Bioenergetics Of Wild Herbivores

Robert J. Hudson

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315891118
• eBook ISBN: 9781351087117
• Topic: Medicine
• Subtopic: Veterinary Medicine

Bioenergetics is an emerging discipline which offers a more profound understanding of the ecology, behaviour, and evolution of wild herbivores. Increasingly, bioenergetic principles have been applied in management since they provide insight into population dynamics and are relevant to manipulation of habitats and assessment of the impacts of resource development. Growing interest in the agricultural potential of wild herbivores has provided further impetus. In spite of this promise, there are few comprehensive syntheses of the concept and its application to wild herbivores. This volume attempts to fill this need. This book provides a great amount of detail but its expressive aim is to lead us to the whole animal, to a herd, to population as integral parts of an ecological entity which in turn is the result of evolutionary forces.The concept of this book promises the realization of an overdue change in the approach to bioenergetics, to nutrition and husbandry, and thus to the management of wild herbivores: the final emancipation from rules and views based primarily on domesticated herbivores or on experimental animals held under unnatural conditions, necessarily impending them behaviourally, physically, and psychically.

Chapter 1

BODY SIZE, ENERGETICS, AND ADAPTIVE RADIATION

R. J. Hudson

TABLE OF CONTENTS

I. Size and Shape

II. Energy Budgets

A. Energy Costs

B. Digestive Capacity

C. Bioenergetic Constraints

III. Productive Functions

A. Growth and Development

B. Reproductive Processes

IV. Life History and Demography

A. Vital Parameters

B. Demographic Strategies

V. Behavioral Ecology

A. Gregariousness

B. Male Mating Systems

C. Home Range

D. Resource Selection

E. Activity Budgets

VI. Grazing Systems Dynamics

A. Biomass and Productivity

B. Plant-Herbivore Interactions

VII. Synthesis

A. Size as an Adaptive Strategy

B. The Large Herbivore Niche

C. Future Directions

References

The ungulates (and subungulates = elephants) include approximately 203 surviving species belonging to 3 orders, 13 families, and 83 genera (Table 1). Their condylarth ancestors appeared in the Paleocene and formed at least five lines; Ceie, Meridungidata, Phenacodonta, Eparctocyon, and Tethytheria.² The latter three gave rise to the modern ungulate orders Perissodactyla, Artiodactyla, and Proboscidea, respectively.

Perissodactyls (odd-toed ungulates) are a very ancient group with an origin dating 50 million years to the late Paleocene. They diversified and became the most numerous herbivore until the Miocene. The emergence of ruminant digestion by the advanced Artiodactyla presumably led to competition and ultimately the demise of this Order.³

Artiodactyls (even-toed ungulates) represent perhaps the most important mammalian radiation. The significant feature of their evolution since their appearance in the Eocene is the development of ruminant digestion. The earliest suborders, Suina (pigs, peccaries, and hippos) and Tylopoda (camels), show various degrees of anatomical elaboration of the digestive tract to accommodate microbial fermentation. The camelids developed a superior pregastric system of fermentation which parallels development of the true ruminant system. But the typical multicompartmented rumen traces its origin to the forest-dwelling tragulids. This digestive theme contributed to the progressive dominance of the Ruminantia in the Oligocene and Miocene.⁴

Proboscids (elephants) originated in the Eocene, perhaps from a semi-amphibious ancestor similar to (but apparently not) Meriotherium, and attained peak diversity in the Pliocene.⁵ There is clear evidence of progressive adaptation to coarser diets in dentition and increasing body size. Pleistocene extinction dramatically reduced their ranks to two surviving species, the African and Asian elephants.

These three orders present remarkable diversity of size, form, and function. An important question relates to the forces forging this adaptive radiation. Another is whether evolution of ungulates can be viewed in terms of individually adaptive traits. Or, are there design constraints which cause such traits to be linked in adaptive suites?

The fitness of genes is tested through the interaction of their carrier, the phenotype, with environment. Ultimately, genetic success is determined by the complex relationship between reproductive rate and reproductive life. Proximally, fitness can be equated with the success of phenotypes in avoiding predation, competing for males,⁶, ⁷, ⁸, ^{and 9} and tranforming resources. We should not expect these factors to be equally important in all stages of evolution. Geist^6-9 argues that during dispersal into rich vacant niches, social factors should emerge as the main selective force while bioenergetic efficiency should be more important under conditions of greater density.

