John Damuth

Supply–demand balance and metabolic scaling

It is widely accepted that metabolic rates scale across species approximately as the 3/4 power of mass in most if not all groups of organisms. Metabolic demand per unit mass thus decreases as body mass increases. Metabolic rates reflect both the ability of the organisms transport system to deliver metabolites to the tissues and the rate at which the tissues use them. We show that the ubiquitous 3/4 power law for interspecific metabolic scaling arises from simple, general geometric properties of transportation networks constrained to function in biological organisms. The 3/4 exponent and other observed scaling relationships follow when mass-specific metabolic demands match the changing delivery capacities of the network at different body sizes. Deviation from the 3/4 exponent suggests either inefficiency or compensating physiological mechanisms. Our conclusions are based on general arguments incorporating the minimum of biological detail and should therefore apply to the widest range of organisms.

On the relationship between hypsodonty and feeding ecology in ungulate mammals, and its utility in palaeoecology

High‐crowned (hypsodont) teeth are widely found among both extant and extinct mammalian herbivores. Extant grazing ungulates (hoofed mammals) have hypsodont teeth (a derived condition), and so extinct hypsodont forms have usually been presumed to have been grazers. Thus, hypsodonty among ungulates has, over the past 150 years, formed the basis of widespread palaeoecological interpretations, and has figured prominently in the evolutionary study of the spread of grasslands in the mid Cenozoic. However, perceived inconsistencies between levels of hypsodonty and dental wear patterns in both extant and extinct ungulates have caused some workers to reject hypsodonty as a useful predictive tool in palaeobiology, a view that we consider both misguided and premature.

The origins and evolution of the North American grassland biome: the story from the hoofed mammals

Abstract The North American grassland biome first appeared around 18 Ma in the mid Miocene. The familiar story of the Neogene evolution of this biome is of the replacement of ungulates (hoofed mammals) having a primarily browsing diet by the more derived grazing ungulates. However, new data show a more complicated pattern of faunal succession. There was a maximum taxonomic diversity of ungulates at 16–14 Ma, including a large number of grazers, and the subsequent decline in overall diversity was largely due to the decline of the browsers, with little corresponding increase in the grazers. Additionally the mid Miocene faunas (∼18–12 Ma) contained a much greater number of browsers than any comparable present-day habitat. We discuss possible explanations for these non-analogous grassland faunas, including the possibility that the primary productivity of the vegetation was greater in the early to middle Miocene than it is today. One possible explanation for increased primary productivity is higher Miocene levels of atmospheric carbon dioxide than in the present day. The proposed difference in vegetational productivity also may explain why horses radiated as the main grazers in North America, in contrast to the radiation of antelope in the Plio–Pleistocene African grasslands.

A general basis for quarter-power scaling in animals

It has been known for decades that the metabolic rate of animals scales with body mass with an exponent that is almost always <1, >2/3, and often very close to 3/4. The 3/4 exponent emerges naturally from two models of resource distribution networks, radial explosion and hierarchically branched, which incorporate a minimum of specific details. Both models show that the exponent is 2/3 if velocity of flow remains constant, but can attain a maximum value of 3/4 if velocity scales with its maximum exponent, 1/12. Quarter-power scaling can arise even when there is no underlying fractality. The canonical “fourth dimension” in biological scaling relations can result from matching the velocity of flow through the network to the linear dimension of the terminal “service volume” where resources are consumed. These models have broad applicability for the optimal design of biological and engineered systems where energy, materials, or information are distributed from a single source.

SELECTION AMONG "SPECIES": A FORMULATION IN TERMS OF NATURAL FUNCTIONAL UNITS

The unit that directly evolves under the action of higher‐level natural selection “among species” must be the higher‐level analogue of the population. Contrary to present formulations of “species selection,” clades (or other higher taxa) do not fulfill the basic structural and dynamic criteria to be so considered. Clades are not localized, their members do not share an environment, and they cannot be said to respond to local selective regimes. Traditional species selection does not provide a causal mechanism for evolutionary change in terms of the interaction of the units of selection with a shared environment in the way that conventional organismic selection does; as used by some authors, species selection is a purely descriptive term. Communities do fulfill the criteria required by a theory of natural selection. Within communities, selection is among the populations of different species that make up the community, here termed “avatars” of those species. Avatars are the closest analogues of individual organisms in traditional selection theory. Just as populations evolve by organismic selection, communities evolve by avatar selection, and more inclusive units, the higher‐level analogues of the species, evolve as their component communities do. This formulation of higher‐level selection reveals a congruence with processes at the lower, organism‐based level and suggests the most profitable direction to be taken in attempts at formal extension of selection theory.

