Introduction to the Tortoises

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Madkins007

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Introduction to the Tortoises


This article is intended to introduce you to the fascinating world of tortoises in general.

Taxonomy and History

[Author's Note: To be honest, this is a terrible time to use scientific names and systematics (grouping animals into classes, families, etc.) since new research is changing things faster than the average person can keep up, and anything I write here can be overturned any minute. For example, Treating turtles as a separate 'class' is not universally accepted. However, we'll continue using the best information I can find for now.]

The Animal Kingdom is divided up in many branches. The first is animals with or without spinal cords. Those with spinal cords are called the Phylum Chordata. The Chordates are further divided by whether they have backbones. Those that do are in the Sub-phylum Vertebrata. Tortoises and humans are both ‘vertebrates’.

Chelonians (Class Chelonia or Testudinia, old class- Reptilia)
Described by Joseph Collins and Travis Taggart in 2002

The vertebrates are divided into Classes. Chelonians (kell-OWE-knee-anz, a generic word for all turtles and tortoises) are usually put in the same class as other reptiles, like snakes and lizards, but new research suggests that chelonians and crocodilians are not really reptiles for several reasons. Chelonian ancestors split off the ‘tree of life’ during the Carboniferous Period about 260 million years ago. Their ancestors, called the Anapsids, have skulls that only have one opening for all facial nerves to go through. The Anapsids developed into several groups but only the chelonians are still around. Later, in the Permian Period, another branch developed 2 skull openings- the Diapsids- and became many other classes, including the reptiles, birds, and dinos. We will just look at the chelonians. The class is pretty easy to identify- just look for a shell! The first true turtle, Proganochelys, seems to have appeared in the Triassic Period, about 213 million years ago.

Turtles originally protected their heads by wrapping them sideways under the overhang of their shells. These turtles were called the Side-necks (order Pleuroira) and are still found today in tropical and semi-tropical fresh water habitats similar to their prehistoric homes.

Hidden-neck Turtles (Order Cryptodira, old order- Testudines)
Described by Edward Cope in 1871

During the Jurassic Period, some 144 million years ago, turtles learned how to pull the head totally inside the shell in a vertical ‘S’ curve, ‘hiding’ the neck totally and giving them the name Cryptodira, the Hidden-neck Turtles. With their increased protection, they were able to colonize more habitats- temperate to tropical, salt water to desert. All tortoises are Hidden-neck Turtles.

The True Tortoises (Family Testudinidae)
Described by August Batsch in 1788

The phrase 'the true tortoises' is used to differentiate the tortoises from other land turtles, especially in those places where species like North American box turtles are called 'tortoises'. Around about the Eocene Period 38 million years ago, the first chelonians that live totally on land, Hadrianus, appear. Prehistoric tortoises ranged much further than tortoises do today- Nebraska, for example, had tortoises back when it also had rhinos. Today, there are about 80 recognized species and sub-species, but as mentioned earlier, this is changing as we speak.

Tortoises are more than just ‘land turtles'. They have several anatomical and behavioral elements that separate them from other turtles: heavy, high-domed shells (except for the Pancake Tortoise); heavy scales on the forelimbs; elephant-like legs without defined or movable toes; several other anatomic features we won't go into here; and terrestrial habits- hunt, feed, mate, nest, and brumate (technical term for reptile hibernation) on land.

Relationship to the Environment

Wild tortoises tend to be shy and easily overlooked. Studies show that most attempts to count them miss between 50-85% of them. (OK, we will temporarily overlook the idea of how the heck they figured that out, but apparently they used trained dogs in this research.) Most tortoises are not very ‘social’- when they come together in groups, it is generally to take advantage of a good shelter, foods, or mating. When they gather, they show little sign of a social structure or pecking order other than the biggest one usually 'wins'.

