Abstract
Many species alter skin color to varying degrees and by different mechanisms. Here, we show that some crocodylians modify skin coloration in response to changing light and environmental conditions. Within the Family, Crocodylidae, all members of the genus Crocodylus lightened substantially when transitioned from dark enclosure to white enclosures, whereas Mecistops and Osteolaemus showed little/no change. The two members of the Family Gavialidae showed an opposite response, lightening under darker conditions, while all member of the Family Alligatoridae showed no changes. Observed color changes were rapid and reversible, occurring within 60–90 minutes. The response is visually-mediated and modulated by serum α-melanocyte-stimulating hormone (α-MSH), resulting in redistribution of melanosomes within melanophores. Injection of crocodiles with α-MSH caused the skin to lighten. These results represent a novel description of color change in crocodylians, and have important phylogenetic implications. The data support the inclusion of the Malayan gharial in the Family Gavialidae, and the shift of the African slender-snouted crocodile from the genus Crocodylus to the monophyletic genus Mecistops.
Highlights
Color change for the purposes of communication, as described by Korzan et al.[8], may signal dominance, aggression, or reproductive state[9,10]
The only two extant members of the Family Crocodylidae, which are not included in the genus Crocodylus, the African dwarf crocodile (Osteolaemus tetraspis) and the African slender-snouted crocodile (Mecistops cataphractus), showed little/no ability to change color
This implies that the common ancestor to the genus Crocodylus developed the capability to alter skin color after 30–40 mya and prior to 12–17 mya, when a rapid divergence of the genus Crocodylus occurred (Fig. 2)
Summary
Color change for the purposes of communication, as described by Korzan et al.[8], may signal dominance, aggression, or reproductive state[9,10]. Saltwater crocodile (Crocodylus porosus) hatchlings developed a dark or light color when raised in dark or light tanks, respectively, for three months. These same animals reverted to the opposite color three weeks after being placed in a tank of opposite color. Since crocodylians are slow, methodic, stalking hunters, the ability to alter skin color to match surroundings would certainly aid in predation[33]. We report the varying abilities of all 24 species of extant crocodylians to alter skin coloration in response to environmental light conditions, as well as the physiological mechanism that regulates color change in the genus Crocodylus. The ability to alter skin color, and type of alteration, is strongly divided along phylogenetic lines
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