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From eminent biologists like Alfred Russel Wallace and Charles Darwin to famous authors such as Rudyard Kipling in his Just So Stories, many people have asked, "Why do zebras have stripes?" There are many explanations, but until now hardly any have been seriously addressed or even tested. In Zebra Stripes, Tim Caro takes readers through a decade of painstaking fieldwork examining the significance of black-and-white striping and, after systematically dismissing every hypothesis for these markings with new data, he arrives at a surprising conclusion: zebra's markings are nature's defense against biting fly annoyance.
Popular explanations for stripes range from camouflage to confusion of predators, social facilitation, and even temperature regulation. It is a challenge to test these proposals on large animals living in the wild, but using a combination of careful observations, simple field experiments, comparative information, and logic, Caro is able to weigh up, scientifically, the pros and cons of each idea. Eventually – driven by experiments showing that biting flies avoid landing on striped surfaces, observations that striping is most intense where biting flies are abundant, and by his knowledge of zebras' susceptibility to biting flies and vulnerability to the diseases that flies carry – Caro concludes that black-and-white stripes are an adaptation to thwart biting fly attack. Not just a tale of one scientist's quest to solve a classic mystery of biology, Zebra Stripes is also a testament to the tremendous value of longitudinal research in behavioral ecology, demonstrating how observation, experiment, and comparative research can reshape our understanding of the natural world.
Preface and acknowledgments
Chapter 1. Stripes and equids
1.1. The question of stripes
1.2. Hypotheses for striping in equids
1.2.a. Antipredator hypotheses
1.2.b. Antiparasite hypotheses
1.2.c. Communication hypotheses
1.2.d. Thermoregulation hypothesis
1.3. Equid evolution
1.3.a. Plains zebra
1.3.b. Mountain zebra
1.3.c. Grevy’s zebra
1.3.d. African wild ass
1.3.e. Asiatic wild ass
1.3.f. Kiang
1.3.g. Przewalski’s horse
1.3.h. Other equids
1.4. Zebra hair
1.5. Conclusion
Chapter 2. Predation and crypsis
2.1. Background matching
2.1.a. Initial discomfort with the idea
2.1.b. Detecting zebras
2.2. Disruptive coloration
2.2.a. Predictions
2.2.b. Sightings at dusk and dawn
2.3. Countershading
2.4. Zebras as seen by nonhumans
2.5. Conclusions
Chapter 3. Predation and aposematism
3.1. Aposematism in mammals
3.2. Signaling component of aposematism
3.2.a. Visibility
3.2.b. Noisy behavior
3.3. Defense component of aposematism
3.3.a. Response to predators
3.4. Conclusion
Chapter 4. Predation and confusion
4.1. Confusion
4.2. Miscounting numbers of prey individuals
4.3. Striping obscuring outlines of fleeing prey
4.3.a. Lines of stripes shown to humans
4.3.b. Lines of stripes in dangerous situations
4.4. Striping preventing a single prey individual being followed
4.5. Dazzle effect
4.6. Motion dazzle
4.7. Misjudging the size of prey
4.7.a. Subjective and estimated heights and girths
4.7.b. Subjective heights and girths and degree of striping
4.8. Quality advertisement
4.9. Conclusion
4.10. Difficulties with the predation hypothesis
Chapter 5. Ectoparasites
5.1. Biting flies
5.2. Behavioral indices of fly infestation in Katavi
5.3. Behavioral indices of fly infestation in Berlin
5.4. Tsetse fly traps
5.4.a. Biconical traps
5.4.b. Cloth traps
5.5. Tabanid traps
5.5.a. Canopy traps
5.5.b. Pelt canopy traps
5.6. Moving objects
5.6.a. Walking in suits
5.6.b. Walking in pelts
5.6.c. Driving with pelts
5.7. Conclusions
5.8. Polarized light
5.8.a. Reflected light
5.8.b. Horvath’s work
5.8.c. Polarization signatures of wild zebras
Chapter 6. Intraspecific communication
6.1. Intraspecific signaling
6.2. Species recognition
6.3. Stripes as a facilitator of mutual grooming and social bonding
6.3.a. Allogrooming
6.3.b. Social bonding
6.4. Stripes as a means of individual recognition
6.5. Stripes as an indicator of quality
6.6. Conclusion
Chapter 7. Temperature regulation
7.1. Black and white surfaces
7.2. Heat measurements in the field
7.3. Heat management
7.4. Conclusions
Chapter 8. Multifactorial analyses
8.1. Comparing hypotheses simultaneously
8.2. The interspecific comparison
8.2.a. Comparative methodology
8.2.b. Overall striping
8.2.c. Striping on different parts of the body
8.2.d. Evaluating the hypotheses
8.3. Conclusions
8.4. The intraspecific comparison
8.5. Concordance on multifactorial analyses
Chapter 9. The case for biting flies
9.1. Last man standing
9.2. Host choice
9.3. Ectoparasite population sizes
9.4. Host seeking
9.5. Parasites and diseases transmitted by bloodsucking diptera
9.6. Mechanistic studies
9.7. Further outstanding issues
9.7.a. Multiple functions
9.7.b. Loose ends
9.8. Conclusion
Appendix 1. Scientific names of vertebrates mentioned in the text
Appendix 2. Nature of wounding seen in African ungulates in Katavi National Park
Appendix 3. Families of insects identified in each type of biconical trap color
Appendix 4. Families of insects identified in each type of cloth trap color
Appendix 5. Photographic sources for comparative analyses
Appendix 6. Derivation of zebra phylogenies
Appendix 7. Phylogenetic analyses
References
Index
Tim Caro is professor of wildlife biology at the University of California, Davis. He is also the author of Cheetahs of the Serengeti Plains: Group Living in an Asocial Species and Antipredator Defenses in Birds and Mammals, both published by the University of Chicago Press, as well as Conservation by Proxy: Indicator, Umbrella, Keystone, Flagship, and Other Surrogate Species.