Tortoises tend to inhabit one of three general biomes- arid, grassland, and forest. While most of this article will discuss things they have in common, there are many differences between these groups and between species.
Arid species (Mediterranean, Desert, and Russian Tortoises, for example) come from places ruled by scrub, cacti and succulents, and little rain- but still many wet micro-climates and a rainy season.
Grassland species (African Spurred, Leopard, Star, Russian, Gopher, and Red-foot Tortoises) live in sunny, grassy plains, savannas, scrub or thorn forests, etc. with more vegetation and rain than the arid locations.
Forest species (Yellow-foot, Hinge-back, Burmese Mountain, etc.) tend to live in warm (but usually not hot) humid forests.
Some species, like the Russian and Red-foot are found in several of these biomes.

Tortoises are well-designed to take advantage of habitats that are low in quality foods. Tortoises from the arid biomes, for example, are often found in places one would swear do not have enough plant life to support insects, much less a thriving tortoise population. A very slow metabolism and a slow-acting 'hind gut fermentation' digestive system combines to let them live quite well on very little. This means that in some habitats, tortoises may be the most common animal there. In fact, forest tortoises are some of the few vertebrate species fully adapted to living on a forest floor. There are many reports of tortoises going months, and in at least a few cases, a year or more, without food (but note that this does not mean they were healthy at the end of the long fasts.)

Being 'poikilothermic', 'ectothermic' or 'cold-blooded', tortoises live by external temperature cycles since they cannot regulate their body temperatures internally. When conditions are right and food is plentiful, they eat, court, mate, and lay eggs. When conditions are harsh, they either aestivate (a short-term form of deepened sleep that conserves resources) or brumate (the reptile version of hibernation, in which the body chemistry changes to keep them in a form of suspended animation for months at a time). While most species aestivate at least occasionally, not all brumate. Even of the species that do brumate, populations that live in nicer conditions may not. Russian Tortoises may actually sometimes brumate longer than they are 'awake' in a year.

Even the tortoise's day is dependent on temperature. They generally sleep until it warms a bit, then graze, hunt, court, fight, or mate until it gets hot. Then they will find shelter, sleep, and digest. If needed, they may do it again when it cools off in the evening before finding shelter for the night. Tortoises are a diurnal, or daytime species, but hot weather can make them act more crepuscular (active dawn and dusk), and they can often be found doing things at night, especially digging nests and laying eggs. Even outside of this rhythm, tortoises will shift activities to warm or cool themselves as needed.

Life Cycle

A tortoise's life starts with hatching, After tearing through the hardened egg shell with the temporary eggtooth on their snout, the hatchling may linger in the egg or nest for several days before venturing further. They will have part of the yolk sack for some time and generally do not stray far until it is absorbed. The nutrients in the sack will sustain the hatchling for some time- some species of chelonians even enter brumation without additional food.

Small tortoises with soft shells are a delicacy and are hunted by almost every predator in their habitat, even invertebrates, so in this stage of life, they spend most of their time hiding. Little is known about the lifestyle of most wild tortoise babies since they are so rarely seen by humans.

The phase from hatchling to sexually mature adult focuses on survival and growth. This is the time that the tortoise will grow the fastest. They are still vulnerable, so still hide much of the time, but must balance that with the need to feed and grow. Tortoises mature more by size than by age, so there is a biological push to hit reproductive age quickly.

Sexually mature male tortoises are driven to mate, often in the spring. Many species hold elaborate courtships and battles to decide who gets to mate. Battles can include ramming, flipping, or wrestling and courtship often involves dances, head bobs, and push-ups. Both can be accompanied by an incredible array of clucks, clicks, hisses, groans, and honks. For a species that is often thought to be mute, these animals can get loud!

Male tortoises mount the rear part of the female's shell and try to balance and support themselves as their long tails intertwine with the female's short one under the female's shell. The female often tries to dislodge the male, even crawling under low branches to knock him off.

