Turtle

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Green turtle, Chelonia mydas and his Total internal reflection. (Source Brocken Inaglory, via Wikimedia Commons)

Introduction

To look at a Snapping turtle with its horny shell and scaley tail, you might imagine that you are glimpsing a dinosaur. In fact, turtles are even older than dinosaurs and were common on earth 50 million years before the first dinosaurs appeared.

The scientific name for turtles, comes from the Latin testudo, which means tortoise. Six reptile families of the order Testudines are represented in Canada: Chelydridae, Emydidae, Dermochelyidae, Cheloniidae, Trionychidae, and Kinosternidae.

 

Morphology

The world's 250 turtle species all share certain traits. They all have four feet, ribs fused to their shell, and leg bones that are tucked into their body cavity. A turtle's head and extremities can ordinarily be pulled into the shell for protection. In species where this is not possible, other methods of defence are used. For example, the snapping turtle has very powerful snapping jaws which protect its exposed head. Although all turtles lack teeth, they possess a horny beak and sharp claws for cutting and shredding their food.

 

 

 

 

 

The turtle shell is divided into an upper carapace and lower plastron. In some, like the box turtle, the plastron is hinged and can completely close over the head and leg openings.

The shell is supported by a structure of vertebrae and plate-like ribs. This boney core is overlayed by an outer horny layer of keratin that is arranged in large, thin scutes. As the turtle grows, fresh keratin is deposited from underneath, and old layers peel off. Melanin pigment in the scutes forms designs and patterns which are unique to each species.

 

 

 

 

Some turtles deviate from this general plan. As their name implies, soft-shelled turtles lack scutes. Instead, their shell is covered with a soft-looking leathery skin. In some soft-shelled species, the plastron is also cartilaginous rather than bony.

 

 

 

 

Respiration

 

Turtles cannot expand their chest to breathe because of their rigid bony shells. They inhale by contracting their limb flank muscles to make the body cavity larger and exhale by drawing the shoulder girdle back into the shell, forcing air out of the lungs. Many turtles do not rely only on their lungs for respiration. Aquatic turtles can remain submerged in water for long periods of time by obtaining oxygen through gas exchange in the cloaca and mouth cavity . The skin in these cavities effectively functions as gills. This kind of respiration is especially important during hibernation, when turtles are underwater for months at a time. Turtles must breathe using their lungs when they are active, however, in order to have sufficient gas exchange in their tissues.

 

Senses

A turtle's brain is small, never exceeding 1% of its body weight. Turtles have a poor sense of hearing, and are virtually mute, although some species vocalize during copulation. A few species, mostly tortoises, also cluck or grunt quietly during courtship. Turtles have acute vision, including good colour perception, and an excellent sense of smell.

Reproduction

Male turtles can often be distinguished from females by their longer, thicker tail, and by the lesser distance between the cloaca and the tip of their tail.

Turtles usually mate in the spring. Courtship precedes mating, and involves different signals between the males and females of each species. Both activities typically take place underwater. These signals include chasing, ramming, head bobbing, water spitting, chin touching, and biting. In painted turtles, the male strokes the female's head region with distinctly elongate foreclaws. The male has a single penis.

Nesting

All turtles lay eggs which they bury on land in nests, typically in sandy or gravelly areas. The embankments of roads, bridges, dams, and railways are often good places to find turtles nesting. Turtles will, however, nest in other areas, depending on 1) the habitats available, and 2) the species involved. For example, spotted turtles in Georgian Bay often lay their eggs under clumps of moss and lichen on rocks, probably because sandy beaches are few and far between in this area. Turtles usually nest in the early morning or just before sunset.

Steps

Turtle nesting behaviour is very ritualized, and can be broken down into a number of distinct steps. The following description is of snapping turtle nesting behaviour, but all turtle species follow this general pattern.

Step 1: Choosing the nesting site

The female turtles often check out potential nesting sites before laying their eggs. This can include observing the site from the water and walking about on land. The turtles may "bulldoze", pushing their heads through the soil, and then raise their earth-covered heads to peer about. This step may take from a few minutes to several hours.

 

 

Step 2: Digging

The next step for the female is to dig a hole. This is done with the hind feet only. Typically the hole is flask-shaped. As the hole gets deeper, the body of the turtle will bob from side to side as it alternately reaches down with each foot to scoop out soil. If the turtle encounters an obstacle such as a root or large rock, it will abandon the nest.

 

 

Step 3: Laying the eggs

Once excavation is complete, the turtle positions its cloaca over the hole and begins laying its eggs, resting a few moments between each egg. Females may use their hind feet to slow the eggs as they drop into the nest.

