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From Jumbo to Dumbo: Cranial Shape Changes in Elephants and Hippos During Phyletic Dwarfing

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Abstract

Members of the mammalian families Elephantidae and Hippopotamidae (extant and extinct elephants and hippos) include extinct dwarf species that display up to 98% decrease in body size compared to probable ancestral sources. In addition to differences in body mass, skulls of these species consistently display distinctive morphological changes, including major reduction of pneumatised areas in dwarf elephants and shortened muzzles in dwarf hippos. Here we build on previous studies of island dwarf species by conducting a geometric morphometric analysis of skull morphology and allometry in target taxa, living and extinct, and elaborate on the relation between skull size and body size. Our analysis indicates that skull size and body size within terrestrial placental mammals scale almost isometrically (PGLS major axis slope 0.906). Furthermore, skull shape in dwarf species differed from both their ancestors and the juveniles of extant species. In insular dwarf hippos, the skull was subject to considerable anatomical reorganisation in response to distinct selection pressures affecting early ontogeny (the “island syndrome”). By contrast, skull shape in adult insular dwarf elephants can be explained well by allometric effects; selection on size may thus have been the main driver of skull shape in dwarf elephants. We suggest that a tightly constrained growth trajectory, without major anatomical reorganization of the skull, allowed for flexible adaptations to changing environments and was one of the factors underlying the evolutionary success of insular dwarf elephants.

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Acknowledgements

We thank Steven van der Mije and Wendy van Bohemen (RMNH), Eileen Westwig and Neil Duncan (AMNH), Laurence Heaney and the late William Stanley (FMNH), Carolina di Patti (MGG), Oliver Hampe, Frieder Mayer and Nora Lange (MFN), Christine Argot and Christine Lefèvre (MNHN), Rainer Brocke and Christine Hertler (SFN), Reinhard Ziegler (SMNS), Darrin Lunde and Nicole Edmison (USNM), Chloe Adamopoulou (ZMUA), Pasquale Raia and Mariella Del Re (MPUN) and Pip Brewer (Natural History Museum, London) for allowing us to study the skulls in their care and their assistance when skulls were too heavy to handle two-handed. We further thank Rutger Vos for clarifying PGLS regressions, and Gert van den Bergh (Centre for Archaeological Science, University of Wollongong), Maria Rita Palombo (‘La Sapienza’ University of Rome), Athanassios Athanassiou (Hellenic Ministery of Culture), Adrian Lister, Chris Stringer and Victoria Herridge (Natural History Museum, London) for discussions we had on island dwarfs.

Funding

GL received support from the SYNTHESYS Project (GB-TAF-6355 and FR-TAF-6549). The research of AVDG has been co-financed by the European Union (European Social Fund—ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)—Research Funding Program: THALIS—UOA-Island biodiversity and cultural evolution: examples from the Eastern Mediterranean, Madagascar, Mauritius and Philippines during the past 800,000 years (MIS375910, KA:70/3/11669).

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Authors and Affiliations

  1. Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA, Leiden, The Netherlands

    Alexandra A. E. van der Geer

  2. Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, Athens, Greece

    George A. Lyras

  3. Department of Theoretical Biology, University of Vienna, Vienna, Austria

    Philipp Mitteroecker

  4. Division of Vertebrate Zoology (Mammalogy), American Museum of Natural History, New York, USA

    Ross D. E. MacPhee

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  1. Alexandra A. E. van der Geer
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van der Geer, A.A.E., Lyras, G.A., Mitteroecker, P. et al. From Jumbo to Dumbo: Cranial Shape Changes in Elephants and Hippos During Phyletic Dwarfing. Evol Biol 45, 303–317 (2018). https://doi.org/10.1007/s11692-018-9451-1

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