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

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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.