We know that animals come in a whole range of different shapes and sizes – there is an immense variety in form, ranging from the smallest of flatworms to the blue whale. One area of real interest to scientists is understanding how form changes with size and why. Are there limits to size that are imposed by the basic materials that make up an animal’s body, or the arrangement of body parts that have been inherited?
Take modern mammals, for example: the Blue Whale at 180 tonnes shows that mammals can grow to a very large size in a marine environment with modifications to the basic mammalian body plan. But on land, elephants are the largest living mammals with a maximum mass of over 10 tonnes – around half the probable mass of Paraceratherium which lived approximately 20-40 million years ago. The largest land animal is likely to have been the dinoasaur Argentinosaurus with a mass of up to 60 tonnes, living 95 million years ago.
Some body parts can be in the same proportion in small and large animals (isometric), but other characteristics will be in different proportion in large and small animals (allometric) – so the diameter of leg bones is thicker in proportion to body size in larger animals than in smaller animals.
A Museum collection provides an ideal resource to compare different body structures and biomechanics, and to try to explain how and why body plans change with size and environment: tens of thousands of individual organisms are available for study, with expert support from curators.
How does internal bone structure change with size? Dr Sandra Shefelbine of Imperial College London with colleagues from IC and the Royal Veterinary College studied bones from ninety species of mammal and bird ranging in size from shrews to elephants, using the Natural History Museum’s collections with other material from the University of Cambridge and the Zoological Society of London.
Their study, funded by the UK Biotechnology and Biological Sciences Research Council (BBSRC) and published in the Proceedings of the Royal Society B, looked at the structure of the spongy bone near joints that helps sustain impact and weight when the animal jumps or walks. They showed that the density of spongy bone near joints was very uniform between species but that the internal struts (trabeculae) that give the bone its spongy appearance got thicker and further apart as species got larger.
What this work helps to demonstrate is that some shared characteristics of different organisms can change more than others as body sizes increase. So the larger animals have larger bones in absolute terms, but the bone does not get more dense to cope with higher body mass – the relative thickness of the trabeculae increases instead. So rather than having denser bone (which would require more resource to grow and require relatively more energy to move) to sustain greater weight, the structure of bone changes allometrically in larger species.
BBSRC have supported this work because of the fundamental interest of the science but also because understanding of the mechanics of bone structures can support other work to combat fractures and osteoporosis. Professor Douglas Kell, BBSRC Chief Executive said: "Bones are remarkably versatile structures able to produce intricate mechanisms in the ear and to support the weight of an elephant. However, in elderly people bones can become fragile and prone to breakages which can lead to serious health problems and drastically reduce quality of life. This research has increased our understanding of how bones have evolved across the animal kingdom and may lead to new insights about how to keep them strong and healthy."
Michael Doube, Michal‚ M. Klosowski, Alexis M. Wiktorowicz-Conroy, John R. Hutchinson, and Sandra J. Shefelbine (2011) Trabecular bone scales allometrically in mammals and birds Proc. R. Soc. B published online before print March 9, 2011, doi:10.1098/rspb.2011.0069