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1 February 2011 The Biology of Small Mammals
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At least 90 percent of the 5416 known species of living mammals are “small.” They are cosmopolitan in distribution and the subjects of extensive research, most notably as model organisms in medical research. Because they are often common members of their communities, they frequently play major roles in the structure and function of community systems. Furthermore, they can invade agricultural crops and carry diseases transmitted to humans.

The Biology of Small Mammals provides an overview of the diversity of life histories exhibited among small mammals, and thereby promotes increased interest, appreciation, and understanding of these creatures that impinge so much on human life. Author Joseph F. Merritt, a senior mammalogist with the Illinois Natural History Survey (University of Illinois, Urbana), is a respected scientist with many years of experience researching these interesting and important organisms. His book aims for a “broad readership” ranging from amateur naturalists and students to wildlife professionals (p. xi, xiii). He makes no pretense to be comprehensive but writes engagingly about the species he simply finds particularly unusual or informative. He defines “small mammal” as those species weighing 5 kilograms (11 pounds) or less; however, most appropriately, he ignores this arbitrary boundary whenever it seems sensible to do so.

After an introductory chapter that lays out the scope and context of the book, Merritt divides 12 additional chapters into three parts: “Modes of Feeding,” “Environmental Adaptations,” and “Reproduction.” These chapters are followed by a one-page list of useful Web sites, a 10-page glossary, literature cited, and an index (17 pages). Some organizational anomalies occur, but this is not surprising since life histories resist being compartmentalized according to just one or a few of their attributes.


The Biology of Small Mammals has many important strengths: (a) In most cases, chapters begin with a brief overview of the taxonomic context of the chapter's subject matter; (b) Merritt often provides a welcome historical context for the topic being discussed; (c) the information is up to date, an example being the discussion of the white-nose syndrome currently devastating bats in northeastern North America; (d) the book contains a plethora of illustrations, their sources carefully documented; (e) there are few typos or spelling errors; and (f) units for body mass, area, and temperature are given in both metric units or Celsius and English units or Fahrenheit.

However, careless errors, contradictory statements, poor word choices, organizational problems, and an apparent lack of serious editing compromise the author's good intentions and enthusiastic treatment of the subject matter. I will mention only a few illustrative examples of these transgressions that unfortunately pervade all chapters. The introductory chapter includes brief overviews of those 14 orders of living mammals that contain small-sized species, and here we find the word “fertility” used in the way that most biologists use the term “fecundity”; neither is explicitly defined. There are also serious errors in the use of the terms “altricial” and “precocial,” although both terms are defined correctly in the glossary.

Part 1 on feeding modes is organized into chapters on insectivory, herbivory, carnivory, and omnivory, and these are particularly rich in information. Chapter 3 on herbivory includes many highlights, including separate sections on colugos (Dermoptera) and hyraxes (Hyracoidea), and a treatment of the interesting but seldom discussed feeding mode of gummivory. Chapter 4 is on carnivory, where the author struggles with, but does not resolve, the semantic issue of what is meant by “carnivore.” Is it members of the order Carnivora, or mammals that “feed primarily on animal material”? Adding confusion, this discussion is followed by a section titled “General Characteristics” that deals only with the order Carnivora, thus equating carnivory with this taxon. Three sections follow: “Flesh-eating Carnivores,” “Piscivores,” and “Sanguinivores.” A reference to sand cats entering torpor implies that aestivation, and by implication, the winter lethargy indulged in by bears, is the same as torpor. The definition of “torpor” in the glossary is no help, as it does not even exclude ordinary sleep.

The final chapter in this first part is on omnivory; it begins with the statement, “Each order of mammals contains omnivorous species.” However, I can think of at least six orders that are not habitually omnivorous. Nevertheless, omnivory is widespread among mammals, and is discussed in three sections: “Omnivorous Carnivores” (meaning members of the order Carnivora), “Mycophagy,” and “Case Studies.” The section on mycophagy focuses mostly on herbivores that include fungi in their diets. Sporocarps are confused with spores in several places, and the widespread mutualism between mycorrhizal fungi and trees is mistakenly termed “a mycorrhiza.” The case studies are about insectivores, not omnivores, plus there is a misplaced paragraph on nectarivorous bats.

