Dichotomous Keys and Keying

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Dichotomous Keys and Keying
A dichotomous key is a tool that allows to
determine the identity of items (e.g. species).
Keys consist of a series of choices that lead the
user to the correct name of a given item.
"Dichotomous" means "divided into two parts".
Therefore, dichotomous keys always give two
choices in each step.
Note that dichotomous key has NOTHING to do with
biology alone. It is simply a tool. You can make
a dichotomous key to describe ANYTHING.
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Dichotomous Keys and Keying

• Key Construction
• Dichotomous keys are a series of parallel and
  mutually exclusive (contradictory) couplets.
• Canine present
• Canine absent
• Each couplet has two parts (e.g. 1 and 1)
• 1) canine present
• 1) canine absent
• At the end of each statement is a link to the
  next couplet
• 1) canine present .2
• 1) canine absent ..6

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Dichotomous key in Mammalogy

• General rules
• Ideally, each statement in the first part of a
  couplet is contradicted by a corresponding
  statement in the second part of a couplet -
  something "is" instead of "is not.
• Postorbital process is present
• Postorbital process is absent
• If possible, start both choices of a pair with
  the same word.
• Postorbital process is present
• Postorbital process is absent
• If possible, start different pairs of choices
  with different words.
• Precede the descriptive terms with the name of
  the part to which they apply.
• Postorbital process is present, interorbital
  region broad
• INSTEAD OF
• Present postorbital process, broad interorbital
  region

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Dichotomous key in Mammalogy

• Selection of Key Character
• Keys may be based on cranial features, skeletal
  features, or external soft anatomy features.
• The character selected should be valid for both
  sexes and all age classes
• The author must keep in mind the portion (s) of
  the organism that the reader of the key will
  examine. For example, a field ecologist can
  utilize a different set of characters than a
  museum curator.
• Key characters should be easily observable
• Characters should exhibit little individual
  variation
• Absolute conditions (such as the presence or
  absence) are the best
• Discontinuous characters (e.g. one versus two
  incisors) are useful
• Continuous characters (e.g. length of a hind
  foot) should be used only when there is no
  overlap
• Characters that call for a value judgment are
  problematic

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Infraorbital foramen on or near zygomatic
plate X Infraorbital foramen on side of rostrum,
anterior to zygomatic plate
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