African Mud Turtles

An Essay on Taxonomy and the Genus Pelusios

by David Kirkpatrick, Ph.D

Taxonomy is the classification of differing organisms into groups based on a set of defined characteristics that is common to all members of the group. Taxonomists examine animals (and plants), determine what features make that particular animal unique, and then compare related animals to verify that the supposedly unique characteristics actually are unique. Until recently, this study involved careful measurement of physical characteristics of an animal - for example, the size of various body parts or the organization and interaction of bones. The advent of molecular biology has added new characteristics for the taxonomist to consider. Chief among them is the degree to which an animal's DNA (the substance that carries the "blueprint" for the animal) is different from that of another animal that superficially may appear similar.

Unfortunately, taxonomists do not always agree as to the proper distribution of groups of animals. Disagreements can arise because of a number of factors. These include a changing evaluation of the relationships between groups, improvement in the techniques or measurements that are being used, a reassessment of the value of various characteristics, and additional data that reduces the significance of previous studies. As differences are brought up, discussed, argued and perhaps resolved, the nomenclature can change, although the organism in question remains the same. This evolution in names means that one species may have been placed into a number of groups and called different names over time. This variation can make it difficult to find information about a specific animal, as it may have been referred to by a number of names since it was initially described. However, even if the nomenclature has changed a number of times, the information is still valid - it just has to be applied to the correct animal!

Although perhaps not directly apparent, taxonomy can have numerous benefits to people (both professional scientists and amateur enthusiasts) and to the organisms themselves. Being able to place animals in groups allows scientists to apply information that they discover about one animal to other, related animals and allows different scientists, perhaps thousands of miles or decades apart, to compare information without fear of inadvertently being led astray. Belonging to a specific group can also benefit a population of animals. Currently there is an ongoing debate as to whether populations of rattlesnakes in various states constitute distinct subpopulations. If they are considered unique, there is a greater chance that they can be protected under the Endangered Species Act in those areas where populations are declining. If those declining rattlesnake populations are not considered to be unique, then the potential loss of that population is not a problem deemed worthy of federal intervention, as there are other rattlesnake populations remaining. Finally, taxonomy can directly benefit a single captive animal and the person caring for that animal. Taxonomy allows a person to identify an animal, and once identified, information already known about that animal can be used to improve its care, monitor its health, and help in setting up a breeding program. Turtles in the African genus Pelusios, the sidenecked mud turtle, are a prime example of the ongoing process that is taxonomy and also demonstrate why and how a knowledge of taxonomy can help captive populations.

Before discussing the genus Pelusios, a quick introduction is in order. Turtles of the world can be classified into two major groups. People unfamiliar with turtles might be inclined to place the land turtles into one group and the aquatic turtles into another. In truth the groups are divided by a more subtle characteristic - the way in which they pull their heads into their shell. Three-quarters of the chelonian species existing today retract their heads straight back into their shell, leaving their eyes and nose pointing directly ahead. The remaining quarter of the turtle species fold their head in sideways so that their nose points to the left or right. This difference reflects a fundamental difference in the way that the animal's spinal column folds. In the first group, the Cryptodira (hidden-necked), the spinal column forms an up and down 'S' pattern, while in the second group, the Pleurodira (side-necked), the spinal column folds into a side to side 'S' pattern. People living in the Northern hemisphere are not as familiar with the side-necked turtles, as they occur much more frequently in the Southern hemisphere. All species of Australian turtle, for example, are pleurodiran. In Africa, one of the larger groups of sideneck turtles is the genus Pelusios (derived from a Greek word for earth or mud).

Side-necked mud turtles can be found throughout most of the sub-Saharan regions of Africa. The preferred habitat of individual species vary, but in general they are semi-aquatic turtles found in marshes, swamps and slow-moving rivers. Some species are quite restricted in range - Broadley's mud turtle (P. broadleyi) is found only in Lake Rudolph (also known as Lake Turkana) in Kenya, while P. seychellensis is found only on the Seychelles Islands. As might be obvious from their common name, many species of Pelusios prefer muddy, soft-bottomed bodies of water. In times of drought, turtles dig into the mud, spending weeks or months buried until the drought is over.

