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類胡蘿卜素作為有機色素，在多種重要的生理功能中發揮作用，而且它還可以作為衡量動物個體質量的指標。動物通過飲食獲取這些類胡蘿卜素，然后在體內經特定的酶促反應生成其它類胡蘿卜素。農場飼養或者野生紅腿鷓鴣的食物主要是富含葉黃素和玉米黃質的谷物。這兩種類胡蘿卜素在紅腿鷓鴣內部組織和血液中含量最高。但是，在紅腿鷓鴣的表皮中蝦青素和蝶刺桐酮（最后被鑒定的）游離存在或者是與脂肪酸發生酯化反應。根據有關動物中類胡蘿卜素代謝機制的文獻，我們認為蝦青素(λ max = 478 nm)和蝶刺桐酮(λ max = 452-478 nm)分別是食物中葉黃素和玉米黃質轉化后顏色趨同的結果。而且，本研究通過液相和質譜相結合的方法第一次在脊椎動物的皮膚（不是羽毛）中鑒定出蝶刺桐酮。蝦青素和蝶刺桐酮的化學分子結構及顏色很相近，這兩種酮類類胡蘿卜素是造成紅腿鷓鴣表皮紅色的主要原因。

Astaxanthin and papilioerythrinone in the skin of birds: a chromatic convergence of two metabolic routes with different precursors?

Carotenoids are organic pigments involved in several important physiological functions and may serve as indicators of individual quality in animals. These pigments are only obtained by animals from the diet, but they can be later transformed into other carotenoids by specific enzymatic reactions. The diet of farm-reared and probably wild red-legged partridges (Alectoris rufa) is mainly based on cereals that contain high levels of lutein and zeaxanthin. These two carotenoids are also predominant in internal tissues and blood of red-legged partridges. However, in their integuments, astaxanthin and papilioerythrinone (the last one identified in this work) are mainly present in their free form and esterified with fatty acids. According to available literature about carotenoid metabolism in animals, we propose that astaxanthin (λ max = 478 nm) and papilioerythrinone (λ max = 452-478 nm) are the result of a chromatic convergence of the transformation of dietary zeaxanthin and lutein, respectively. Moreover, the results obtained in this work provide the first identification by liquid chromatography coupled to accurate mass quadrupole time-of-flight mass spectrometer system of papilioerythrinone (m/z 581.3989 [M + H](+)) in the skin (i.e., not feathers) of a vertebrate. Astaxanthin and papilioerythrinone are very close in terms of chemical structure and coloration, and the combination of these two keto-carotenoids is responsible for the red color of the ornaments in red-legged partridges.