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Most crea­tures need a good sense of bal­ance — espe­cially tree-​dwellers that swing among high branches. In mam­mals, the abil­ity largely comes from three loop-​shaped struc­tures in the inner ear called semi­cir­cu­lar canals; in most species, the size, shape, and arrange­ment of those loops (inset) is extremely con­sis­tent from one indi­vid­ual to another.

But in three-​toed sloths (such as Brady­pus var­ie­ga­tus, the brown-​throated three-​toed sloth), many pro­por­tions of the semi­cir­cu­lar canals are sur­pris­ingly vari­able from one sloth to another. The over­all vari­abil­ity is at least twice that seen in other species of mam­mals the team ana­lyzed, researchers report online 1^st August in the Pro­ceed­ings of the Royal Soci­ety B. That high degree of vari­a­tion stems from the sloths’ lan­guid lifestyle, the researchers sug­gest. Sloths, which move extremely slowly when they move at all, don’t require the sense of bal­ance that a swift, agile crea­ture such as a pri­mate needs. The find­ing sup­ports one of Charles Dar­win’s notions about evo­lu­tion: If an organ isn’t cru­cial, vari­a­tions in its struc­ture or per­for­mance aren’t lost over time, keep­ing the pot­pourri in the pop­u­la­tion.

The above news item is reprinted from mate­ri­als avail­able at Sci­enceShot. Orig­i­nal text may be edited for con­tent and length.

(Source: Sci­enceShot, 31.07.2012)