Size change and the development of mammalian body form: a morphometric and ancient DNA study of island dwarfing

Variation in the form of animals of different species must, of necessity come about by differences in the growth of the organism from egg to adult. Since, on a much longer time-scale, the diversity of animal species arose by a process of evolution, it follows that the evolution of a new body form must have entailed changes in the pattern of individual growth. In this project, we will examine a natural experiment in evolution: the origin of dwarfed mammals on islands. This was a common phenomenon during the Ice Ages (the last two million years or so of Earth history), when dwarfed species of elephants, deer, hippopotami, and other mammals arose from isolated populations on many islands around the world. Are they miniature replicas of their full-sized continental ancestors, or do they differ in shape, and if so, how and why? A remarkable collection of ancient animal remains was discovered on the island of Madagascar. These include a hundred or more beautifully preserved skulls of dwarf hippopotamus, which evolved from animals of the large, mainland African species that had become isolated on the island. We will visit museums to take numerous measurements of these skulls, as well as a sample of male and female skulls of both species of living African hippopotamus. The skulls represent animals from new-born to adult, and by computer-based analysis of the measurement data, we will quantify the differences of size and form between the mainland and island forms, and learn how those differences came about in the growth of the individual animal. In this way we will understand how evolution modifies developmental processes to create new species. One particular idea is that, especially for rapid evolutionary changes, the growth changes are an extension of the differences in development between males and females of the ancestral form. Recent technological advances now mean that it is possible to extract genetic material (DNA) from fossil bones. We will take small samples of bone or tooth from up to 50 skulls of Madagascan dwarf hippo, and submit them to a biochemical procedure that extracts minute quantities of DNA and multiplies the molecular fragments up into millions of copies so that the DNA sequence can be determined. One gene will be compared to those of the two living species, and by determining how different they are, the length of time since they separated in evolution, and hence the duration of the dwarfing process, can be determined. It should also be possible to confirm the suggestions based on skull anatomy, that there was more than one species of dwarf hippo on Madagascar. Radiocarbon dating of the bones will help with these deductions. Another gene, present on the sex chromosomes, can be used to determine the sex of the individual skulls, of importance since in fossils, sexual differences are often confused with differences between species. We will also look briefly at other examples of island dwarfing, to see if similar patterns emerge. These include a dwarf elephant, and other species of dwarf hippo, from Sicily in the Mediterranean. Some of these had longer to evolve than the Madagascan form, so it will be interesting to see if their growth has been modified in different ways. Overall, the research will explore the ways in which the widespread phenomenon of size change is intimately linked to the production of diversity in the living world.

An ontogenic model for rapid phyletic body size change (e.g. dwarfing) will be formulated, based on known modes of developmental modification commonly associated with variation in mammalian body size. A suite of hypotheses, suggesting that rapid body size adjustments produce particular types of developmental changes, will be tested using morphometric methods, statistical growth models, and development sequence data. The aim is to establish whether particular types of developmental modification are linked with rapid body size change, using both interspecific examples of size change such as dwarfing on islands, and intraspecific examples of size change such as sexual dimorphism. The hypothesis that the phenomena of insular dwarfism and sexual size dimorphism share the same developmental basis will thus be tested. A case study of the recently extinct subfossil dwarf hippos from Madagascar will form the core of the project. A large series of exceptionally well preserved crania of different ontogenetic stages, conserved in museums around the world, will be recorded by the capture of a series of x,y,z landmark co-ordinates. These data will be converted to linear distances and statistically analysed to bring out growth-related (allometric) shape variation in comparison with samples of modern African hippopotamus, the putative ancestor. Individual traits will be plotted against each other and against a dentally-determined age axis. Several possible patterns of developmental change will be examined, such as the extension or truncation of a common ontogenetic trajectory; the decoupling of scaling between traits (e.g. between teeth and skulls); and the timing of the closure of cranial sutures (one of the last stages in mammalian skeletal ontogeny). The morphological study will be complemented by the extraction and analysis of ancient DNA from bone and dental samples, including, for the first time, to our knowledge, genetic control of sex for morphological comparisons of fossil mammals. First, the complete mitochondrial cytochrome b gene (1140bp) will be sequenced on all available specimens of Malagasy hippopotamus, and analysed in conjunction with homologous sequences from the two living species. Sequence variation and divergence among samples will provide information on phylogenetic relationships and evolutionary rate. This will allow us to establish the taxonomy of the dwarf forms and, in conjunction with radiocarbon dating of bones, place the morphological changes in a chronological context. Second, the nuclear amelogenin gene, which occurs on both sex chromosomes and shows, almost universally among mammals, variation between x and y, will be sequenced to allow sexing of fossil specimens, so that sexual dimorphism can be explicitly taken into account in the morphological analysis. In addition to the main research on Malagasy hippos, we will examine subfossil dwarf hippos and elephants (Palaeoloxodon, in collaboration with V. Herridge) from Mediterranean islands (chiefly Sicily), where time of isolation was longer. This will help to test the generality of patterns found in the hippos, and bring out possible differences of pattern relating to differing durations of isolation. Finally, we will utilise our existing datasets on modern mammals (primates, rodents and elephants) to provide comparative and baseline analyses of cranial ontogeny and sexual dimorphism.