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ScienceWeek
EVOLUTIONARY BIOLOGY: A GIANT FOSSIL RODENT
The following points are made by R. McNeill Alexander (Science 2003 301:1678):
1) The rodents we are familiar with are small. Mice weigh about 30 g, and rats about 300 g. By comparison, the largest living rodent, the South American capybara, seems gigantic. Its adult body mass is about 50 kg, as much as that of a typical sheep. But some extinct rodents were even larger. Phoberomys, the largest of all, is a member of the Caviomorpha, the group that includes the guinea pig. Until recently, it was known only from isolated teeth and bone fragments. But Sánchez-Villagra et al (1) have recently reported that most of the skeleton of one individual and part of the skull of another have been found in Upper Miocene rocks in Venezuela. The Miocene Epoch extended from 24 million to 5 million years ago, and these particular rocks seem to have been formed in wetlands. The newly described fossils greatly extend our knowledge of this giant rodent and show that it must have weighed about 700 kg, as much as a buffalo.
2) The body mass of terrestrial vertebrates is often calculated from the diameters of their leg bones, because these bones must be thick enough to support the animal's weight. The calculations would be easy if large animals were exact, scaled-up models of their smaller relatives. If they were, we could estimate that Phoberomys, with a femur diameter 18 times that of a 0.5-kg guinea pig, had (18)^(3) or about 5800 times the mass of the guinea pig. Measurements of modern caviomorphs show, however, that body mass is not proportional to (femur diameter)(FD)(3), but instead is approximately proportional to (FD)^(2.5) (2). This is the basis for the mass estimate of 700 kg ((18)^(2.5) = 1400) by Sanchez-Villagra and colleagues.
3) Relationships between bone dimensions and body mass are not precise. Some animals have unusually thick bones for their body mass, and some have unusually thin ones. This leads to widespread uncertainty when, as in the case of Phoberomys, body mass is estimated by extrapolation from much smaller animals. The uncertainty is increased by variation in body proportions (3). Phoberomys has relatively weak forelegs, and a mass estimate based on humerus diameter is only 440 kg. The mass estimate based on femur size is judged to be the more reliable one, but even the mass estimate based on humerus size is huge by rodent standards. The obvious reaction to the Phoberomys size estimate is amazement. What enabled this giant to so vastly exceed the size range of normal rodents? What are the problems and benefits of being a giant?
4) The relationship between bone diameter and body mass suggests at first sight that Phoberomys might have had trouble supporting its own weight. Its femur had a cross-sectional area about (18)^(2) or 324 times that of a guinea pig femur, but had to support a body estimated to have been 1400 times as heavy. That may seem to be a recipe for disaster. Large mammals, however, stand with their legs much straighter than small ones. For example, small rodents have a crouching stance, with their elbows and knees strongly bent, but elephants keep their legs almost straight, like supporting pillars. This relationship between size and posture ensures that similar stresses act in the leg bones of mammals of all sizes(4). Unlike smaller rodents, the capybara stands on relatively straight legs, much more like a sheep than a guinea pig. Phoberomys presumably stood with its legs even straighter. Seen from a distance, it would have looked much more like a buffalo than like a scaled-up guinea pig.(5)
References (abridged):
1. M. R. Sánchez-Villagra, O. Aguilera, I. Horovitz, Science 301, 1708 (2003)
2. A. R. Biknevicius, D. A. McFarlane, R. D. E. McPhee, Am. Mus. Novit. 3079, 1 (1993)
3. M. Fortelius, J. Kappelman, Zool. J. Linn. Soc. 108, 85 (1993)
4. A. A. Biewener, Science 245, 45 (1989)
5. I. J. Gordon, A. W. Illius, J. Anim. Ecol. 65, 18 (1996)
Science http://www.sciencemag.org
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SIZES OF PLANTS AND ANIMALS
The following points are made by John Damuth (Proc. Nat. Acad. Sci. 2001 98:2113):
1) The relationship of body size to the anatomical, physiological, behavioral, and ecological characteristics of animals has long been a focus of interest in zoology. As one considers animal species of different sizes, regular, predictable changes are seen in the relative proportions of the body's organs and the relative rates of physiological processes such as metabolism and growth. Students of zoology are familiar with these scaling relationships (also called "allometries") and many of their ecological and adaptive implications. For example, the relative scaling of metabolism versus that of the volume of the digestive tract affects the potential diets of herbivorous mammals, which in turn influences their social behavior.