If mating is successful, the female will later dig a nest in warm soil, usually at night. She will use her hind legs to dig and move the soil, urinating on it to soften it and possibly scent mark it. She will deposit several whitish, oval eggs and cover them over- often well enough that even someone who knows where the nest is can have troubles relocating it. The size, color, shape, and number of eggs depends on many things- species, size, age, how the season has gone so far, etc. Some species can store sperm and have more eggs later without mating again.

All chelonian eggs are a delicacy, so the nest is at great risk. Females may dig practice or decoy nests, or use a couple different nests at the same time, etc. to improve the odds. Some tortoises reproduce so slowly- few eggs, infrequent mating cycles, etc. that disruptions to their lifecycles can seriously jeopardize entire populations.

After several reproductive seasons, tortoises get old. Most stories of tortoises living past 200 seem to be exaggerated, although several individuals are documented to be over 100. A more reasonable estimate for most small, more common tortoises is 30 to 80 years. Most wild tortoises will be attacked by predators or succumb to disease or injury. Predators of adult tortoises range from predatory birds that like to drop them on rocks to break open the shell to big cats that gnaw through the heavy shell like a chew toy. In many places, humans are the biggest threat, collecting them for food, pets, or to be used for medicines and souvenirs.

Senses

Tortoises can hear some things, but generally do not use hearing for much. They seem most responsive to the sounds young tortoises and mating males make, and seem to be able to make and hear noises at frequencies below what a human can hear. Anyone who keeps tortoises knows that they can indeed make noise, especially when interested in mating. They are also very sensitive to touch and vibration.

They have a well-developed sense of taste and smell- some species even leave and use scent trails. Red-foots, for example, are believed to use these trails to help relocate shelters and possibly food sources- helpful in a place with few visual landmarks.

Tortoises do not seem to see as far or in as much detail as humans. They can see all the usual colors, and into the ultra-violet. This probably helps them locate basking areas, and may also help in food or mate selection (lots of flowers, etc. look a lot different under UV than they do otherwise). Their eyes are positioned so they can see in stereo straight ahead, and have decent peripheral vision as well.

Tortoises have a reasonably high intelligence, often equated to a white rat, and can obviously learn and remember things.

Anatomy

The shell is the most distinctive feature of a tortoise. It is made up of several layers of keratin- similar to human fingernails- built on a foundation of living, growing bone formed from the ribs, vertebra, pelvis, sternum, and other bones. The bottom layers are living and have blood vessels while the outer layers are 'dead'. The keratin form scales, called 'scutes' when they are on the shell. As the shell grows, new keratin develops around each scute making a pale growth ring. The color pigments in the scutes will soon turn the new ring the right color and pattern.

The heavy shell of the tortoise is a mixed blessing. It offers great protection from most predators but it takes a lot of effort to create and haul around. It also limits movement, so a tortoise on its back might have to work at righting itself. Contrary to popular myth, tortoises do not quickly die on their backs- but if they cannot right themselves, they will eventually die of 'exposure'- a combination of heat stroke, stress, pressure on the internal organs, dehydration, hunger, etc. Giant tortoises used as a food source on old sailing ships were just piled up on their backs, alive and often usually unfed, for months at a time.

Each scute on the shell has a name. Those on bottom edge of the carapace (back shell) are the 'marginals'. If there is a scute directly above the neck, it is the 'nuchal'. The one over the tail is the 'supracaudal'. There are usually five 'centrals' or 'vertebrals' down the back and four pairs of 'costals' on the sides.

On the plastron (bottom shell), the pairs of scutes, from front to back, are called the 'gulars', 'humerals', 'pectorals', 'abdominals', 'femorals' and 'anals'. The carapace and plastron are joined by the 'bridge'. We can often identify a chelonian just by the color or arrangement of these scutes. Some tortoises have flexible 'hinges' between parts of the shell. Hinge-back Tortoises (Genus Kinyxs) can flex the back section of their carapace over their hindquarters, while other tortoises have a little flexibility in the rear part of the plastron to allow eggs to pass.

