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The cross taxa information is conflicting in some respects. Some of the taxa do show competition in the uptake of the carotenoids as the carotenoids are transported with the small lipid micelles. In some other taxa like iguanas show selective uptake in different sections of the digestive tract, see for example Selective absorption of carotenoids in the common green iguana (Iguana iguana)

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Note that vitamin A2 is not derived from carotenes and that the iguanas show selective uptake of xanthophylls as opposed to carotenes.
To some extent this does mimic some of what happens in anurans.

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Interesting stuff:

I have been supplementing some pumilio with a mix of carotenoids(Repashy superpig in the FF media, dusting with paracoccus powder and spirulina)...and I swear it looks like the female is turning more yellow orange over time(when I first got her she was more of a reddish orange). I am a bit red/green colorblind-but I have found it has become more difficult for me to tell her apart from the male who is a yellow/gold colored.
This could be due to different demands placed on the female for provisioning the eggs versus the male. I found that adding straight astaxanthin to the dusting supplement produced the greatest changes in reds. When I tried this with caucheros the frogs would develop very small bright red flecks on their backs. If I then stopped adding the additional astaxanthin to the supplement, the red flecks would decrease over time.

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Ed
 

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Again a wild source of astaxanthin for frogs seems to be lacking.
I have to admit I'm really puzzled that you made this statement. Astaxanthin is very important for the proper metabolism and pigmentation for a wide number of insects and has been found in many different species including things that don't feed on things in an aquatic environment like grasshoppers.
Frogs don't appear to have the ability to modify carotenoids like we seen in birds but insects do have this ability.
There are pathways to produce astaxanthin from other carotenoid such as keto- oxidation of zeaxanthin to astaxanthin... Zeaxanthin is fairly commonly available carotenoid in the environment....

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Ed
 

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Ed: Hmm...is their any creature that is high in astaxanthins that would be suitable for culturing to feed frogs? I recall hobby isopods have been found not to contain carotenoids the way shrimp do. I'd bet terrestrial amphipods are no different.
In general the insects that have been analyzed to date have mainly been pretty poor sources of carotenoids.
Heating the isopods did not change their color which indicates that carotenoids are not bound up in the exoskeleton as it occurs with shrimp, crabs and lobsters. In those crustaceans, the carotenoids are bound to a heat labile protein that seperates from the carotenoid when heated providing the pink to red coloration. In at least one marine isopod, canthaxanthin and lutein have been found in the tissues. Totally ruling out isopods is premature at this time.

Also, it is interesting to look at old debates on "enhancing" frogs to become more colorful than in nature. Theoretically, this is possible. Apparently the wingtip colors of Cedar Waxwings change from sulphur yellow to orange when they feed on a newly introduced berry apparently rich in carotenoids that can be turned into red pigment.
Theoretically yes but we have not seen this to occur to this date. We have seen an intensification of coloration so it approaches the color seen in wild collected frogs. This is probably due to several factors in play such as inefficient uptake, and high demand on the stores in the tissues. Carotenoids have multiple roles in various tissues and are under demand for both as a previtamin A as well as provisioning of the eggs. In other tissues, the carotenoids act as precursors for rhodopsins, oil droplets for visibility into the red end of spectrum, as well as antioxidants.
In the tissues carotenoids that have reacted with antioxidants are not regenerated unlike some other antioxidants in the tissues. This is visible as a progressive bleaching of the carotenoids.. To some extent this can be used to explain why some frogs in captivity change the hue and intensity of their color over time (such as the red forms of pumilio).

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