"Long before it's in the papers"
January 27, 2015


Riddle of zebra’s stripes “solved”

April 1, 2014
Courtesy of UC Davis
and World Science staff

Sci­en­tists say they have fi­nally fig­ured out why ze­bras have stripes, but the an­swer raises a new ques­tion.

Ze­bras have stripes be­cause these help ward off bit­ing flies, ac­cord­ing to a new stu­dy. As to why stripes ward off bit­ing flies, that is not so clear.

Plains zebras in Ka­ta­vi Na­tion­al Park, Tan­za­nia. (Cre­dit: Tim Ca­ro/UC Da­vis)

“No one knew why ze­bras have such strik­ing col­ora­t­ion,” said Tim Car­o, a bi­ol­o­gist at the Uni­vers­ity of Cal­i­for­nia Da­vis, lead au­thor of the stu­dy. The find­ings are pub­lished April 1 in the on­line jour­nal Na­ture Com­mu­nica­t­ions.

Car­o and col­leagues found that bit­ing flies, in­clud­ing horse­flies and tset­se flies, are the ev­o­lu­tion­ary driv­er for ze­bra’s stripes. Ex­pe­ri­ments had pre­vi­ously shown that such flies tend to avoid black-and-white striped sur­faces, but many oth­er hy­pothe­ses for zeb­ra stripes have been pro­posed since Al­fred Rus­sel Wal­lace and Charles Dar­win de­bat­ed the prob­lem 120 years ago. These in­clude: a form of cam­ou­flage; con­fus­ing preda­tors; a mech­an­ism of heat man­age­ment; hav­ing a so­cial func­tion; and avoiding “ec­topar­a­site” at­tack, such as from bit­ing flies.

The team mapped the ge­o­graph­ic dis­tri­bu­tions of the sev­en dif­fer­ent spe­cies of ze­bras, hors­es and as­ses, and of their subspe­cies, not­ing the thick­ness, loca­t­ions, and in­tens­ity of their stripes on sev­er­al parts of their bod­ies. Their next step was to com­pare these an­i­mals’ ge­o­graph­ic ranges with dif­fer­ent vari­a­bles, in­clud­ing wood­land ar­eas, ranges of large preda­tors, tem­per­a­ture, and the ge­o­graph­ic dis­tri­bu­tion of glossinid (tset­se flies) and tab­a­nid (horse­flies) bit­ing flies. They then ex­am­ined where the striped an­i­mals and these vari­a­bles over­lapped.

Af­ter an­a­lyz­ing the five hy­pothe­ses, the sci­en­tists ruled out all but one: avoiding blood-sucking flies.

“I was amazed,” said Caro. “A­gain and again, there was great­er strip­ing on ar­eas of the body in those parts of the world where there was more an­noy­ance from bit­ing flies.”

Why would ze­bras evolve to have stripes where­as oth­er hooved mam­mals did not? The study found that, un­like oth­er Af­ri­can hooved mam­mals liv­ing in the same ar­eas as ze­bras, zeb­ra hair is shorter than the mouth­part length of bit­ing flies, so ze­bras may be more sus­cep­ti­ble to an­noy­ance by bit­ing flies.

So why do bit­ing flies avoid striped sur­faces? Caro and col­leagues hy­poth­e­size that bit­ing flies are un­able to see striped sur­faces as po­ten­tial prey, but just why will have to await fur­ther stud­ies.

* * *

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Scientists say they have finally figured out why zebras have stripes, but the answer raises a new question. Zebras have stripes because these help ward off biting flies, according to a new study. As to why stripes ward off biting flies, that is not so clear. “No one knew why zebras have such striking coloration,” said Tim Caro, a biologist at the University of California Davis, lead author of the study. “Again and again, there was greater. “But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it.” The findings are published April 1 in the online journal Nature Communications. Caro and colleagues found that biting flies, including horseflies and tsetse flies, are the evolutionary driver for zebra’s stripes. Experiments had previously shown that such flies tend to avoid black-and-white striped surfaces, but many other hypotheses for zebra stripes have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago. These include: a form of camouflage; confusing meat-eating predators; a mechanism of heat management; having a social function; and avoiding “ectoparasite” attack, such as from biting flies The team mapped the geographic distributions of the seven different species of zebras, horses and asses, and of their subspecies, noting the thickness, locations, and intensity of their stripes on several parts of their bodies. Their next step was to compare these animals’ geographic ranges with different variables, including woodland areas, ranges of large predators, temperature, and the geographic distribution of glossinid (tsetse flies) and tabanid (horseflies) biting flies. They then examined where the striped animals and these variables overlapped. After analyzing the five hypotheses, the scientists ruled out all but one: avoiding blood-sucking flies. “I was amazed,” said Caro. “Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies.” Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be more susceptible to annoyance by biting flies. So why do biting flies avoid striped surfaces? Caro and colleagues hypothesize that biting flies are unable to see striped surfaces as potential prey, but just why will have to await further studies.