Saving their skins: how animals protect from the sun.

Abstract

Adistinctive evolutionary changeexperiencedbyour ancestors as they branched away from their fellow apes was the loss of body hair. While this may have had certain benefits, our species has also suffered drawbacks; forone,without thick furorhair toscattersunlight,ourskin ismore susceptible to burning in the sun. Indeed, sunburns have been observed in other relatively hairless members of the animal kingdom, including whales, dolphins, fish, elephants, and rhinoceroses. But just as humans have evolved physiologic and behavioral adaptations to protect from the sun, other creatures too have developed their own. Whaleshavehairless, streamlinedbodiesand frequently travel to the sea surface for air, making them particularly vulnerable to UV-induced skin damage. The spermwhale,which spends up to6 hours at a time at the surface, is known to activate genotoxic stress pathways in response to persistent UV exposure. While the fin whale possesses constitutivelyhigh levelsofmelanin, thebluewhalemodulates its skinmelanin throughout the year, becoming darker when UV levels are highest from February to May. This adaptive pigmentation has also been observed in hammerhead sharks and some fish. EquatorialAfrica,which receivesabundantdaily sunlightyear round, is home to theworld’s 3 largest pachyderms: the elephant, rhinoceros, andhippopotamus, forwhomlearnedbehaviorsareessential forsunprotection.TheAfricanelephantuses itsprehensile trunkto throwdustonto itsback, the rhinoceroswallows inandcoats itselfwithmud,and thehippopotamus submerges itself in water. The hippopotamus also possesses a curious adaptation found in no other animal—it manufactures its own sunscreen. The hippopotamus’s “blood sweat” is a thick, reddish fluid that is neither blood nor true sweat, as the animal’s skin contains no sebaceous glands. It is secreted by subdermal glands through large skinpores andcontains2pigments, hipposudoric acid andnorhipposudoric acid, whose polymerization gives the fluid a reddish orange color and whose absorption of UV light makes this substance a highly effective sunscreen. Someanimalsarewell coveredwith furbutmayneedtoprotectother body parts from the sun. A giraffe’s long tongue, frequently outstretchedtoforagethetreetopsfor leaves, iscoloredblackishpurple from thecircumvallatepapillae to the tip,believedtoprotect it fromsunburn. The meerkat’s eyes are surrounded by dark bands to reduce sun glare, and the nictitating membrane covering a polar bear’s eye filters bright sunlight reflected by the snow. Increasing UV exposure from a depleted ozone layer will certainly exert a selective pressure on animals to evolvemore sophisticated skin defenses. Studying thesecreaturesmayprovidegreater insight into skin physiology and how our own species will adapt to a changing solar environment.