 

 

 

Step 4: Burying the eggs

Finally, the female turtle scoops soil back into the hole with her hind feet, burying the eggs. Towards the end of this process, she may pack the earth into the nest by kneading it with her feet. After this is complete, the turtle abandons her eggs to their fate, without ever having seen them.

 

 

 

Predation

The fate of turtle eggs is usually not a happy one. Foxes, skunks, and raccoons frequent nest sites during the laying season, and dig up and eat many of the eggs. Turtles compensate for this predation by living a long time, and by making repeated attempts at producing offspring. However, the presence of humans in an area artificially increases populations of scavengers such as raccoons, and this creates additional pressure on the turtle populations.

 

 

 

 

 

 

Hatching

The eggs undergo development through the remainder of the summer, and in fall the hatchlings break out of their eggs, dig their way to the surface, and ordinarily make for the water. However, the hatchlings of painted turtles can overwinter in the nests, because they have the ability to survive partial freezing of their body tissues.

 

 

 

 

 

 

Temperature

In most animals, sex is determined at fertilization. For example, in humans, if the sperm which inseminates an egg carries an X-chromosome, the offspring is female; if it carries a Y-chromosome, the offspring is male. All crocodiles, most turtles and some lizards, sex is determined by the temperature experienced by the embryos as they develop. This is called temperature-dependent sex determination (TSD).

There are three types of temperature-dependent sex determination: pattern Ia, pattern Ib and pattern II.

Most turtles exhibit Pattern Ia, where males are produced at cooler incubation temperatures and females at warmer ones. It is usually observed in species where males are smaller than females. In painted turtles, males are smaller than females, and are produced at lower temperatures.

Pattern Ib is just the opposite of type Ia: females are produced at cooler temperatures and males at warmer temperatures. This is common in a number of lizard species.

The rarer Pattern II is usually observed when males are larger than females, or there is no sexual dimorphism. Common in crocodiles and some turtles, this type of TSD produces males at intermediate temperatures and females produced at the coolest and warmest ones. The stinkpot turtle exhibits this pattern of sex determination.

Ecology

Turtles are found in nearly all freshwater habitats in Canada. Some species prefer deep water, but most occur only in relatively shallow water and nearly all avoid swift currents.

Most turtles are omnivorous, feeding on both animals and plants. They eat various vertebrates, invertebrates, algae, and the leaves, stems, and fruits of terrestrial and aquatic plants. Some species have narrower food preferences, consuming only plant material or mollusks, while other species show shifting food preference with age, tending toward carnivory while they are growing, and becoming more herbivorous at maturity. Turtles may actively stalk their prey, or simply lie in ambush. Large pieces of food are seized and torn apart by the beak and claws before being swallowed. Smaller prey is swallowed whole.

Turtles are active in Canada region between April and October. For the remainder of the year, they hibernate in the soft bottom sediment of lakes or under soil and debris in woodlands.

 

 

Conservation

Five Canadian species of turtle are recognized by COSEWIC as being at risk:

Leatherback turtle (Dermochelys coriacea), spotted turtle (Clemmys guttata), wood turtle (Clemmys insculpta), and eastern spiny softshell (Apalone spinifera). The population of Blanding's turtles (Emydoidea blandingi) in Nova Scotia is also considered threatened.

Turtle populations are declining because of habitat destruction, pollution, road kill and the pet trade. Click on the titles below to learn more about these factors.

Habitat Destruction

Habitat destruction, such as clear-cutting woodlands, and draining wetlands. Wetlands are considered marginal, undesirable land by humans, but are essential to the lives of creatures such as turtles. As an example, southwestern Ontario was 65% wetlands before European settlement, but presently it is only 3% wetlands.

 

 

Pollution

Turtles bio accumulate pollutants such as chlorinated hydrocarbons in their body fat. These chemicals may gradually poison them or impair their reproduction.

 

 

 

Road Kill

Thousands of female turtles are killed every year crossing highways in search of nesting sites. The survival of turtle populations depends on the naturally low mortality of adult turtles. However, turtles have not evolved to cope with road kill mortality, and so road kill hits turtle populations especially hard.

 

 

 

 

Pet Trade

Collection of turtles for food and the pet trade. Almost all turtle species are protected by law, but there still is a black market trade. Collectors especially favor rare, endangered, and charismatic species such as the spotted and wood turtles. If collectors find a communal hibernating site, they can clear out an entire population in a few days. If a pet store is selling an adult turtle, it probably came from the wild.

 

 

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Biodiversity Institute of Ontario. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Biodiversity Institute of Ontario should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

Glossary

Citation

Ontario, B. (2013). Turtle. Retrieved from http://www.eoearth.org/view/article/156743