According to the biological species concept, horses and donkeys are separate species because they

Why can two different species (such as wolves and dogs) breed together, and still be considered different species?

Heather Bird, Rochester United States

• If their offspring is fertile then the animals are of the same species; if the offspring is sterile they are of different species. A half-dog half-wolf will always be sterile, as will a mule (half donkey, half horse), but a half-alsatian half-poodle should be fertile.

  Nigel, Cockfosters UK

• According to Steve Jones in his book "Alomst Like a Whale" the international committee which rules on taxonomic problems like this has decided that dogs are really just domesticated wolves that have been artificially bred into funny shapes, and consequently the species designation Canis domesticus (dog) has been formally abolished: they're just a subspecies of Canis lupus (Wolf). The interesting bit of all this is that there is such a committee.

  Christopher Young, Sheffield England

• The dividing line between species is not always clear-cut, but is usually drawn at the ability to interbreed. Precisely for this reason, the domestic dog and the wolf used to be considered as seperate species, "canis familiaris" and "canis lupus", but the dog is now regarded as a sub-species of the wolf species "canis lupus familiaris".

  Campbell McGregor, Glasgow Scotland

• Some groups that we classify as species are able to breed together, because they're physically able to get together and genetically similar enough to produce a viable egg. The 'biological species concept' says that two animals are of separate species if they are not able to breed together to produce a fertile hybrid. However, in practice, some closely related groups that we consider to be individual species are prevented from breeding with one another through geographical or other isolation rather than through physical constraints. Given sufficient time in isolation, these groups would be likely to become genetically incompatible with another.

  Flit, Leeds UK

• Nigel is wrong. A wolf/dog hybrid is fertile and is in fact not a hybrid at all because wolves and dogs are exactly the same species. The dog is now known scientifically as Canis Lupus Familiaris and not just Canis Familiaris (as it is in older textbooks) in recognition of this fact.

  Susan Deal, Sheffield UK

• Definition of a species is more complex than whether it can interbreed. Some same species cannot (maybe geographical change), some different species can and there are reasons for this anomaly but I am not sure why. Ask a taxonomist, but understand that as many opinions as experts.

  Steve, Leeds, England

• Nigel sorry but you are wrong both about horses, donkeys, dog, wolf. It is true that a mare female horse crossed with a jack male donkey will produce a infertile mule, but when you cross a stallion male horse with a female donkey you can get a fertile hinny/jennet female mule. Plus dogs and wolf offspring can be fertile and reproduce.

  Sandra Fuller, Panama City, US