Why ze­bras have stripes

It’s not to keep them cool.

Cosmos - - Digest -

Pon­der­ing the pur­pose of ze­bra stripes has been a pop­u­lar pas­time for nat­u­ral­ists stretch­ing back to Charles Dar­win.

As many as 18 the­o­ries have been pro­posed. Now a team led by Ga­bor Hor­vath at Eötvös Loránd Univer­sity in Hun­gary, has suc­ceeded in re­duc­ing the num­ber by one.

Their find­ings pub­lished in Sci­en­tific Reports in July, demon­strate that the stripes do not help the an­i­mals stay cool be­neath the African sun.

The most pop­u­lar ex­pla­na­tion for the func­tion of ze­bra stripes is that they pro­vide cam­ou­flage. But even Dar­win had a hard time see­ing how stripes of­fered pro­tec­tion on the open plains. And re­cent stud­ies have found that ze­bra stripes, whose dis­tinct­ness varies across African herds from north to south, do not show a strong cor­re­la­tion with the pres­ence of large preda­tors. They did how­ever cor­re­late with the pres­ence of swarms of bit­ing horse flies and tse tse flies. That favoured the view that stripes dis­suade flies, a find­ing borne out by stud­ies in the lab.

Stripes also cor­re­late with cli­matic con­di­tions - herds in the warmer north­ern re­gions have more dis­tinct stripes than those in the south­ern cooler climes. That has favoured the the­ory that stripes help cool the ze­bra. How could stripes pro­vide cool­ing? Given that the an­i­mals’ black and white bands ab­sorb and re­flect heat at dif­fer­ent rates, the tem­per­a­ture gra­di­ent be­tween the zones might cre­ate a tiny breeze that cools the an­i­mal down.

To test this the­ory, Hor­vath and col­leagues used wa­ter-filled metal bar­rels, each cov­ered by a pelt from ei­ther a horse, cow or a ze­bra. The skins cov­ered the spectrum from solid black, solid white, grey and stripey.

The bar­rels were left out in the sun for four months and their core tem­per­a­tures mea­sured con­tin­u­ally.

Alas, even af­ter runs of sev­eral hot days in a row, the tem­per­a­ture of the ze­bras­triped bar­rel was not much dif­fer­ent to that of the grey cow and grey horse stand ins.

The au­thors re­ported the av­er­age core tem­per­a­ture of the bar­rels in­creased as fol­lows: white cow, grey cow, ze­bra, grey horse, black cow.

One the­ory down, 17 to go.

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