Tortoise front legs are generally club-like, with a rounded foot and no visible toes. They are usually covered with heavy scales that protect the front shell opening when the tortoises is pulled in. The hind legs are usually columnar, with flat soles and no visible toes.

The tail has an opening called the 'cloaca' which is used for defecation, urination, mating, and laying eggs. Generally, male tails will be much longer than female.

The tortoise head features a toothless jaw, and a pair of small holes, called 'nares' that the tortoise breathes and smells through. Tortoises breathe quite differently than humans since their ribcages cannot move. They force air in and out of the lungs by working their limbs which can look like shrugging their shoulders. They smell by pumping their throat to move air past their Jacobsen's Organ- the scent organ most reptiles use.

The tortoise neck has five vertebrae, just like most other animals, yet they can bend it in a sharp 'S' curve to bring it all the way in the shell (and they can touch their elbows in front of the nose and point their hands to either side as they do it. Try that for yourself to see how odd it is.)

The internal structures of a tortoise are interesting- the familiar organs are there, but in unusual ways. Most of the top half of the shell is filled with the lungs. Chelonian lungs are built like sponges, so that punctures are not as serious as they are for many other animals, and their respiratory system is driven by a need for oxygen rather than getting rid of carbon dioxide like other animals. This gives them a great ability to hold their breath. The ratio of lung size to body weight lets some species of tortoise float and swim, while others sink- although many 'sinkers' can walk underwater for a short distance. Much of the rest of the shell is filled with the stomach and intestines.

Tortoises have digestive systems similar to other plant eaters, but that work even slower to extract as many nutrients as possible from poor foods, If they do get better quality food however, they can actually send that food through a faster process. Many tortoises routinely eat their droppings, probably to extract more nutrients or to get the healthy microbes in it.

The tortoise urinary system usually stores water in the bladder and concentrates their uric acids to make urates. It will expel the accumulated materials periodically. Depending on species and hydration levels, this can be anything from clear water to an almost toothpaste-like substance (often a little of both.) If the white urates are dry and gritty, the tortoise may be dehydrated. Some tortoises live in such arid locations and water is so scarce that they carry some in their bladders for a long time, and making one of these tortoises void their precious water can threaten their lives! Tortoises can recharge the bladder by taking water in orally, generating 'metabolic water' from food, or by soaking- which can recharge the bladder but does not rehydrate the tortoise otherwise.

Tortoises and Human Society

Tortoises today are generally threatened, and many are critically endangered. Habitat loss, the pet trade, and bush meat are the biggest problems, although many of the species also have low reproductive rates. Burmese Star, Southern Speckled Padlopper, Ploughshare, Flat-tailed, Geometric, and Egyptian Tortoises are all considered critical in the wild. The Abingdon Island Galapagos is down to one known tortoise (Lonesome George), and the Seychelles and Arnold's Giant Tortoises may still exist in zoos, but the evidence is unclear.

Chelonians of several types have long played a major role in human society. We eat them and their eggs; we have used them for medicines and oils and harvested their shells to make tortoiseshell, rattles, and more. At the same time, they play in many of our myths and stories. In India, the world rides on four elephants on a turtle's back. In China, tortoises are a symbol of good luck and wisdom. Only a few species of chelonians are considered harmful or dangerous, most are well-thought of, even lucky somehow.

And, of course, we keep tortoises as pets. They are a rather unique kind of pet- more 'handle-able' and personable than fish or most reptiles; as intelligent as many mammals; generally low odor and non-allergenic; and some are fairly easy to care for once the right habitat is established... which may take quite a bit of work and money.

Whether you are interested in tortoises as pets or just in tortoises in general, I hope this brief introduction has sparked your interest and answered some questions.

Main Sources
Pritchard, Peter and Trebbau, Pedro, 1984. "The Turtles of Venezuela"
Thompsen, J.S., 1932. "The Anatomy of the Tortoise"
Bonin, F., Devaux, B., Dupre, A. 2006. "Turtles of the World" (Translated by Peter Pritchard)
 
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