Thursday, November 11, 2010

Why whales get sunburn, but pigs and hippos don't


Cool pig: Mammals such as pigs avoid sunburn by utilising mud to protect their skin

Sunburn, it is commonly thought, is a uniquely human phenomenon, the result of over-exposure by fair-skinned people to ultraviolet solar radiation against which they have no natural defences.

But new scientific research has this week shown that this is not the case. Whales studied in the Sea of Cortez, off the west coast of Mexico, were also found to suffer from severely sun-blistered skin.

The fact that whales can get sunburn should not surprise us. Like humans, whales and dolphins have smooth, fur-free skin (seals and their relatives, on the other hand, have thick fur).

And, as with humans, the darker the whale’s skin the less susceptible it is to sun damage because the dark pigment (melanin) protects against ultra-violet radiation.

This new discovery raises the fascinating question of why some animals suffer from sunburn and why some don’t.

Of course, fur, especially dark fur, provides the ultimate protection. In compensation, hairless mammals have evolved clever strategies to cope with fierce sunlight.

It’s a particular problem for animals such as hippos, rhinos and elephants, which are large and have a relatively small area of skin from which they can lose heat.

The solution for hippos is to secrete a red substance (called ‘hippo sweat’) which is an oily liquid containing microscopic particles that are able to scatter ultraviolet light — a highly effective natural sunscreen.

The liquid also acts an insect repellent and antiseptic, warding off infections and helping the skin to heal itself if damaged. No wonder several cosmetic firms are trying to copy its properties — as long as they can find a way of eliminating the pungent odour.

Elephants and rhinos, on the other hand, use mud and waterbaths to keep the sun at bay.

The same principle applies to wild pigs, which have evolved to defend themselves by growing thick fur.

Blisters: Visible marks on the skin of a whale shows it is suffering from sunburn

But domestic pigs are bred with much less hair and consequently suffer sunburn and sunstroke. Many farmers have to use suncream to protect their livestock.

Even dogs, goats, cattle and sheep, which usually have plenty of fur, can suffer from sunburn — especially breeds with light hair and (usually) corresponding light skin underneath.

Newly-shorn sheep are vulnerable to the sun, as are light-coloured dairy cows. It is thought the high levels of chlorophyll (the pigment that gives plants their green colour and turns sunlight into sugars) in the grass they eat exacerbates sunburn by changing the chemical make-up of the skin in some mysterious way.

Animals that are deliberately bred to be hairless — such as breeds of dog including the Chinese Crested and Mexican — are most vulnerable of all.

Their thin, pale, exposed skins are totally unadapted to sunlight. Their owners must, therefore, plaster them with high-factor sunscreen even in moderate sunshine.
Of course, humans are particularly vulnerable to the sun, especially pale-skinned people who evolved in northern latitudes.

Humans who migrated from our original home in Africa to northerly latitudes evolved pale skin. Its extra transparency allows the body to use what limited sunlight there is to manufacture Vitamin D, which is vital for wellbeing. But the downside is a vulnerability to radiation damage.

As with all species, the rare mutation which causes albinism — a lack of pigmentation in the skin, fur and iris in the eye — can leave the animal terribly exposed to the sun.

For example, Snowflake, an albino lowland gorilla that lived in Barcelona Zoo until its death in 2003, was very sensitive to sunlight and developed a form of skin cancer. It is highly unlikely that he would have lived for long in the wild, certainly not for the 38 years he survived in captivity.

Fish of all colours, meanwhile, can suffer sunburn, as can amphibians. Reptiles and birds, however, are largely protected by thick scales and feathers.

As for the poor sunburned whales of western Mexico, it is not clear whether this is a new phenomenon or something that had just never been noticed before.

So far, scientists don’t know if there is a simple explanation such as that ultraviolet levels have increased recently.

Whatever the reason, rubbing sunscreen or calamine lotion on a 40-tonne whale is probably impractical. Finding a cool saltwater bath — often prescribed in cases of severe sunburn — is much easier.

source: dailymail

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