Body Size Energetics and Food Acquisition

All organisms require energy for the essential activities of survival, reproduction, and growth. Consequently, knowledge of energetics is central to an understanding of the selective forces that shape an organism's physiology, natural history, and evolution. The environment imposes intense selective pressures on organisms over both short and long time intervals. The occupation of novel environments, or abrupt environmental alterations, for example, can radically alter the pattern of energetic allocation between the essential activities of survival, reproduction, and growth.

The rate at which energy (E) is acquired, transformed, and used is known as the metabolic rate (MR); it drives the rate of all biological activities of and within the organism. Biologists often measure metabolic rate in calories (cal; defined as the energy required to heat 1 g water by 1 °C) or joules (1 cal = 4.1840J). More fundamentally, the metabolic rates of organisms reflect their energetic demands or footprint on the environment. Strikingly, the metabolic rate of mammals scales consistently with body size with an exponent of 3/4 (Figure 2); this is often referred to as the 'mouse to elephant curve'. The 3/4 scaling relationship between metabolic rate and body size was first proposed by Max Kleiber in 1932 and has been the object of intense debate and study. Much of the controversy centered on whether the relationship was related to surface area, which would result in an exponent of 2/3, or whether it reflected other constraints. Several comprehensive studies have firmly demonstrated a 3/4 scaling exponent and extend the relationship to organisms as diverse as microbes, invertebrates, to the largest mammals and trees. Over 90% of the

o 15

-5

Y = 0.71X + C

□ Endotherms (C = 19.50)

Fish (C = 18.47)

Amphibians (C = 18.05)

Reptiles (C = 18.02)

Inverts (C = 17.17)

O Unicells (C = 15.85)

Plants (C = 16.35)

Figure 2 The logarithmic relationship between temperature-corrected metabolic rate (in watts) and body mass (in grams) for various taxa, ranging from unicellular organisms, invertebrates, and different groups of vertebrates, to plants. The overall slope provides an estimate of the allometric exponent; the intercepts are the normalization or proportionality constants for each group (see text). Differences in the intercepts reflect taxon-specific biology. The observed slope of 0.71 ± 2 is close to the predicted value of 3/4. Drawn with permission from Brown JH, Gillooly JF, Allen AP, Savage VM, and West GB (2004) Toward a metabolic theory of ecology. Ecology 85: 1771-1789.

variation in metabolic rate across species can be explained by body mass, with the residual variation reflecting unique evolutionary or biological adaptations specific to particular groups. Although still somewhat controversial, recent studies convincingly demonstrate that the underlying constraints are a result of the design properties of the vascular system. Specifically, the mechanism involves limitations on rates of uptake of resources across surfaces and rates of distribution across fractal-like branching networks within organisms.

The ecological and evolutionary consequences of allo-metric scaling of metabolism are profound. In practical terms it means that each gram of an animal the size of a mouse or shrew uses 20 times more energy than a gram of elephant or giraffe. Thus, food acquisition, processing, and passage rates are typically much more rapid for small animals (Figure 3), and the type of digestive strategy that can be utilized is heavily influenced by body size. True herbivory - that is, the ability to obtain energy from plant structural materials as opposed to relying largely on the easily digestible cell contents - is largely controlled by residence time in the gut or fermentation chamber. If passage rates are rapid as they are for small animals, insufficient time may elapse for the microbes to ferment plant materials and consequently limited energy can be obtained from this digestive strategy. Some small herbivores have evolved specialized adaptations to get around these constraints. These include a highly convoluted cecum and microvilli (increasing surface area), or shunts for selectively retaining materials to effectively increase residence time. While such adaptations may allow more efficient use of plant structural materials, the consequence is that small

Figure 3 While metabolic rate scales allometrically with mass to the 3/4 power, gut capacity is isometric with body mass. Thus, the ratio of metabolic rate to gut capacity scales negatively with mass to the 1/4 (e.g., M0J5/MA = M~025). This fundamental constraint mandates higher-quality food and/or high passage rates for small animals to meet their higher per gram metabolic requirements. Food that takes longer to process (i.e., plant structural materials such as leaves) becomes progressively more difficult to handle and digest. Consequently, the smallest vertebrates are insectivores, subsisting on a ubiquitous and high-energy food source that is relatively easy to process.

Figure 3 While metabolic rate scales allometrically with mass to the 3/4 power, gut capacity is isometric with body mass. Thus, the ratio of metabolic rate to gut capacity scales negatively with mass to the 1/4 (e.g., M0J5/MA = M~025). This fundamental constraint mandates higher-quality food and/or high passage rates for small animals to meet their higher per gram metabolic requirements. Food that takes longer to process (i.e., plant structural materials such as leaves) becomes progressively more difficult to handle and digest. Consequently, the smallest vertebrates are insectivores, subsisting on a ubiquitous and high-energy food source that is relatively easy to process.

herbivores are often energetically limited, which in turn influences other essential activities such as reproduction. Most small animals are much more selective and forage on higher quality resources.

The Influence of Temperature

Temperature plays a crucial role in energetics. The total energy required by an animal is not only a function of size, but is also dependent on whether it maintains a constant body temperature, that is, whether it is endo- (endo = inside) or ectothermic (ecto = outside, thermic = to heat). Endotherms maintain their body temperature within a narrow range by means of heat generated by their metabolism. Maintaining homeothermy (a constant body temperature) consumes ^90% of the energy intake of the animal, but allows activity largely independent of environmental temperatures. Only birds and mammals have adopted this evolutionary pathway - mammals typically maintain core temperatures of ^37-40 °C, while birds maintain slightly higher core temperatures of ^39-43 °C. Ectotherms, such as reptiles, fish, and other taxa, do not utilize metabolic energy to maintain a constant body temperature. Consequently, their absolute energy requirements are considerably less. However, ectotherms are generally incapable of intense activity over sustained periods of unfavorable environmental temperatures. Moreover, the metabolic rate of ectotherms is influenced by ambient environmental temperatures. Some ectotherms behaviorally thermoregulate by shifting among different microclimates to maintain a more consistent core temperature. The thermal inertia resulting from huge body masses achieved by sauropods in the Mesozoic probably meant that they were, for all practical purposes, homoeothermic. The implications of endo-versus ectothermy go beyond differing metabolic requirements. In general, ectotherms grow slower and mature at a larger body size in colder environments. Fecundity is related to adult body size, with larger individuals having larger clutches.

It is well known that rate of biological activity rises exponentially with temperature. Mechanistically, this is because the increase in the kinetic energy of molecules results in substrates colliding with active sites more frequently. Physiologists express the relationship between metabolic rate and temperature as the Q10, the rate increase for each 100 rise in temperature. A Q10 of 2, for example, means that the metabolic rate doubles for each 10 rise.

Recently, investigators have incorporated the important influence of temperature on metabolic rate (MR) by adding the Boltzmann factor to the allometric equation relating it to body size. In this formulation, the relationship is stated:

where E represents the activation energy, k is the Boltzmann's constant, and T is the absolute temperature in degrees kelvin. Similarly, the rates of many other fundamental life-history traits demonstrate temperature dependence. Thus, the addition of the Boltzmann factor into allometric relationships allows comparisons across the entire range of living organisms, regardless of thermal regulation regime, life history, or size (Figure 2). The robust relationships found suggest that the combined effects of body size, temperature, and resource supply constrain metabolic and other fundamental biological rates for all taxa.

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