If you are trying to create a dichotomous key for a series of animals, you might see that some have feathers, some swim, some walk on legs, etc. For example, if you are trying to differentiate a set of big cats, you might note that some are brown, some are black, some have stripes, some have spots, some have long tails, some have short tails, and so on.
However, a trait all of the animals share is not a good distinguishing factor. For example, since all big cats are warm-blooded, you wouldn’t want to use that trait on your dichotomous key.
When making your dichotomous key for big cats, you may find that some of the cats you are analyzing have dark fur, and some have light fur. You may also see that all of them have short hair. Finally, you see that some of them have long tails, but some of them have no tails at all. You would start your key with a question/statement about fur color. You wouldn’t need to ask a question about fur length, since all of the examples have short fur. You would follow up with a question about tail length, since tails are not common to all of the cats, and therefore are a less general characteristic.
For example, “The cat has solid-colored fur” or “The cat has patterned fur” are statements that can be used to break specimens down into two groups. Include the question “Does the cat have solid-colored fur?” in your dichotomous key to divide the animals. If the answer is “Yes,” then the cat belongs in the solid-colored fur group. If the answer is “No,” then the cat belongs in the patterned-fur group.
For example, separate your cats based on whether they have solid or patterned fur. Similarly, if you note that all of your specimens have either feathers or scales, then these could be your groups A and B. You can begin your key with the question “Does the animal have feathers?”
For instance, you might notice that some of the animals in group A swim, and others don’t. This differentiation can form level C/D for group A. Likewise, you might see that some of the animals in group B have legs, and others don’t. This differentiation can form level C/D for group B.
Some specimens will be differentiated before the end, as you work through your contrasting characteristics. For instance, you might find yourself looking at some birds, and some reptiles. You will break them down into these groups, then sub-divide the birds. Two of the birds swim, but one of them does not. The single land bird will be identified as such, but you will have to further differentiate the swimming birds. In this case, you notice that one of the swimming birds is ocean-going, and one is not. This characteristic can allow you to identify them more precisely (e. g. , as a gull and a duck).
You could also try organizing your key in the style of a flow chart. For instance, have a box that asks a question like “Does the cat have dark fur?” Then, have a “Yes” arrow leading one way, and a “No” arrow leading another way. The ends of the arrows can lead to new boxes where you ask the next questions.
Q: ”Does the animal have feathers?” A: “No” (it has scales, so it is a reptile). Q: “Does the reptile have legs?” A: “No” (it is a snake, either a cobra or a python, given your specimens). Q: “Does the snake have a hood?” A: “No” (so it is not a cobra). Your specimen is identified as a python.
For instance, “Does the cat have solid-colored fur or stripes?” is not a useful question for a dichotomous key. This question might differentiate solid-colored and striped cats from cats with spotted fur. However, since solid-colored and striped fur are themselves very different, this is not a useful category to work with. Instead, you might first have a question that asks about solid-colored vs. patterned fur, then follow up with another level of questions like “Does the cat have black fur?” and “Does the cat have stripes?”
