Marlene Zuk
Harcourt, Inc. Orlando
2007; 328 pgs
Price: $25.00 (hard)
ISBN 13-978-0-15-101225-1

In Riddled with Life, Marlene Zuk translates complex scientific information about parasites, disease, evolution, behavior and related topics into understandable and interesting reading for a non-scientist. Although this book is not primarily about insects, they are used as examples to make her points on several topics. The underlying theme seems to be: “why worms and germs (or most of them) are good for you.” She strongly defends her premise that microorganisms and parasites are not bad for us; indeed, most are beneficial. The book gives a long-term, evolutionary perspective on the interactions of animals, plants, and humans with pathogens and parasites. Some examples include the benefits and costs of fever, human physiology and behavior related to changes in disease from our hunter-gatherer to agricultural past, and an evolutionary perspective on genetic diseases that had some benefit in earlier times (e.g., sickle cell anemia). She relates the rapid increase in immune diseases such as asthma in developed nations to our obsession with cleanliness. The complex topic of evolution of pathogen/host balance and evolution of virulence is described in a clear and understandable way, with a balanced description of opposing theories.

Insects such as aphids are used as examples to show why sexual reproduction continues to exist, even though asexual reproduction is more efficient at passing on the parent’s genes. Zuk makes a strong point that without disease, there would be no sex, as sexual recombination leads to genetic diversity, with some members of the population exhibiting resistance to parasites. As we know, organisms that are basically clones (for example, agricultural crops) can be devastated by disease. Ladybugs and mites also provide interesting examples of how sexually transmitted diseases influence sexual behavior, leading to longer mating periods and potentially more disease transmission, but at the same time, reproduction is more evolutionarily important to the hosts and worth the risk. A fungus disease of flies that makes the dead fly attractive to males because it looks like an egg-filled female is another unusual example. 

Male/female differences in vulnerability to disease, length of lifespan, and behavior are discussed with understandable and often amusing examples. Zuk addresses the very human question: why do females survive much longer than males? She includes a clear description of the difference between “average” life expectancy, which is strongly influenced by childhood mortality, and statistics on adult life span. Behavioral differences (e.g., risk taking and roaming) can result in earlier mortality in males. In male vertebrates, testosterone weakens the immune system, so that males with attractive behaviors and external features that signal “good” genes may also have a higher parasite load resulting from immune suppression.  Zuk presents data from her own experiments with field crickets and birds that show mate choice can be influenced by parasite load. Female bumblebees that mate with multiple males produce offspring more resistant to intestinal parasites, apparently due to increased likelihood of choosing at least one male with genes for resistance to the parasites. Crickets, beetles, bedbugs, and butterflies are also used as examples of how the strength of the immune system can influence mate choice. Once again, parasites and pathogens may control this basic aspect of life.

In later chapters, Zuk addresses how folk medicine and animal behaviors reflect the influence of parasites and pathogens throughout evolutionary history. Birds, monkeys, and other mammals chew certain plants and groom, probably leading to reduction of bacteria, fleas, and lice. However, she also expresses sensible skepticism toward much folklore about diseases. Emerging diseases are a further topic, with a realistic description of how human population increase, international travel, and our propensity for demanding antibiotics have resulted in “an evolutionary experiment on a grand scale.” She raises the interesting question of whether other diseases generally not thought to be of infectious origin, such as cancer and heart disease, may actually have microbial origins. 

The final chapter includes several fascinating examples of parasite influence on behaviors of the host, using invertebrate examples. A cricket infected with a horsehair worm is induced to seek out water, where the worm emerges to complete its life cycle; a spider is forced to build a “home” for its wasp parasite to pupate; and mosquitoes, sand flies, and fleas infected with malaria, leishmaniasis, or plague are induced to bite repeatedly due to their guts being blocked by the parasites. Her final example is toxoplasmosis, a fairly common parasite of rodents, cats, and humans. Rats infected with this parasite display behaviors that lead to more frequent predation by cats. Zuk then describes a Czech study of potential changes in personality of men infected with this organism, leading to more risky behavior. Are we influenced by our parasites, pathogens, and commensal microorganisms more than we would like to admit? Marlene Zuk would like us to believe that we are—and have been forever.

Riddled With Lifetakes a complex topic to the general public in an understandable and amusing way. I found myself laughing out loud many times—how often does this occur when reading about science? It was refreshing to see the scientists and their institutions mentioned along with their work, which is sadly rare in much writing for the general public. If Dr. Zuk’s classes are as interesting and amusing as this book, I think I’ll sign up.

Elizabeth W. Davidson
School of Life Sciences
Arizona State University
Tempe, AZ 85287-4501
E-mail: e.davidson@asu.edu

American Entomologist
Vol. 54, No. 4, Winter 2008