The Jarman–Bell principle seeks to explain why smaller herbivore species tend to select higher-quality forage (high protein and high fiber digestibility) than larger herbivore species. This principle may also provide insight into intraspecific differences in resource use in species with pronounced sexual size dimorphism. We examined the relationship between body mass and diet quality in a sexually dimorphic macropodid community comprising larger (17–55 kg) eastern and western gray kangaroos, Macropus giganteus and M. fuliginosus, and smaller (7–23 kg) red-necked and swamp wallabies, M. rufogriseus and Wallabia bicolor. We determined diet composition through microhistological analysis of fecal pellets, and then assessed diet quality by measuring available nitrogen and fiber (cellulose and hemicellulose) in the plants consumed. We predicted that the smaller species and smaller individuals within species would consume the highest-quality diets. The concentration of fiber consumed differed significantly among species; available nitrogen concentration did not. As predicted, the lowest-fiber diet was consumed by a wallaby (small species) and the highest-fiber diet was consumed by a kangaroo (large species). Contrary to predictions, we detected no sex-based differences in diet quality. However, large male red-necked wallabies consumed diets containing harder-to-digest fiber than small male red-necked wallabies. Body mass and diet type were related between and within species. However, body mass did not account for all differences in diet quality between species; swamp wallabies consumed less fiber than similarly sized red-necked wallabies. These differences are likely related to other adaptations that allow swamp wallabies to better tolerate tannin-rich browse-dominated diets.
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Vol. 99 • No. 2