Collectively, the imperatives of security, social dominance, and efficient resource transformation confront wild herbivores with a number of potentially complex compromises. An important area of inquiry probes the adaptiveness of traits which determine the way animals experience these tradeoffs and hence conduct their daily lives. This question can be addressed in two ways.

The first is to examine the association of phenotypic traits with ecological circumstances.¹² The potential problem with this a posteriori method is one of confusing correlation with causation particularly when it is uncritically accepted that all traits are adaptive. Of course, neutral or even maladaptive traits may be present because evolutionary change may lag behind a changing environment. Certain traits may simply be genetically linked to others which have adaptive significance. The tendency to imagine adaptations where they do not exist has been termed the Panglossian Fallacy.¹¹

The second approach is predictive rather than deductive. It is based on a priori consideration of the sorts of traits which would enhance fitness followed by the construction of testable hypotheses. There are several reasons for modest progress in this area. First, there is the uncertainty of how natural selection operates on quantitative traits whose phenotypic expression varies with environmental opportunities.⁹ Second, the adequacy of widely applied optimizing criteria remains open to question (Chapter 3). Third, it is difficult to select an appropriate proxy for neo-Darwinian fitness given the divergent nature of selective forces.

Table 1
TAXONOMY OF EXTANT UNGULATES AND SUBUNGULATES

This volume deals primarily with the success of large herbivores as resource transformers. Although a variety of nutrients are relevant to the efficiency of this transformation, energy is used as a convenient tangible currency. This chapter evaluates the linkages of bioenergetics, ecology, and social behavior, and probes their allometric basis in body size.

I. Size and Shape

Compared with other mammals, ungulates and subungulates are relatively large.² Body weights of modern species (data available for 177) form a log-normal distribution. However, there are several discontinuities in untransformed distributions which allow identification of three or perhaps four groups (Figure 1). Most ungulates weigh less than 50 kg and are selective feeders. Medium ungulates range from 125 to 325 kg and include a variety of feeding styles. Large ungulates weighing up to 625 kg are mostly nonselective grazers. Megaherbivores are scattered upwards in weight distribution and include an assortment of browsers, grazers, and mixed feeders.

Sexual dimorphism in weight varies widely. In many forest-dwelling microherbivores, females are similar to or even slightly larger than males¹⁴ but, in most medium and large species, males are larger because of the importance of size in social dominance and mating enhancement. In highly gregarious species with little social segregation, sexual dimorphism in size and coloration may again decline to confer the advantages of “hiding” in groups. Since males seem to be large for social reasons, females probably are close to the bioenergetic optimum size for each species and their weights rather than average weights appear most appropriate for allometric scaling.

FIGURE 1. Frequency distribution of body weights in living ungulates. Parameters of the log-normal distribution are χ = 216 kg, SD = 442. n = 177.

A persistent theme in evolution has been to increase body size, a tendency which has been accompanied by consistent changes in body shape and adornment with horns, antlers, beards, or manes. Gradients of size and conformation form can be arranged with fossil as well as living forms at almost any taxonomic level although the clearest picture exists within phyletic lines.^6-9 These evolutionary changes are associated with the expansion of vacant habitats such as grasslands, deserts, floodplains, and tundra-steppes (Figure 2). A common feature is that new opportunities are afforded in habitats with a relative surfeit of potential forage. Typically, these habitats are open which encourages gregariousness to minimize the individual’s risk of predation. Under conditions of forage abundance, herbivores are released from bioenergetic constraints so fuller expression of genetic potential for growth and development can be achieved. Thi...

Table of contents

• Cover
• Title Page
• Copyright Page
• Foreword
• Preface
• Editors
• Contributor
• Table of Contents
• Chapter 1 Body Size, Energetics, and Adaptive Radiation
• Chapter 2 Forage and Range Evaluation
• Chapter 3 Foraging Behavior: Dynamics of Dining Out
• Chapter 4 Regulation of Forage Intake
• Chapter 5 Digestion
• Chapter 6 Maintenance Metabolism
• Chapter 7 Incremental Cost of Activity
• Chapter 8 Thermoregulation in Ungulates
• Chapter 9 Growth and Development
• Chapter 10 Pregnancy and Lactation
• Chapter 11 Assessment of Nutritional Status
• Chapter 12 Computer Simulation of Energy Budgets
• Index