"The purpose of zebra stripes has perplexed science at least since Charles Darwin debated the matter with fellow Victorian naturalist Alfred Russel Wallace. Today, according to Caro, no fewer than four groups of researchers study the question. They do not lack for hypotheses: In Caro's new book, Zebra Stripes, he outlines nearly twenty of them [...] He decided to test each of these ideas one by one, in the field, over a decade of summers. This was the scientific method on repeat: Trial and error after trial and error. And because studying large wild mammals is no easy task, Caro had to get creative. Coming up with experiments was, he said, 'the thing that taxed me most.' [...] Caro's [is a] systematic, sometimes wacky quest, which he refers to as a 'personal discovery.'"
– Karin Brulliard, Washington Post
"Caro's study exemplifies how one should conduct such an investigation [...] This is an exemplary study. This is how science should be done: patient, systematic, careful and comprehensive. Charles Darwin referred to his work On the Origin of Species as 'one long argument' for evolution by natural selection. Caro's Zebra Stripes is one long argument also – for this is in essence a 300-page scientific paper – and as such makes a fine academic behavioral ecology monograph."
– Tim Birkhead, University of Sheffield, Times Higher Education
"Zebras are weird: the familiar shape of a horse packaged in a color scheme rarely seen beyond the coral reef. For that reason, there has been no shortage of theories for why the stripes evolved, with Rudyard Kipling's story of camouflage for an originally forest-dwelling beast by no means the most fanciful. Through his decades of fieldwork in Tanzania while building an enviable reputation as a world-leading behavioral ecologist and conservationist, the puzzle of zebra stripes has gnawed at professor Tim Caro. His book brings together a wealth of historical and scientific information, coupled with previously unpublished data, to suggest that an answer is at hand: deterring biting flies. This might sound like a Just So Story, but Caro's evidence is compelling, from his own comparative data showing an association across zebra races between stripiness and fly distribution, to experimental studies with painted targets, to mathematical modeling of the fly visual system. This book will interest any sensory or behavioral ecologist interested in animal coloration, but is also an insight into how to succeed in science: be curious, read widely, tolerate failure, and think broadly."
– Innes Cuthill, University of Bristol
"A tour de force that characterizes the biology, morphology, physiology, and behavior of the equid genus as a starting point for examining in detail the unique features of its striped members. Caro clearly and succinctly presents the competing hypotheses put forward over the last one-hundred-fifty years (some even earlier) for why zebras are striped. The mix of methods with facts and interpretation is compelling, making what could be a complex and tedious presentation one that is clear, engaging, and to the point. Zebra Stripes is an easy read, full of authoritative documentation from the literature bolstered by clever experiments (with Caro putting himself literally in a zebra's shoes – or actually a pelt), and constructed in a didactic, hypothetical, deductive way that gives it credibility. Its completeness and attention to detail will make it a must read."
– Daniel I. Rubenstein, Princeton University
"Zebra Stripes is an attempt to answer the mysterious, longstanding, and popular question of why zebra have strips. A personal journey in addition to a scientific appraisal of the evidence, it is highly original in its discussion of stripe hypotheses and its personal aspect; I am not aware of any work of comparable synthesis in depth or scope. It is comprehensive, up-to-date, and authoritative, and also contains a wealth of unpublished work and observations. Well written in a very readable style, interesting, and clear, Zebra Stripes will unquestionably be of importance to any specialist in the field and of much interest to biologists more broadly, especially those interested in animal coloration, behavior, and evolution. Highly successful."
– Martin Stevens, University of Exeter, author of Cheats and Deceits: How Animals and Plants Exploit and Mislead
