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Many animals have red tongues, though not all of them - blue-tongued skinks and giraffes come to mind. Are they red because of the blood (and therefore haemoglobin) in them?

It sounds plausible, but it also sounds like a post-hoc explanation.

I'd be especially curious about any other species having blue blood, like horseshoe crabs, but still having red tongues, unlike horseshoe crabs, which apparently don't even have tongues.

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i would think so, but if a tongue is not red, its probably because there are other colored molecules in the tongue besides iron globins - it wouldn't necessarily mean the blue tongues have less red coloring. –  shigeta Nov 22 '12 at 12:54
    
The tongue is just a huge muscle, so it should have its colour from the same source as other muscles - which to my knowledge is blood. Unless it is pigmentated in its skin :) Human tongue I'm pretty sure is due to blood. –  Armatus Nov 22 '12 at 19:43

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up vote 6 down vote accepted

Mammalian tongues are red because of haemoglobin. Blue/black colouration is due to the additional presence of melanin.

At the boundary between the epidermis and dermis are melanophores, cells that contain melanin. This brown pigment absorbs ultraviolet radiation from the sun that might otherwise damage the underlying dermal tissue. Melanin in the dark tongues of giraffes (Giraffa camelopardalis) protects the tissue from sunburn as the animals lift their heads into the top branches of trees to forage for leaves. In humans, synthesis of melanin increases upon exposure to sunlight and produces tanning. Extensions of melanophores reach into the epidermis where they inject their pigments into developing hair cells. In a few cases, such as the bright ischial callosities of baboons, pigmented patches of skin are used as visual signals. The blue and red coloration on the scrotum and perineal region of male vervets (Chlorocebus aethiops) is used in dominance displays.

From: Mammalogy: Adaptation, Diversity, Ecology by George A. Feldhamer, Lee C. Drickamer, Stephen H. Vessey and Joseph F. Merritt (2007) p 97 The Johns Hopkins University Press; 3rd edition edition

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