动物会晒伤吗？

埃玛·布赖斯

For lots of animals—humans included—lazing about in the sunshine is one of lifes greatest pleasures. But unfortunately， this pastime comes with a cost： the skin-sizzling ordeal known as sunburn. And， while its most likely victims are the fairer-skinned among us， animals are at risk of sunburn， too.

But if this can happen to animals too， why， then， dont we ever see sunburned fish， or scarlet elephants？

“If you think of it， the sun has been here forever in terms of our planet， and all individuals have been exposed to it，” said Karina Acevedo-Whitehouse， a molecular epidemiologist from the Autonomous University of Queretaro， in Mexico. “So， its a pretty strong selective pressure that the sun has imposed on animals and that has led to many mechanisms of counteracting it.”

Some of these mechanisms are obvious： Hair， fur， wool， feathers and scales on many creatures create a barrier between sunshine and skin. These adaptations are so effective that the only time they really fail is when humans intervene. For instance， domesticated pigs—bred to have less hair—are more sensitive to sun damage than their wild cousins.

Animals with naturally hairless， unscaled skin must resort to other methods of self-protection. Elephants and rhinos not only have thicker hides; they also regularly coat themselves in dust or mud to create a rudimentary sunscreen. When conditions are extreme， most animals retreat to the shade or take refuge in burrows. “All of that is helping animals to cope， so we dont see a lot of sunburn [in these species]，” said Acevedo-Whitehouse.

Some species up the ante by producing a unique brand of sunscreen from their own cells. Taifo Mahmud， a molecular biologist at Oregon State University， has discovered genetic traits in fish， birds， reptiles and amphibians that enable them to produce a compound called gadusol， which creates protection against ultraviolet （UV） rays in the sun. “Most vertebrates， except mammals， have the genes responsible for producing gadusol，” said Mahmud. So far， theyve proved that only zebrafish actually use this compound as a protectant against UV rays. But now scientists are looking into how humans might exploit this trait for our skin.

Why dont we—and other mammals—produce gadusol？ “It has been proposed that early mammals were nocturnal. Was that because they lost the genes that produce gadusol？ We dont know，” Mahmud said. “I think it would be interesting to figure out if fur and thicker skin were developed later in their evolution.”

Not to be outdone， gadusol-deprived mammals have developed their own sophisticated mechanisms of protection. Hippos are known to secrete a scarlet liquid from their pores that looks just like blood—and it wasnt until 2004 that a group of Japanese scientists discovered that the orangey-red compounds in this skin-coating fluid protected hippos from UV rays， according to a report in the journal Nature. Other animals focus their sun protection in the most vulnerable parts of their bodies. Giraffes， for example， produce more protective melanin in their tongues—giving them a darker hue—because they spend most of their lives with their tongues exposed to the sun as they pry tender leaves off trees.

So， do animals ever get sunburned？ Yes. “Marine mammals， and specifically cetaceans [whales， dolphins and porpoises]， are an exception because they dont have fur; they dont have scales，” said Acevedo-Whitehouse， who has been studying sunburn in whales for over five years.

In skin samples taken from the backs of blue， sperm and fin whales on their cross-ocean migrations， Acevedo-Whitehouse and her colleagues discovered signs of sunburn from the whales hours spent breathing and socializing at the surface. But crucially， they also discovered that whales have specialized mechanisms that help them counteract this burn. “The common adaptation of cetaceans is that they appear to be very effective at repairing damage，” she said.

Some whales generate pigments that darken and protect their skin; others have genes that trigger a protective stress response in the skin. There are even whales that have developed a hard， keratinized layer that protects the delicate skin below. “We were excited to see there isnt really evidence of skin cancer in whales，” Acevedo-Whitehouse said. Now， theyre trying to understand precisely how those healing mechanisms work.

From protective coats， to self-made sunscreen， to rapid healing， these sun-smart animals may one day give us the clues we need to save our own skin.

