Island rule and bone metabolism in fossil murines from Timor

Skeletal growth rates reconstructed from bone histology in extinct insular hippopotamids, elephants, bovids, and sauropods have been used to infer dwarfism as a response to island conditions. Limited published records of osteocyte lacunae densities (Ot.Dn), a proxy for living osteocyte proliferation, have suggested a slower rate of bone metabolism in giant mammals. Here, we test whether insularity may have affected bone metabolism in a series of small to giant murine rodents from Timor. Ten adult femora were selected from a fossil assemblage dated to the Late Quaternary (ca. 5–18 ka). Femur morphometric data were used in computing phylogenetically-informed body mass regressions, although phylogenetic signal was very low (Pagel’s lambda = 0.03). Osteocyte lacunae densities from midshaft femur histological sections were evaluated against bone size and estimated body weight. Weight estimates calculated from these femora ranged from 75g to 1188g. Statistically significant (p < 0.05) and strongly negative relationships between Ot.Dn, femur size, and estimated weight were found. Larger specimens were characterised by lower Ot.Dn, indicating that giant murines from Timor may have had a relatively slow pace of bone metabolic activity, consistent with predictions made by the island rule.