2) Plant biology, on the other hand, does not have a long history of investigation of issues involving the scaling of physiological processes versus body-size, despite a wealth of detailed data on plant morphology and function. This situation is perhaps because plants are seen to exhibit degrees of modular construction, indeterminate growth, and variety of form greater than those shown by animals, so the idea of a plant species even having a "body size" strikes some as problematical. Nevertheless, plant species do have characteristic shapes and sizes and span 20 orders of magnitude in body mass. Niklas's 1994 book on plant allometry has been described fairly as the first attempt to provide a unified treatment of plant form and function from an allometric perspective. However, until even more recently, the scaling of such basic processes as metabolism and growth had remained undocumented for a representative sample of plant species. New analyses reveal that growth scales among plants in the same way that it does among animals, and further underscores the growing realization that the same scaling rules may apply to both animals and plants, and for much the same reasons.
3) Growth rates, or rates of production of new biomass, are of fundamental importance in linking physiological processes to adaptively important features such as reproductive rates and other life history variables. Among animal species, rates of biomass production and growth are proportional to metabolic rate, which scales as the 3/4 power of body mass (M). This proportionality, where organismal growth rate scales as M^(0.75), makes intuitive sense. Cells should divide or otherwise do work at rates roughly proportional to the rates at which they are supplied with energy. Across different species, these rates should be the rates of metabolism, less the energy used for physiological maintenance and ecological demands, and energy lost as heat. Previous work has strongly suggested that plant nutrient flux used for photosynthesis scales as M^(0.75). This result implies that plant growth rates should also scale as M^(0.75), a value confirmed by Niklas and Enquist (2001). Further emphasizing this connection between plant metabolic processes and growth rates is the additional demonstration that the anatomical measures of an individual's photosynthetic pigment volume (and thus its presumed ability to obtain energy) also scale as M^(0.75).
Proc. Nat. Acad. Sci. http://www.pnas.org
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DINOSAURS, DRAGONS, AND DWARFS: THE EVOLUTION OF MAXIMAL BODY SIZE
The following points are made by G.P. Burness et al (Proc. Nat. Acad. Sci. 2001 98:14518):
1) The size and taxonomic affiliation of the largest locally present species ("top species") of terrestrial vertebrate vary greatly among faunas, raising many unsolved questions. Why are the top species on continents bigger than those on even the largest islands, bigger in turn than those on small islands? Why are the top mammals marsupials on Australia but placentals on the other continents? Why is the world's largest extant lizard (the Komodo dragon) native to a modest-sized Indonesian island, of all unlikely places? Why is the top herbivore larger than the top carnivore at most sites? Why were the largest dinosaurs bigger than any modern terrestrial species?
2) A useful starting point is the observation of Marquet and Taper (1998), based on three data sets (Great Basin mountaintops, Sea of Cortez islands, and the continents), that the size of a landmass's top mammal increases with the landmass's area. To explain this pattern, they noted that populations numbering less than some minimum number of individuals are at high risk of extinction, but larger individuals require more food and hence larger home ranges, thus only large land masses can support at least the necessary minimum number of individuals of larger-bodied species. If this reasoning were correct, one might expect body size of the top species also to depend on other correlates of food requirements and population densities, such as trophic level and metabolic rate. Hence the authors assembled a data set consisting of the top terrestrial herbivores and carnivores on 25 oceanic islands and the 5 continents to test 3 quantitative predictions:
a) Within a trophic level, body mass of the top species will increase with land area, with a slope predictable from the slope of the relation between body mass and home range area.
b) For a given land area, the top herbivore will be larger than the top carnivore by a factor predictable from the greater amounts of food available to herbivores than to carnivores.
c) Within a trophic level and for a given area of landmass, top species that are ectotherms will be larger than ones that are endotherms, by a factor predictable from ectotherms' lower food requirements.
3) The authors point out that on reflection, one can think of other factors likely to perturb these predictions, such as environmental productivity, over-water dispersal, evolutionary times required for body size changes, and changing landmass area with geological time. Indeed, the database of the authors does suggest effects of these other factors. The authors point out they propose their three predictions not because they expect them always to be correct, but because they expect them to describe broad patterns that must be understood in order to be able to detect and interpret deviations from those patterns.
Proc. Nat. Acad. Sci. http://www.pnas.org
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