Turtles: How have these armoured reptiles lived for millions of years?

One of the most notable things about turtles is their endurance. Arising over 100 million years ago, turtles have enjoyed a fairly stable existence, standing by as the dinosaurs rose and fell, and changing very little across the many millennia.

Specialised adaptations allow turtles to live in a diverse array of habitats. Many freshwater turtles must hibernate to survive the winter, slowing their metabolism down so far that they can go months without eating or drinking. On the other hand, the leatherback sea turtle speeds its metabolism up, enabling it to survive in water far too cold for any other reptile.

Some turtle species are the territorial and may have dominance hierarchies, but they form no social bonds. Typically turtles come together only because they share habitat or for mating. Freshwater species tend to breed annually, whereas marine turtles may only reproduce every few years. Female turtles are capable of storing viable sperm in their oviducts for months, or even years! When ready to lay her eggs, she leaves the water and seeks a suitable substrate for digging on land. Freshwater turtles make nests in the soil along the waterways and ponds where they live. Marine turtles, on the other hand, travel widely over the course of a year, following ocean currents, but will swim hundreds, or even thousands, of miles to return to their natal beaches.

Once her eggs are buried, a female turtle’s parental responsibilities are over. Incubation time varies by species and climate, but is around two to three months. When the baby turtles are ready to hatch they use a sharp egg tooth to cut through their shells and then, as a group, they dig their way out and make for the relative safety of the water. Marine turtles typically emerge at night and instinctively crawl en masse towards the brightest horizon which on a natural beach is towards the ocean.

Young turtles are vulnerable to dehydration, as well as a variety of predators; relatively few survive to adulthood. Even adult freshwater turtles may fall prey to animals such as raccoons or herons, but adult marine turtles have few natural enemies. Although the occasional shark or killer whale may attack, by far the biggest threat to marine turtles is humans. Thousands of turtles are injured by boats or killed in fishing nets each year, while others are hunted for their shells or meat, either for food or for traditional medicines.

Today these fascinating animals are among the most threatened in the world, with more than half of turtle/tortoise species facing extinction. All seven species of marine turtles are endangered or critically endangered, and the total populations of certain rare freshwater turtles number in the single digits.

Anatomy of a turtle
Anatomy of a turtle
Baby hawksbill turtles make the most perilous journey of their lives as they crawl from their nest to the sea

Keeping clean

When you’re covered by a shell and your flippers won’t hold a scrub brush, you need a little help to stay clean. Over time, turtles can become covered with algae, ectoparasites and barnacles. Cleaner fish – notably wrasses, but also species such as the pictured tangs – swim in to remove the debris, earning an easy meal in the process. Although this
relationship is apparently mutualistic – ie beneficial to both creatures – there are concerns that cleaner fish may spread disease among turtles.

An inner compass

Sea turtles can travel all over the world and yet many species somehow find their way back to the beach where they hatched, sometimes laying their own eggs mere metres from where they emerged. Research indicates that many species use magnetic fields to create a mental map that pinpoints their current location relative to their goal, as well as providing them a compass bearing. Exactly how they do this is unclear, but scientists have found tiny crystals of the mineral magnetite in the brains of turtle species known to perceive magnetic fields.

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