Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis

Abstract

The diminutive hominin Homo floresiensis, first described in Nature in 2004, lived on the island of Flores in Indonesia until about 14,000 years ago. The cover shows the partial skeleton of the type specimen, LB1. It preserves enough material to permit partial assembly of the foot, and that assembly is now reported by Jungers et al. LB1's foot had human-like fully adducted big toes, but they were longer relative to the rest of the lower limb than in modern humans, instead resembling some apes. The idea that H. floresiensis was a diminutive hominin met with some scepticism, in particular its small brain size was attributed by some to pathology. New work by Eleanor Weston and Adrian Lister shows that the brains of extinct dwarf hippos from Madagascar were disproportionately a lot smaller than those of their mainland relatives. This supports suggestions that island dwarfism breaks the 'rule' that body size reduction in mammals is accompanied by only a moderate reduction in brain size. In News & Views, Daniel Lieberman discusses these papers and a special issue of The Journal of Human Evolution, and concludes that H. floresiensis probably is a bone fide — and very interesting — species of hominin. Body size reduction in mammals is accompanied by only a moderate reduction in brain size, so that the brains of dwarfs are proportionately larger than those of giants. Here, the brains of extinct dwarf hippos from the island of Madagascar are shown to be disproportionately very much smaller than those of their closest mainland relatives. If this trend no longer holds true on islands, it may explain the exceptionally small brain size of the diminutive hominin, Homo floresiensis. Body size reduction in mammals is usually associated with only moderate brain size reduction, because the brain and sensory organs complete their growth before the rest of the body during ontogeny1,2. On this basis, ‘phyletic dwarfs’ are predicted to have a greater relative brain size than ‘phyletic giants’1,3. However, this trend has been questioned in the special case of dwarfism of mammals on islands4. Here we show that the endocranial capacities of extinct dwarf species of hippopotamus from Madagascar are up to 30% smaller than those of a mainland African ancestor scaled to equivalent body mass. These results show that brain size reduction is much greater than predicted from an intraspecific ‘late ontogenetic’ model of dwarfism in which brain size scales to body size with an exponent of 0.35. The nature of the proportional change or grade shift2,5 observed here indicates that selective pressures on brain size are potentially independent of those on body size. This study demonstrates empirically that it is mechanistically possible for dwarf mammals on islands to evolve significantly smaller brains than would be predicted from a model of dwarfing based on the intraspecific scaling of the mainland ancestor. Our findings challenge current understanding of brain–body allometric relationships in mammals and suggest that the process of dwarfism could in principle explain small brain size, a factor relevant to the interpretation of the small-brained hominin found on the Island of Flores, Indonesia6.