A Sawback is a Sawback is a Sawback: Diet and Habitat-Related Variation in Body Size of the Turtle Graptemys oculifera Show It Is an Ecological Analogue of Its Two Closest Relatives

Abstract

The map turtles and sawbacks (Graptemys) sort by female head width into narrow-headed (microcephalic) females of 5 species that eat few mollusks, moderately broad-headed (mesocephalic) females of 4 species that eat moderately high amounts of mollusks, and broad-headed (megacephalic) females of 5 species that eat mollusks almost exclusively. The microcephalic species include a clade of 3 sawbacks, Graptemys nigrinoda, Graptemys flavimaculata, and Graptemys oculifera. The first 2 are sponge specialists, but 2 previous dietary studies of G. oculifera did not report sponges; both also lacked comparisons between the sexes. Both of the other sawbacks also feed on brackish-water mussels near the coast, but this phenomenon has not been examined for G. oculifera. Increased body sizes occur in coastal populations of both other sawbacks as well. We examined G. oculifera diets using fecal samples and found that contrary to the earlier results, they also fed primarily on sponges, as well as insects. Females also consumed algae, Asian clams, and, in a coastal population, mussels. In low-gradient river reaches near the coast, G. oculifera achieved large body sizes compared to populations inhabiting sandbar-dominated inland reaches; they were also large at the confluences of the Pearl River and a tributary creek with the Ross Barnett Reservoir. The fact that G. oculifera at the 2 confluences with a large inland reservoir showed the same trend in body size as they did at the coastal sites suggests that the primary reason for large body size may be thermal stability and/or slowing current that promotes biological production, rather than consumption of brackish-water mussels in coastal populations, as the mussels were absent near the reservoir. In addition, high predation by alligators may also occur at the reservoir confluence and coastal sites and select for large body sizes. Overall, the 3 allopatric sawbacks show a pattern of vicariant divergence as ecological analogues, consistent with findings for other allopatric sets of related species.