Part 2 is vaguely titled “Environmental Adaptations.” Its five chapters (6–10) concern only adaptations to temperature extremes and water scarcity. The introductory comments include inaccurate statements, such as: in deserts “small mammals...conserve water by...using metabolic water.” This assertion perpetuates the common error that only desert mammals use metabolic water, whereas all mammals do this, and moreover, such usage does not conserve water. Conservation is accomplished by a suite of adaptations discussed in the ensuing chapter. Chapter 6 on endothermy effectively summarizes the basic features of temperature regulation in mammals. Chapter 7 is an excellent summary of heterothermy, but leaves photoperiod out of a list of environmental cues for hibernation, and fails to mention the spleen in a discussion of arousal from hibernation in bats.

Chapter 8, “Coping with Cold,” contains much useful information, but lacks any mention of high altitudes as relevant cold places. Chapter 9 treats coping with heat and aridity, and is a generally well-written and informative chapter. It includes, however, a statement explaining that oxidation of foods “creates” metabolic water, “especially carbohydrates,” but Merritt does not mention fats, which actually generate more water per molecule than do carbohydrates. Then there is the careless claim that a group of desert rodents “do not require water.” Chapter 10 is devoted to “Ecogeographic Rules” (Allen's, Gloger's, and Bergmann's), a topic that seems to be mainly of historical interest. The chapter starts boldly with the statement that these rules “explain morphological variation of animals on a geographic scale,” but fails to incorporate the critical words “in some traits and in endotherms.”

The final part is about reproduction, and consists of three chapters: “Reproductive Variations” (chapter 11), which is limited to modes of gestation; “Mating Systems and Reproductive Strategies” (chapter 12), which highlights the strategies of six carefully selected taxa; and “Population Cycles: Lemmings and Snowshoe Hares” (chapter 13), which is a brief discussion of multiannual cycles. The introduction to this final part contains an erroneous definition of “cloaca” that leads to the incorrect labeling of the marsupial urogenital sinus as a cloaca. Ironically, the glossary definition is correct. The six groups of mammals chosen to illustrate various reproductive strategies in chapter 12 are well chosen. Tenrecs represent a high fecundity strategy, and indeed boast a record litter size of 32. Elephant shrews or sengis (Macroscelidea) exemplify a monogamous reproductive strategy. “Absentee maternal care” is the undefined term given for the tree shrew's (Scandentia) strategy. Leking behavior is illustrated by the hammer-headed bat (Hypsignathus monstrosus), and eusociality by the naked mole-rat (Heterocephalus glaber). Lastly, semelparity is illustrated by the marsupial Antechinus stuartii. The final chapter on population cycles is notable for its treatment of early myths about lemmings, and its inclusion of recent data on the disappearance of multiannual cycles from some regions.

Overall, The Biology of Small Mammals is a complex mix of favorable and unfavorable features. The subject matter is inherently interesting and important, and for the most part has been well chosen. Merritt writes with evident enthusiasm for his subject, he is impressively up to date on most topics, and the book is copiously illustrated. Care is exercised in providing supporting documentation and in the use of scientific names. All of these favorable attributes would seem to guarantee a superb product; unfortunately, hurried writing and lack of comprehensive editing can compromise even a good plan. A more subtle critique of this book is that it misses opportunities to encourage a scientific perspective for the nonscientist readership that it covets. The author hardly mentions that the majority of small mammal species are little known. He misses multiple opportunities to educate by making statements that falsely imply complete knowledge about something. For example, the statement, “Of the 376 species of shrews, only 7 cache food” could easily have been transformed into a factual one— with a science lesson as a bonus—by adding the words “are known to.”

All things considered, I can recommend this well-conceived book to the broad audience intended, but with the caveat that one must be wary of the widespread lapses in scientific and editorial rigor that weaken its impact.

William Z. Lidicker Jr. "The Biology of Small Mammals," BioScience 61(2), 155-157, (1 February 2011).
Published: 1 February 2011

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