Diets of syntopic black-knobbed sawbacks in the Alabama River

The emydid turtle genus Graptemys is characterized by intra- and interspecific dietary diversity. Sympatric species pairs typically differ in trophic morphology and dependence on bivalve mollusks in female diets, yet there is a lack of comprehensive comparisons of the diets of sympatric species. I collected feces from 92 black-knobbed sawbacks (G. nigrinoda) and 54 Alabama map turtles (G. pulchra) from the Alabama River at a site in Autauga and Lowndes counties, Alabama. Samples were analyzed separately for unsexed juveniles, adult males, juvenile females larger than the smallest mature males, and adult females of each species. Sponges, aquatic insects (particularly caddisfly larvae), and filamentous algae were the primary foods of G. nigrinoda. Native mussels, invasive Asian clams, and aquatic insects (but few caddisfly larvae) were the primary foods of G. pulchra. Each of the four sex and size classes exhibited substantially greater similarity with conspecific classes than with classes of the other species. Intraspecific interclass similarity in diet showed size-structured patterns in both species but with opposite patterns. In G. nigtinoda the classes that were most similar in diet were the smallestbodied classes, unsexed juveniles and adult males, which fed most heavily on sponges; the largest-bodied class, adult females, fed most heavily on filamentous algae and was least similar in diet to other classes. In G. pulchra the classes that were most similar in diet were the two largestbodied classes, adult and juvenile females, which fed most heavily on bivalve mollusks; the smallest-bodied class, unsexed juveniles, was least similar in diet to other classes. These results reinforce the importance of body size in determining Graptemys diets and suggest dietary differentiation of sympatric Graptemys species may extend beyond differentiation in adult females more often than is generally thought.