Postnatal growth rates of the dwarf hippo Phanourios minor from the Pleistocene of Cyprus

Cyprus, in the Pleistocene, was extremely isolated from a geological and biogeographical point of view. Only two macromammals successfully colonised the island before the Holocene: Elephas cypriotes, approximately 1.4 metres tall at the withers, and Phanourios minor. The latter is the smallest dwarfed hippo ever found; it stood 70 centimetres at the withers and weighed an estimated 200 kilograms, approximately 10% of its mainland ancestor’s weight. The main objective of this study was to determine the mode of dwarfing in hippopotami from Cyprus using bone histology. Radii and several other long bones of P. minor, and their normal sized relatives (Hippopotamus amphibius), were thin-sectioned and studied under a polarizing optical microscope. Type of bone matrix and counts of lines of arrested growth (LAGs) served as proxies for development rate and time. Histological analyses suggest that Cypriot dwarfed hippos had slower postnatal growth rates than their normal-sized relatives. In fact, Cypriot dwarfed hippos would have needed at least two decades to reach adult size as demonstrated by the radius. Slow growth might have been selected for in a resource-limited but predator-free environment