What makes a turtle a turtle and a tortoise a tortoise is not always clear, says Ellen Husain
Tortoises are actually turtles. Numbers are somewhat contested but the tortoise family, Testudinidae, includes 65 species belonging to 18 genera. This is just one of 11 families in the turtle order, encompassing 365 species. But what makes a turtle a turtle and a tortoise a tortoise is not always turtle-y clear.
What’s the difference between turtles and tortoises?
While British English speakers call land-dwelling turtles tortoises and ocean-dwellers turtles, Americans use the term turtle more prevalently, including for some land animals.
All turtles have a body encased in a hard, bony shell made up of plates, but while water-based turtles tend to be streamlined with flattened flippers, the land-dwelling tortoises usually have more domed shells and stockier ‘elephantine’ legs.
Tortoises are not good swimmers, but sometimes enter water to drink or bathe. They have a largely vegetarian diet, whereas turtles are mainly omnivorous with marine species eating jellyfish. Both lay their eggs on land.
How do tortoises and turtles breathe?
While most vertebrates inflate their lungs using muscles between their ribs to increase their chest volume, this is not an option for tortoises and turtles with their rigid ribcages. Instead they have a muscular sling attached to the shell that contracts to expand their chests into the opening through which their heads and forelimbs protrude.
Fossils show this sling appeared even before a rigid shell, paving the way for modern forms. Some species, such as Australia’s Fitzroy river turtle, use their rear ends too, ‘inhaling’ oxygen-rich water via the cloaca (the exit for the digestive, urinary and reproductive systems), where blood-rich tissues perform the necessary gaseous exchange
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