Turtles in the genus Pelusios grow to a moderate size, with the largest (P. sinuatus) reaching a length of approximately 18 inches. The smallest, P. nanus, only reaches a size of 4.5 inches. A strong hinge on the plastron is a distinguishing feature of the genus Pelusios. African mud turtles have carapaces that range in shape from oval to almost round; the carapace itself is not strongly domed. Generally they resemble the sliders and painted turtles of North America. Most species of Pelusios are not strongly colored; the carapace usually is some shade of brown or black, while the plastron ranges from yellow to black. The skin color ranges from gray through brown to yellow. While most species are relatively undistinguished, some do possess some attractive patterning. Some animals have distinctive dark and light patterns on the plastron; these can be useful in distinguishing between species. P. b. bechuanicus has a contrasting pattern of yellow stripes on its face, while the faces of a number of species have a fine yellow vermiculation on a dark background. Many species have two or three short barbels on the chin and a bright ring around the iris of the eye.

African mud turtles have been reaching North American markets in increasing numbers in the past few years. As they are relatively hardy, of moderate size, and fairly abundant in their natural range, they have been collected in large numbers, and so enter the pet trade as a cheap exotic turtle. Their natural ability to survive periods of deprivation probably contribute to their increasing exploitation. Fortunately, because of this hardiness, most turtles generally arrive in reasonably good condition. Most of the species of Pelusios are relatively undemanding in their needs, and so usually prosper under conditions suitable for North American semi-aquatic turtles such as sliders and painted turtles.

In their natural environment, African mud turtle species are carnivorous or omnivorous. In captivity they have been reported to take many of the foods normally fed to North American turtles, including various vegetables, fish, earthworms, insects, and prepared foods such as turtle or trout chows. However, the proportion of vegetable to animal matter preferred will vary between species. Pelusios species can be maintained at temperatures that are suitable for sliders and painted turtles: from the mid 70's to the mid 80's Fahrenheit. Captive African mud turtles have been known to estivate in underground burrows when the temperature gets too warm. When conditions return to a suitable level the estivating turtles re-emerge and become active once again.

Although maintenance of Pelusios chelonians presents little difficulty, breeding is a different matter. Although African mud turtles have been bred in captivity, much of this breeding has occurred in zoos and other institutions that acquired a documented population of a specific species. In contrast, many of the turtles entering the country for the pet trade will be sold as individuals with little accompanying documentation. Also, differences between species of Pelusios can be relatively subtle. The combination of these two problems means that it may be quite difficult to locate a suitable mate for an individual African mud turtle. At this point, the taxonomy of the genus Pelusios becomes quite important!

The status of Pelusios species has been the subject of much debate. Given the sometimes quite subtle differences between species, different authorities have divided up the genus in various ways. More and more species of Pelusios are being recognized, as subspecies are determined to be independent species and new populations of mud turtles are discovered. Although all of this argument over minor variations may seem too esoteric to be interesting, it has a number of practical benefits, as mentioned earlier. In order to place a turtle into a new species or combine two existing groups, taxonomists must rigorously show that the division or combination is warranted. This proof involves stating which characteristics of the turtles in question are being considered, how they vary from all other species in the genus, and why they indicate that the turtles should be grouped in a particular way. The end result of all of this taxonomic wrangling is a very good set of characteristics that can be used to definitively identify a particular turtle as belonging to a particular species.

Often the set of identifying characters is compiled into a "key" for the genus. Keys are a list of 'either/or' statements. An example might be - "Line 1: If the length of the plastron is more than twice the width, go to Line 2, otherwise go to Line 5". Each either/or statement narrows down the possibilities until you are left with a single species. A number of books on turtles contain keys, some very general, others quite specific. "Turtles of the World" by Carl Ernst and Roger Barbour is a good example, as it contains a key for most chelonian species, divided up by genera. Unfortunately, some keys use features that are not effective when trying to identify a live turtle - for example, it is hard to determine the shape of a particular bone in a live turtle! Features of a particular turtle that you might consider to be easy to use to identify the turtle might vary too much between individual turtles of the same species and so are not discussed by taxonomists. A knowledge of turtle anatomy (especially the names of different scutes and scute boundaries) is essential for using keys. Luckily, most good turtle books give detailed diagrams of the various body parts. With some work and practice, it is relatively easy to become skillful with a key. The more precisely a turtle can be identified the more information you can acquire about its specific requirements, and therefore the better the care that you can give it. This benefits both the turtle and yourself - a turtle receiving the proper care is apt to be healthier and more willing to breed in captivity.

Taxonomy is a tool that just about everyone uses, generally without realizing that they are doing so. Differentiating between organisms, placing them into groups - all of this comes under the heading of taxonomy. As with most tools, the more you learn about it, the easier it becomes to use. For the genus Pelusios, a knowledge of the taxonomic relationships within the genus can lead to better care for individual turtles, better success in breeding, and a greater appreciation of the special place each species has in its native environment.

This article copyright 1995 by David T. Kirkpatrick. Originally published in Reptile & Amphibian Magazine, March/April 1995, pages 32-40.

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