Circadian colour change in Rana graeca in Albania
by Henrik Bringsoe - 30/4/2010 12h09For decades I have made several field trips to mainland Greece, mostly the Peloponnese, and I have often watched the stream-dwelling frog Rana graeca. It is commonly seen in clean mountain stream and generally has a brown dorsal and lateral coloration, ranging from greyish brown to dark brown, sometimes reddish brown or brown tinged with yellow. I have noticed that coloration during the day and at night.
From 28 March through 12 April 2010 my wife and I were in Albania with the purpose of herp watching and bird watching. We checked a number of mountain streams but only at one locality we found Rana graeca. That was in a mountain stream in southwestern Albania, i.e. between Vlorë and Sarandë. Altitude approx. 200 m. The air temperature during the day was approx 20 degr. C and at night approx. 8-10 degr. C.
Our first visit at that locality was in the afternoon on 2 April. I observed approx. four or five R. graeca and photographed three of them. I did not really pay much attention to them as they seemed quite normal, as I had seen them in Greece.
At night I made another excursion along the stream. That was from 21:30 to 23:30. One major purpose was to find adult Salamandra salamandra, but I did not find any. The night was quite dry and there was no dew on the vegetation which was probably an important reason.
But I saw in total five Rana graeca which had a totally different coloration from what I saw at daytime. And they were very different from what I have seen previously in Greece (day and night trips). All five had a distinct yellowish coloration. The dark bars on the hind legs and other dark parts were not so clear anymore and had also turned yellowish.
I will here present photos of three of the frogs (i.e. three different frogs) from the night excursion. None were captured.
Indeed I did not mark any of the four or five frogs which I found during the day, so I cannot say for sure whether it was the same frogs which had turned yellow at night. But I find it highly likely that those frogs which were brownish at daytime had become yellow at night.
Has anybody ever seen any such circadian colour change in R. graeca? Or in other brown frogs? I know several tropical treefrogs which change colour from day to night (for instance from dark to light) and such changes also occur in several Middle American plethodontids such as the genera Bolitoglossa and Chiropterotriton. But these amphibians are all nocturnal, i.e. inactive during the day. In the case of R. graeca, I found them during their activity at daytime and at night which is quite normal.
What is the explanation for such a change from day to night? When we look at the rocky substrate in the Albanian stream, it also looks yellowish. From that point, the yellowish coloration of the frogs might have been crypsis (camouflage). But in my opinion it does not offer a full explanation because I think a cryptic coloration would be much more useful during the day. I think most anuran predators will use olfaction (smelling) for night hunting. For instance snakes. During my night trip I found one adult Natrix natrix persa along the banks of the stream, most probably hunting frogs.
Also a few freshwater crabs, probably Potamon fluviatile, were observed and one was photographed. They will indeed eat tadpoles and small frogs. I do not know to what extent they can catch and eat adult R. graeca, and I do not know whether crabs mainly use vision or smelling for detecting frogs.
A my night excursion I also saw one larva of S. salamandra.
Any comments will be very welcome!
Kind regards,
Henrik Bringsøe
Ilian Velikov |
Henrik Bringsoe |
Ilian Velikov |
Henrik Bringsoe |
Ilian Velikov |
Henrik Bringsoe |
Henrik Bringsoe |
Jeroen Speybroeck |
Henrik Bringsoe |
Jeroen Speybroeck |