Why are some Snakes Black?

Red shouldered hawk. ©Bill Dunson

It is a normal reaction for the human mind to become accustomed to certain things and not to question them. So among the myriad of things I come into contact with every week I often see a large black snake whether I am in VA or FL. In VA this is a black rat snake and in FL a black racer. Although they resemble one another superficially they are not closely related and differ in several significant details. For example the black rat snake is a very accomplished climber and feeds heavily on birds (see photo right, black rat snake eating a starling) it is a relatively slow mover, it constricts its prey, has keeled scales on the back, and has flecks of white intermingled with its shiny black exterior. The black racer can move very quickly and tends to remain on or near the ground, it does not constrict its prey, it has smooth scales, and is colored a dull black color. So why are both of these two snakes so similar in color, and why black?

The story becomes even stranger when one looks at the juvenile snakes (see photos). Both are mottled in coloration and again somewhat similar. In addition to the differences in the shapes of the head and body (rat snake has a "loaf of bread" configuration, the better to climb trees), the blotched pattern is somewhat different. For example look at the top of the tail where the racer has no pattern and the rat snake has blotches. The purpose of this immature pattern seems clear as a means of camouflage. Indeed another member of the rat snake clan, the southern yellow rat or chicken snake has a similarly blotched, if yellowish juvenile.

Juvenile Black Racer. ©Bill Dunson

The purpose of the black coloration in the adults is less clear. In the north there are both black rat snakes and black racers, often in the same general areas. In the far south, in addition to plentiful black racers, there are yellow, red (corn) and gray rat snakes but no black rat snakes. So there is some geographic effect on the presumed value of the black coloration. The purpose of the coloration seems most likely to be related to protection from predators and/or a means of camouflage while the snake is hunting prey. One intriguing theory I have heard is the possibility that, depending on color, the unusual sinuous movement of snakes can make it difficult for predators to focus on a spot to attack. So snakes which are black or striped may receive some protection from capture by a predator such as the red shouldered hawk. These medium-sized hawks spend a lot of time perched in trees watching the ground for any movement. Some hesitation in attacking because it was hard to focus on the prey might be crucial in allowing a snake to escape. At least this is a plausible theory that might be tested. What are your favorite theories?

Bill Dunson

Galax, VA & Englewood, FL

The bagworm- it builds a house and comes to eat your plants

I came across an old acquaintance today while walking near Arcadia,FL, the bagworm. This curious caterpillar of a moth with an unpronounceable name, Thyridopteryx ephemeraeformis, has a very strange habit that makes it very distinctive and easy to identify. As its name implies it makes a case or bag out of silk and bits of plant material and crawls around with only its head and front end exposed. The bag is an ingenious contraption of small pieces of stems and leaves of the plant that the caterpillar is feeding on. In this case the plant was a hog plum, but this moth feeds on a wide variety of plants, causing it to be a serious pest in some situations. Clearly the bag serves to protect the caterpillar in two ways; it is both a camouflage and a physical protection against attack by predators such as birds. However the caterpillar must come at least partly out to feed and can be attacked then.

For some amazing photographs of the Bagworm check out Victor Engel's work: http://bugguide.net/node/view/135982/bgimage

The breeding habits of the bagworm are quite peculiar- the female is flightless and remains in the case for her entire adult life. The adult male must fly to her location and fertilize her eggs, which are laid in the bag where they remain all Winter. They hatch in the Spring and the young larvae disperse and make their first bags. Although this species is named for a mayfly (which the adult male slightly resembles), there is a greater resemblance to the caddisfly larvae which lives in water but also constructs a case out of sticks, sand or small pebbles. Now although caddisflies and bagworms are somewhat related, they are in separate groups and it seems unlikely that a common ancestor had the habit of constructing larval cases. So we may conclude that their shared habit of constructing cases for their larvae is probably an example of separate but convergent evolution.

So if you see bagworms on one of your valued plants you might just want to move them someplace else since they can do some serious damage. The simplest time to do this is in Winter when the bags contain hundreds of eggs and can easily be picked off the plants. But in any event let us marvel once again at the remarkable diversity of nature.


Bill Dunson
Englewood, FL & Galax, VA