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Friday, September 13, 2013

Sunburned whales 

Most of us have experienced a sunburn or two in our lives. But we don’t usually think about other animals getting sunburned. Especially not animals that spend the bulk of their lives under water. Apparently, as little time as whales are exposed to the sun is long enough for them to get sun damage. So say researchers led by Laura M. Martinez-Levasseur of the Zoological Society of London and Queen Mary University of London.

In humans, mitochondrial DNA (mtDNA) is a great biomarker for UV sun damage. MtDNA is both more easily damaged and less readily repaired than nuclear DNA. Thus, a lifetime of sun exposure can be seen in the accumulating mutations of skin mtDNA.

It’s also true in humans that darker skinned individuals (those with more melanin) suffer from less sun damage than lighter skinned individuals. Whales too come in different colors, though their differences tend to be species specific. Therefore, the authors used the three whale species shown below: blue whales, sperm whale and fin whales for their sunburn study.

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(A) From top to bottom:
a blue whale (pale grey skin colour, the lightest species)
a sperm whale (dark grey skin colour) 
a fin whale (black skin colour, the darkest species). 
Photograph taken by Diane Gendron of Instituto Politecnico Nacional.
(B) Differences in density of melanocytes (thick bars) and melanin (thin bars) amongst the three studied species 
(C) Association between melanin abundance and melanocyte counts in whales. Grey dots correspond to blue whales, black dots to fin whales and crosses to sperm whales. 
Scientific reports, 3 PMID: 23989080.

Sure enough, fin whales got the least sunburn.

For the light-skinned blue whales, the amount of sun damage, both in terms of visible lesions and mtDNA mutation levels was inversely proportional to the amount of melanin they had. That is, specimens that were a bit darker had less damage. Older whales had more lesions, suggesting that the damage was accumulating in the whales, just as it does in humans.

There are many factors involved in determining how much sun a whale is exposed to. Different whale species spend vastly different percentages of their time at the surface. Some species are highly migratory leading to seasonal differences in the intensity of the UV radiation they are subjected to. And of course, depletions in the ozone layer have led to increased skin damage in whales.

Luckily, there is some evidence that whales, like people, can acclimate to these changes by increasing the production of melanin. In other words, whales are able to tan. 

Martinez-Levasseur LM, Birch-Machin MA, Bowman A, Gendron D, Weatherhead E, Knell RJ, & Acevedo-Whitehouse K (2013). Whales use distinct strategies to counteract solar ultraviolet radiation. Scientific reports, 3 PMID: 23989080.