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I think there have been some misinterpretations with my observation in color change. I think, it is not stress, but diving into the (very) cold water - therefore a temperature problem ?

That could be the reason why "my" arvalis did not change colour when dive. They´re mating in very shallow water near bank of the pond where water is not so cold even if they dive.

Bert Vandebosch wrote:So to conclude: my theory is just plain old camouflage! What do you think?

I have a few issues with this theory:

1) Why would only males develop camouflage? That would not be a very effective strategy. Mostly females would be killed by predators as a result of their more "conspicuous" coloration. And males in a breeding pool already outnumber females by far. I'm not convinced that females would rely on having a male to "camouflage" them on their backs.

2) From a bird's-eye point of view, in other words from high above, the water would not look blue, it would be transparent so potential predators would see clearly what's under the surface. Anyone that's ever looked down in a river from a bridge would know that. Therefore blue frogs would stick out against the brown/dark bottom. To see the sky's reflection in the water you have to be low down (or at eye level), and at an angle to the surface. And overall the idea that a coloration would serve as a camouflage only at a particular angle/point of view doesn't make sense for me.

3) Animals that rely on camouflage would usually adopt a defense strategy of staying completely still when there's potential danger. Breeding arvalis and other frogs do exactly the opposite they escape immediately by diving as quickly as possible. Not to mention the whole commotion involved in breeding. Even if they were transparent it would be hard to miss tens to hundreds of frogs jumping and wrestling with each other in the water.

So, after all this exchange of facts and educated guesses and opinions (not the first one...), how much wiser we are?
What's for sure?
WHY RANA ARVALIS TURNS BLUE?

I think there have been some misinterpretations with my observation in color change. I think, it is not stress, but diving into the (very) cold water - therefore a temperature problem ?

Yes that could be the explanation.

@Ilian

1) Evolution is no strategy: Why don't females turn blue? Probably it's impossible. The mechanism of color change in this case requires most likely some high level in male hormones. Females only spend a minimum of time in the water. After spawning they flee the harassment of the males. Chances of being killed by predators while in the water are thus much lower for females.
I think we can assume that arvalis and temporaria share a common ancestor. So the question is: why does arvalis turns much more blue than temporaria? Probably at one point they were separated and evolved in different habitats. I'll bet that arvalis occupied a colder climate in a sparsely vegetated habitat. It is called moor frog for a reason. Therefore in general they breed very early in the season (if seasons are short, they would need to), they prefer bogs and heathland as a habitat and they might be found breeding during the day more often because nights are too cold for activity.
Temporaria can also be found breeding during the day but much less than arvalis and always more hidden between vegetation or in small pools covered in shade. Arvalis has real peak moments of breeding activity at high noon and they venture out more in the open.

2) and 3) If you look straight down from a bridge in the shadow of the bridge, yes it doesn’t look blue. But I stood on a higher point overlooking a bog/pond recently. It appeared very blue, more so than looked at from the water’s edge. Even with a little bit of wind (or even frogs moving and creating small waves), you will see a lot of “sun glitter”; and for me it seems logic that a blue frog is less conspicuous in open water during a bright sunny day. I don’t say they become invisible and never get eaten. They just might get overlooked more often by a flock of hungry cranes or whatever flew around when they evolved this feature. If I only had a drone to test my crazy theory …
A fleeing reaction doesn’t contradict with camouflage. It’s a very basic instinct that acts supplementary to camouflage. When you walk passed a water’s edge with a lot of pelophylax around,they all flee by diving in the water while they were perfectly camouflaged and readily overlooked.

Of course I realize that my theory can be completely wrong. I just wanted to share a completely different view point. In these cases not much is for sure. Maybe turning blue lost its relevance over time in changing environmental circumstances but if it doesn’t present a disadvantage, such a feature can linger in a species.

Bert, thanks for sharing your thoughts. Much of what you said is true and I'm not disregarding your theory but still personally for me the the immediate sex recognition as a means of saving energy (for something such as breeding which needs a lot of it) after months of starvation seems more plausible.

Hi, I will use this topic - did you ever seen male arvalis in mating colours with blue eyes? Never saw it by myself, but sometimes I see that on photos of other people.
I wonder, if it is just an effort of photographers to make them even more bluish (very blue throat which is normally white and whole photo tint looks bluish to me) or if it is possible and some males are so blue?

Example:


Thanks,
T.

oh yes! they might be really so blue.
Look at these pictures from our arvalis excursion in the surroundings of Vienna
http://herpetozoa.at/index.php/veranstaltungen/17-exkursionen/106-die-moorfrosch-expedition-der-oegh-2018