Morphological and Ontogenetic Skin Color Changes in the American Alligator (Alligator mississippiensis)

SIMPLE SUMMARY: The skin of American alligators became lighter when animals were exposed to light colored environments. This change occurred over 2–3 months, and was caused by increased pigmented cells in the dermal layers of the skin. Alligators hatch with bright vertical yellow and black stipes that presumably function as camouflage in emergent vegetation. However, the yellow stripes gradually fade/darken until alligators become a homogenous dark gray/black color as adults. Because alligators live in more temperate climates than other crocodilians, we propose that the loss of yellow stripes might help alligators thermoregulate more efficiently during colder months. We measured the brightness of stripes on wild alligators of a broad spectrum of sizes from three different latitudinal locations within their natural range: northern (South Carolina), central (south Louisiana/southeast Texas), and southern (South Florida). The alligators in South Carolina lost stripes faster as they grew larger, while the alligators in the middle portion of the range became darker at a slower rate, and animals in south Florida lost their stripes at an even slower rate. The loss of stripes at different rates might allow alligators to be more efficient at thermal regulation in the different areas that they inhabit. ABSTRACT: To assess skin color change in alligators, we maintained animals in differently lighted environments and also measured skin colors in an ontogenetic series of wild animals. Juvenile alligators maintained in black enclosures exhibited a gradual lightening of skin color when shifted to white enclosures, and these observed changes were reversible. A histological examination of the skins of alligators maintained in dark tanks showed that the dermis exhibited a dense layer of pigmented cells, while samples from the same animals in light environments exhibited a more diffuse pigmented layer. As alligators grow, they exhibit an ontogenetic loss of stripes that may aid in crypsis and predation. Hatchlings have intense black and yellow vertical stripes that darken with age; adults are a more homogenous black/gray color. Since alligators live in temperate climates and adults have lower surface area/volume ratios, which can be detrimental for the absorption of radiant energy, the darker color of larger animals may also aid in thermoregulation. Alligators at the northern end of their range, with colder climates, exhibited darker skin tones, and the ontogenetic extinction of stripes occurred at a more accelerated rate compared to animals in southern, warmer regions, supporting the idea that latitude-dependent ontogenetic color shift has a role in thermoregulation.