Blondes beat brunettes on beach

The joke around some beach communities is that you can never be too thin, too rich, or too blonde. I don’t know about the first two, but a new paper suggests being blonde on a beach may be good, but you can be too blonde – if you’re a mouse.

Animal colours provide some classic cases of adaptation and natural selection. For instance, most people with even a passing familiarity with biology know about peppered moths and industrial melanism. Fur colour in mice isn’t quite as famous as moth colours yet, but it might be getting there. I discussed mouse colour being hyped as “one for the textbooks” in a previous post. I suggested that the field experiments supporting some of the work were a little obscure and not well described.

Some people agreed with me. Alright, nobody said anything to me personally, but it would explain a new paper from the same lab. Sacha Vignieri, Joanna Larson, and lab leader Hopi Hoekstra go back to the field to get some more direct evidence for how mouse coat color influences survival in the field. The first couple of paragraphs do an excellent job of explaining why this story is worth revisiting 60 years after experiments showing that different fur colours were advantageous in different environments.

The team went to northern Florida, the scampering grounds of a field mouse (Peromyscus polionotus; pictured) that shows two distinct fur colours. They needed to go there not for the mice, but because that’s where the mouse predators live.

The authors didn’t need any mice, because they made their own.

Now, before you get this image of some sort of Frankenmouse experiment (“It’s alive... and squeeking!”), Vignieri and colleagues made models of mice out of plasticine.

Okay, they were a little more realistic than that.

You might think plasticine is a poor thing to make a mouse out of. Mice are furry, and plasticine... isn’t. True, but from a distance, the shape alone is enough to fool some predators.

(I can just imagine some poor, hungry predator coming down thinking, “Food, food, at long last, food!—Wha...!? This isn’t food! Damnit!”)

The big advantage to using plasticine models over other kinds is that when something attacks them (“So... hungry!”), they leave behind too or claw marks. This way, you can actually make a reasonable guess as to whether the predator was, say, a mammal or a bird. They also measured the colour of the soil they placed their plasticine models on, so they were able to test how closely they matched.

They tested two locations. On an beachfront island, the soil was very light coloured. Some ways inland in a park, the soil was quite a bit darker.

It might be to the predators’ credit that the non-moving, non-furry plasticine mice didn’t get attacked all that often. Of 2,688 chances predators had at them, 28 models were recovered with predator marks; another 12 went missing. And, as expected, the mice models that didn’t match the soil colour had the most attacks. The light coloured mice on dark soil getting the worst of it, but the authors attribute this to a higher rate of predation overall in habitats with dark soils.

But light coloured mice on light soil suffered from increased attacks if the mice were too much lighter than the soil. This is a nice example of stabilizing selection, where predation is constantly nudging mice to match their local environment as closely as possible.

In other words, it was possible for mice to be “too blonde.”

But before brunettes start gloating: the same was true for very dark coloured mice on dark soil.

But while blonde mice may have a selective advantage over brunette mice on the light-coloured beaches, brunettes could console themselves by remembering there’s a lot more inland than there is beach.

Reference

Vignieri, S., Larson, J., & Hoekstra, H. (2010). The selective advantage of crypsis in mice. Evolution DOI: 10.1111/j.1558-5646.2010.00976.x

Mice with coin photo from here. Plasticine mouse photo from here. Blondes from here.