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Insular dwarfism in hippos and a model for brain size reduction in 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.

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Figure 1: Relationship between brain size and cranial size for an intraspecific ‘late ontogenetic’ model of dwarfing.
Figure 2: Relationship between brain size and cranial size for an intraspecific ‘ontogenetic’ model of dwarfing.


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We thank A. Currant, C. Lefèvre, C. Sagne, E. Gilissen, F. Renoult, H. Chatterjee, J. Ashby, M. Nowak-Kemp, M. Harman, P. Jenkins, P. Tassy, R. Sabin, R. Symonds and S. Stuenes for facilitating access to museum collections; A. Rasoamiaramanana, G. Ravololonarivo, H. Andriamialison, T. Rakotondrazafy, M. Ramarolahy and S. Bourlat for permission and/or assistance with study of the subfossil material held in the University of Antananarivo and the Académie Malagache; B. Ramanivosoa, D. Gommery, C. Guérin and M. Faure for allowing the study of material at the Akiba Museum, Mahajanga, Madagascar; R. Portela Miguez for assistance with recording endocranial capacity measures from H. amphibius specimens in the Natural History Museum, London; A. Friday for assistance with data collection in the University Museum of Zoology, Cambridge; C. Anderung, J.-R. Boisserie, S. Walsh and V. Herridge for discussion and helpful comments; and J. Kappelman, J. Niven, D. Lieberman and A. Gordon for comments on earlier versions of this manuscript. This research was supported by the Biotechnology and Biological Sciences Research Council.

Author Contributions E.W. and A.L. designed the study. E.W. collected and analysed the data and drafted the paper. Both authors discussed the results and edited the manuscript.

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Correspondence to Eleanor M. Weston.

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Weston, E., Lister, A. Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis. Nature 459, 85–88 (2009).

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