Do Americans use the world 'turtle' as a generic word to mean 'tortoise'?
Solution 1:
It all depends on how technical you want to be.
(I am writing as the spouse of a nationally recognized expert on wild turtles, tortoises and terrapins in the US.)
In the US there are 50 species of "chelonians" excluding sea turtles. Among experts they are identified by strict Latin taxonimic names when precise species identification is required. There is also a looser English taxonomy, but there is enough regional variation that experts fall back on Latin when confusion arises.
But, even experts in "casual" conversation use the word "turtle" to include "turtles, tortoises and terrapins"; e.g. "I study turtles."
Some non-experts refer to the shelled animals that live primarily in water (lakes, rivers and streams) as "turtles" and those that live primarily on land as "tortoises". However, in the US northeast, the most common land dweller is the "box turtle".
Bottom line, in casual conversations in the US the word "turtle" encompasses all those shelled creatures.
In discussion with passionate "turtle people" try to use the correct taxonomic name.
Solution 2:
Your colleague is correct to an extent. Generally in British English, tortoise refers to the land animal, turtle to the marine animal, and terrapin to the freshwater animal.
In U.S. English, turtle is used as a catch-all term for all animals of that type, but distinctive tortoises like the giant Galapagos tortoise will be called such.
Solution 3:
Yes. American English often uses "turtle" for tortoise, in British English the two terms are not interchangeable, and (for what it's worth) Indian English often uses tortoise as the generic term for both turtles and tortoises. (The Hare and the Tortoise is never *The Hare and the Turtle, and Indian languages—and Japanese is similar according to a comment above—usually have the same words for both, and this is mapped to tortoise, not turtle.) So it varies from region to region.
As evidence, the Compact Oxford English Dictionary (UK) says, under tortoise that it's "Called turtle in North America", and the American Heritage Dictionary of the English Language gives, under tortoise, definition 1b saying "Chiefly British A terrestrial or freshwater chelonian."
That they are distinct in British English can be seen from this exchange from Alice's Adventures in Wonderland:
“When we were little,” the Mock Turtle went on at last, more calmly, though still sobbing a little now and then, “we went to school in the sea. The master was an old Turtle—we used to call him Tortoise–—”
“Why did you call him Tortoise, if he wasn’t one?” Alice asked.
“We called him Tortoise because he taught us,” said the Mock Turtle angrily. “Really you are very dull!”
Solution 4:
In the NOAD is reported that turtle means:
- a slow-moving reptile, enclosed in a scaly or leathery domed shell into which it can retract its head and thick legs.
• Family Testudinidae: numerous genera and species, including the European tortoise (Testudo graeca). - (also sea turtle) a large marine reptile with a bony or leathery shell and flippers, coming ashore annually on sandy beaches to lay eggs.
• Families Cheloniidae (seven species) and Dermochelyidae (the leatherback). - a freshwater reptile related to the turtles, typically having a flattened shell. Called terrapin in South Africa and India and tortoise in Australia.
• Order Chelonia: several families, in particular Emydidae and Kinosternidae.
• any reptile of this order, including the terrapins and tortoises.
It's then true that in American English the term turtle can be used to refer to an animal that in other English languages is called tortoise.
This is not something that happens only in American English, though. Also other languages have a word equivalent to turtle (In Italian tartaruga) and one equivalent to tortoise (in Italian testuggine), but the first can be used instead of the second (even if it is less accurate).
To notice also that, for example, in Italian the European tortoise is called tartaruga.