burrows Podomys is reported to use. Twelve of those fourteen sites contained fossils of Gopherus. The two remaining sites contained fossils of the other candidate taxa, consistent with the hypothesis that if Podomys floridanus did not use the burrows of Gopherus, it could have used the burrows of other species, as it does in modern ecosystems. Among the sites that are from the same spatiotemporal extent and contain Gopherus, fossils of Podomys were no more likely to be present at a site than would be expected by chance. Overall, we find evidence that Gopherus and Podomys have a long-term association consistent with their current relationship that extends at least to early Irvingtonian faunas.