Melanistic Garter Snake

For half of this summer, I'll be living up in Northern Ohio helping with some salamander research as a field assistant. It's only been 2 weeks so far, but we've found so many interesting and wonderful things. The best find of last week came from one of the field sites on South Bass Island in Lake Erie.

Olivia Brooks, who is the one behind the nature-oriented Twitter account Wild Earth, is the other field assistant. We're always on the lookout for snakes, and while in the middle of trying to find salamanders, Olivia yelled "snake!" We ran toward Olivia and were met by a very young Eastern Garter Snake. I've covered Eastern Garter Snakes before, and you might notice that this one doesn't really look like a garter at all. The coloration is all wrong, right? Garter snakes aren't black?!

This is indeed an Eastern Garter Snake, Thamnophis sirtalis sirtalis, but it is an individual with melanism. Melanism is essentially the opposite of albinism. While albinism is the absence of melanin (a dark colored pigment found in skin), melanism is the overabundance of melanin, leading to an individual with an abnormal amount of black coloration. Melanistic Eastern Garter Snakes can be found along the Western Basin of Lake Erie and on some of the Lake Erie Islands. Some populations can be up to 50% melanistic in this area, meaning there will be a mixture of normal-looking and melanistic individuals within one area.

The photo above shows what a normal Eastern Garter Snake looks like. This normal-colored individual was found at the same exact location on South Bass Island as the melanistic individual we found. I have no clue as to what the exact ratio between the two types is, but it would be very interesting to do a survey and get an estimate!

So why is there such a high concentration of melanistic garters in Northwest Ohio? Mutations that lead to melanism can arise randomly in any animal that has melanin; however, mutations that cause melanism and albinism are very rare. This leads to sporadic occurrences of the color abnormality that randomly occur across multiple populations. For a trait like melanism to "build up" in a population, there must be something else going on. More specifically, being melanistic must benefit the individual in some way. If the trait was bad for the individual, it would quickly be selected out of the population. If it had a neutral effect, individuals with the trait would only exist in very small numbers. Adaptive melanism can be adaptive for many different reasons. For example, in Black Panthers, which are actually just melanistic Jaguars, melanism leads to better camouflage as these predators hunt in the dark of night. In the Eastern Garter Snake, being melanistic makes it easier for an individual to warm up when basking in the Sun. The color black absorbs all the wavelengths of the white light of the Sun, resulting in the black individual gaining relatively more heat energy than the predominantly brown individuals. Snakes are ectotherms ("cold-blooded"). This means that they do not hold a constant internal temperature like we do, but instead rely on the ambient heat of the environment to regulate their internal temperature. For a snake to be able to hunt successfully, their body temperature must reach a certain threshold. By being able to reach that threshold quicker and easier, melanistic garter snakes have an advantage in being able to find and eat more food than the normal individuals, who have to wait longer for their temperature to reach that threshold.