Rib remodelling in fossil insular dwarf and mainland elephants from Greece

Abstract In the Mediterranean, during the Middle and Late Pleistocene, populations of straight-tusked elephants (Palaeoloxodon antiquus) evolved repeatedly and independently into endemic dwarf species on several islands, including P. creutzburgi on Crete (Greece). Here we test whether this body size reduction was accompanied by changes in bone remodelling. Bone histology of non-weight-bearing ribs was compared between Cretan dwarf (N = 5) and mainland P. antiquus (N = 20) specimens from Greece. Rib cortical bone in both elephant samples displayed dense Haversian bone with evidence of remodelling up to the periosteum. We adjusted osteon densities and area, and Haversian canal to osteon ratio by cortical width. Results were statistically significantly (P < .01) higher in P. creutzburgi than in P. antiquus. This suggests remodelling in the smaller ribs was characterized by larger and more numerous osteons than in P. antiquus. We interpret these findings as indicative of a bone metabolic adaptation to the insularity-driven reduction of body mass in P. creutzburgi. This is possibly a response to its need for deposition of a higher quantity of bone within osteons and through maintaining osteon numbers that are comparable to the mainland ancestor. We propose that despite the size reduction of the skeletal frame of P. creutzburgi, bone quantity was maintained at higher levels to ensure metabolic viability.