對于包括人类在内的很多动物而言，懒散地享受日光浴是生活中的一大乐事。可惜，这种消遣是要付出代价的，代价就是皮肤会遭受烧灼之苦，我们称之为晒伤。虽然最易被晒伤的是皮肤相对白皙的人，动物也难逃被晒伤的风险。

可是，如果动物也会被晒伤，为什么我们从来没见过晒伤的鱼或晒得红通通的大象呢？

“想想看，对地球来说，太阳始终存在，所有物种一直在接受阳光的照射。”墨西哥克雷塔罗自治大学分子流行病学家卡琳娜·阿塞韦多-怀特豪斯说道，“因此太阳就对动物施加了较强的选择压力，使动物进化出了许多应对机制。”

部分此类机制显而易见：众多生物身上的毛发、毛皮、绒毛、羽毛、鳞甲等，在阳光和皮肤间形成了一道屏障。这类适应机制非常有效，实际上，除了遭受人类干预外，它们从未失效过。例如，驯养的猪毛发减少，与其野生近亲相比，对晒伤更加敏感。

天生无毛、无鳞甲的动物，必须通过其他方法保护自己。大象和犀牛不但皮更厚，它们的身上还经常裹着一层尘土或泥浆，形成一种粗陋的防晒霜。日照极为强烈时，多数动物会躲避到阴凉处，或藏匿在洞穴里。“这些手段全都有助于动物抵御烈日暴晒，所以我们很少见到（这些物种）晒伤。” 阿塞韦多-怀特豪斯说。

一些物种则更进一步，它们的细胞会产生一种独特的防晒霜。俄勒冈州立大学分子生物学家泰福·马哈茂德发现，鱼类、鸟类、爬行动物和两栖动物体内存在一些基因特征，使这些动物能够分泌出一种叫作gadusol的化合物，这种化合物会对阳光中的紫外线产生防御效果。“除哺乳动物外的大多数脊椎动物，都有负责产生gadusol的基因。” 马哈茂德说。迄今他们已证实，只有斑马鱼实际利用了这种化合物来抵御紫外线。不过，现在科学家正在研究人类能否利用此基因特征防止皮肤晒伤。

为什么我们和其他哺乳动物不会产生gadusol呢？“有说法称，早期哺乳动物是夜间活动的。是不是因为它们失去了产生gadusol的基因呢？我们不得而知。” 马哈茂德说道，“毛皮和厚皮是不是它们进化后期出现的？把这个问题搞清楚，我想会挺有意思。”

失去gadusol生成能力的哺乳动物不甘示弱，它们发展了自身特有的复杂保护机制。我们知道河马会从毛孔分泌出一种血状的红色液体。然而，《自然》杂志的一份报告称，直到2004年，一群日本科学家才发现，覆在河马皮肤上的这层液体所含的橙红色化合物可以防御紫外线。其他动物则把防晒重点放在身体最脆弱的部位上。例如，长颈鹿的舌头会产生较多的保护性黑色素，因此舌头颜色较深。之所以如此，是因为长颈鹿要取食树上的嫩叶，舌头长期暴露在烈日下。

那么，动物究竟会晒伤吗？会的。“海洋哺乳动物，尤其是鲸类动物（鲸、海豚和鼠海豚）就是例外，因为它们既没有毛皮，也没有鳞甲。” 阿塞韦多-怀特豪斯说。她研究鲸的晒伤现象已有五年多时间。

从跨洋迁徙的蓝鲸、抹香鲸和长须鲸背上采集的皮肤样本中，阿塞韋多-怀特豪斯和她的同事们发现了晒伤的痕迹，鲸间或会浮上海面呼吸和相互交流，晒伤就是在那些时候造成的。而至关重要的是，他们还发现鲸有专门对抗这类晒伤的机制。“鲸类动物共有的适应机制是它们似乎具备非常有效的损伤修复能力。” 阿塞韦多-怀特豪斯说。

一些鲸会产生色素，使皮肤颜色变深，为皮肤提供保护;另一些鲸体内的基因会在皮肤内触发保护性应激反应。有的鲸甚至还进化出坚硬的角化层，以保护下层的脆弱皮肤。“我们激动地发现，没有任何实质证据表明鲸会罹患皮肤癌。” 阿塞韦多-怀特豪斯说。现在，他们正试图确切掌握这些修复机制的作用原理。

从防护涂层、自制防晒霜，到快速修复，这些动物用各种巧妙手段抵御日晒，也许有朝一日能给予我们启发，帮助我们拯救自己的皮肤。

（译者为“《英语世界》杯”翻译大赛获奖者）