Natural History

Tortoises are well-adapted to live in fringe or marginal habitats in temperate, desert, and tropical zones around the world. They have specialized in living in areas with amazingly poor food options: desert and arid species that thrive in places with almost no plant life at all, or forest species that live in thick rainforests that are surprisingly short on nutritious foods. Their slow metabolism means they do not need a lot of calories, and their ability to become dormant when the weather is too hot or too cold to support food gives them a real edge in their chosen habitats.

The famous shell of the tortoise is a distinctive feature, but is also a mixed blessing. The shell is an effective defense mechanism against most predators, although some have learned how to defeat it, either with the strong jaws of the jaguar, the narrow muzzle and strong hands of raccoons, or the technique some birds have developed of picking them up and dropping them on rocks. Some tortoises have developed useful features that make the shell more helpful: some species have raised pyramidal shapes that make them almost impossible to turn over, while others have hinged sections that either allow the shell to cover their hindquarters better or let them pass eggs through what would otherwise be too small openings. The pancake tortoise even has a softened shell that it can inflate while hiding in rocks to make it nearly impossible to be moved.

The shell makes the tortoise slower and easier to succumb to overheating. It also interferes with things like breathing, absorbing UVB rays from the sun (to make vitamin D in the skin), and even mating. But… they have used their shells for millions of years, so it must be working for them!

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.) Many tortoises have been documented as spending over 70%
of their waking day under heavy vegetation cover. Most tortoises are
not very “social”- when they come together in groups, it is generally to take advantage of a good shelter, food, or mating. When they gather, they show little sign of a social structure or pecking order other than the biggest one usually “wins”.

Tortoises are “poikilothermic (a more accurate term than “cold-blooded”) and so 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 or brumate (See the section on “Dormancy” below). While most species aestivate at least occasionally, not all brumate. Of the species that do brumate, populations that live in more favorable conditions may not. Russian tortoises may actually sometimes brumate longer than they are “awake” in the course of a year.

Forest tortoises

Tropical and semitropical forest habitats are generally warm, humid, and so overgrown that much of the sun’s rays are blocked out. There is usually relatively little variation in temperatures during the day or between seasons, The two main seasons are the wet/winter and dry/summer. Heavy rains in the wet season flood lowland areas and fuel massive growth including flowers, fruits, while the water levels fall and growth slows during the dry season. These forests are amazingly low in nutritious foods: the constant rain and sandy soils means nutrients are quickly washed away. They also have an amazing diversity — a single acre can hold hundreds of species of trees, but often only a couple of each kind. This means that during the fruiting season, there may only be a few trees fruiting in a large area, so competition for food is fierce. While most tortoises live mostly on grasses and vegetation, forest species eat a wider variety of foods to improve their chances for better nutrients, especially components like calcium and proteins.

Forest-dwelling tortoises are designed for fringe habitats like this, though. The fact that they are smallish animals with very slow metabolisms, non-specific diets, and a strong defense against most predators enables them to live in places where there is barely enough food for most other animals, while also allowing them to get in and compete for things like carcasses or fruit falls. In some places, where nutritious food is especially scarce, tortoises may be the most common vertebrate in the area. Tortoises generally feed in two ways: wandering long distances and nibbling at a wide variety of relatively low-value foods, or finding a bonanza of good food and gorging, then sleeping it off and gorging again. They may sleep for a month or longer after a big meal, and at least a few have been recorded as not eating again for a year!

Most forest tortoises do not burrow on their own and often even struggle to dig egg nests, but often use burrows other animals dig, even if they do flood on occasion (researchers have found Red-footed Tortoises in flooded burrows, noses pressed to the ceiling). Forest chelonians can all swim and have even been seen swimming in fast rivers and big lakes. Many of the terrestrial turtles and some forest tortoises actually require good access to “swimmable” water in captivity.

Dormancy

Animals use various forms of dormancy to avoid periods of poor weather or food availability, etc. Tortoises, like many reptiles, use several strategies to help them cope with tough times. Dormancy is different from regular sleep in that something actually changes in the animal’s metabolism: chemical processes, digestive processes, etc. will change how they work to protect the animal. Keep in mind that not all species use all of these techniques, and even in a species that does utilize some of this, not all individuals in all places will.

Diapause is a strategy that allows eggs to stop development until certain conditions are met that help guarantee the hatchling’s survival. Some tortoise eggs go through this if the weather turns cool or the nesting areas flood, etc.

Aestivation is a common response to dry, hot conditions, and the animal’s sleeping metabolism will change to let it survive this period more safely. Many forest tortoises practice aestivation in the summer.

Brumation is the reptile version of hibernation, as different metabolic changes occur than in hibernation. Many species of tortoises brumate during the cold months, including forest species in the sub-tropical areas where it can get quite chilly sometimes.

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