Texas Parks & Wildlife Magazine   


April 2009 cover image dorrado fish in the gulf

Skill Builder: Unlocking Keys

Guides present simple questions that can help you identify wildlife.
By Dyanne Fry Cortez

Let’s say you have a brand-new field guide. You open it to find pictures and descriptions of trees, turtles or whatever type of wild thing sparks your interest. Depending on the guide, you may also find a section called a “Key to Species” or “Identification Key.”

Scientists have long used keys to identify animals and plants collected in the field. If you haven’t seen one before, it may look like gibberish at first. Once you understand the system, you’ll find that a key often works better — and faster — than flipping through pages in a pictorial guide.

Keys are typically “dichotomous,” meaning “divided into two parts.” The classic dichotomous key is a series of couplets, or paired lines. Each couplet presents an either/or choice based on observable characteristics. You choose the line that best describes your specimen. A number at the end of the line directs you to another couplet in the key.

Here’s an example from The Mammals of Texas. Let’s say you’ve found a small rodent. You turn to the “Key to Rodents” and start with couplet 1:

1. Presence of external, fur-lined cheek pouches: 2
Absence of external, fur-lined cheek pouches: 16 Your animal appears to have cheek pouches, so you proceed to couplet 2:

2. Front feet much larger than hind feet; ear (pinna) short and inconspicuous; tail about half the length of head and body: 3
Front feet much smaller than hind feet; ear (pinna) conspicuous; tail as long as (or longer than) head and body: 5
Your animal fits the second description. Skip to couplet 5:

5. Hind legs more than twice as long as front legs; tail long and bushy at end; head broad, 25 mm or more in width (kangaroo rats): 6
Hind legs less than twice as long as front legs; head about 15 mm in width (pocket mice): 10
Your specimen has long hind legs and a tuft at the end of the tail, so it’s probably a type of kangaroo rat. Couplet 6 and those that follow ask you to compare other details.

Think of each couplet as a fork in the road. You choose a path, follow it to the next fork, and choose again. Eventually, the road will lead to one of the five species of kangaroo rat that can be found in Texas.

To complete the identification process, find that species in the field guide and compare your specimen with the written description. If it doesn’t match, you may have taken a wrong turn. If you weren’t sure about some of your choices, back up and try again.

Tips for Using Keys
Keys work best for organisms that will hold still long enough to be examined: trees and flowers, fish or small animals that you can catch and temporarily restrain, or any critter that’s dead when you find it.
Some keys use terms that only a scientist would understand. Others are written for the casual naturalist and may have illustrations to help. Look for a key that’s geared to your level of expertise.
Don’t guess! Use a magnifying lens if you need it. If you see a word you don’t know, look it up.

Some Keys to Try
•   David Schmidly, The Mammals of Texas. University of Texas Press, 2004. (Also available online at: www.nsrl.ttu.edu/tmot1/)
•   Alan Tennant, A Field Guide to Texas Snakes. Taylor Trade Publishing, 2006.
•   Texas Turtle Regulations and Identification Key. (www.tpwd.state.tx.us/regulations/)
•   Roy A. Lehman, Ruth O’Brien and Tammy White, Plants of the Texas Coastal Bend. Texas A&M Press, 2005.


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