CLIMATE CHANGE AND SIZE EVOLUTION IN AN ISLAND RODENT SPECIES: NEW PERSPECTIVES ON THE ISLAND RULE

Abstract As stated by the island rule, small mammals evolve toward gigantism on islands. In addition they are known to evolve faster than their mainland counterparts. Body size in island mammals may also be influenced by geographical climatic gradients or climatic change through time. We tested the relative effects of climate change and isolation on the size of the Japanese rodent Apodemus speciosus and calculated evolutionary rates of body size change since the last glacial maximum (LGM). Currently A. speciosus populations conform both to Bergmanns rule, with an increase in body size with latitude, and to the island rule, with larger body sizes on small islands. We also found that fossil representatives of A. speciosus are larger than their extant relatives. Our estimated evolutionary rates since the LGM show that body size evolution on the smaller islands has been less than half as rapid as on Honshu, the mainland‐type large island of Japan. We conclude that island populations exhibit larger body sizes today not because they have evolved toward gigantism, but because their evolution toward a smaller size, due to climate warming since the LGM, has been decelerated by the island effect. These combined results suggest that evolution in Quaternary island small mammals may not have been as fast as expected by the island effect because of the counteracting effect of climate change during this period.

History matters: ecometrics and integrative climate change biology

Climate change research is increasingly focusing on the dynamics among species, ecosystems and climates. Better data about the historical behaviours of these dynamics are urgently needed. Such data are already available from ecology, archaeology, palaeontology and geology, but their integration into climate change research is hampered by differences in their temporal and geographical scales. One productive way to unite data across scales is the study of functional morphological traits, which can form a common denominator for studying interactions between species and climate across taxa, across ecosystems, across space and through time—an approach we call ‘ecometrics’. The sampling methods that have become established in palaeontology to standardize over different scales can be synthesized with tools from community ecology and climate change biology to improve our understanding of the dynamics among species, ecosystems, climates and earth systems over time. Developing these approaches into an integrative climate change biology will help enrich our understanding of the changes our modern world is undergoing.

A Macroevolutionary Explanation for Energy Equivalence in the Scaling of Body Size and Population Density

Across a wide array of animal species, mean population densities decline with species body mass such that the rate of energy use of local populations is approximately independent of body size. This “energetic equivalence” is particularly evident when ecological population densities are plotted across several or more orders of magnitude in body mass and is supported by a considerable body of evidence. Nevertheless, interpretation of the data has remained controversial, largely because of the difficulty of explaining the origin and maintenance of such a size‐abundance relationship in terms of purely ecological processes. Here I describe results of a simulation model suggesting that an extremely simple mechanism operating over evolutionary time can explain the major features of the empirical data. The model specifies only the size scaling of metabolism and a process where randomly chosen species evolve to take resource energy from other species. This process of energy exchange among particular species is distinct from a random walk of species abundances and creates a situation in which species populations using relatively low amounts of energy at any body size have an elevated extinction risk. Selective extinction of such species rapidly drives size‐abundance allometry in faunas toward approximate energetic equivalence and maintains it there.

Scaling of growth: Plants and animals are not so different

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 bodys 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 (1–3). 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 (4, 5).

Population ecology: Common rules for animals and plants

The relationship between the size and the number of plants in natural stands of vegetation has generally been taken to be governed by geometric considerations. On this basis, theory has it that average plant size scales as the -3/2 power of population density. If, however, energy use is taken as the determining factor, the exponent becomes -4/3 — which happens to be the exponent that applies to comparable relationships in animals.