Why are there different ways of indicating gender for animals?

Why are there different ways of indicating gender for animals? For instance, by inflexion we get:

lion (male) & lioness (female)

where the female is distinguished from the male. Here the male is also known by the general (have I used this correctly?) term for this type of animal, i.e. lion.

But, in another instance, we have:

duck (female) & drake (male)

where the male is distinguished from the female by a distinct or different word and also from the general term for this type of fowl, i.e. duck.

Then, for kangaroos and rabbits, we have:

buck (male) & doe (female)

where neither are known by the general term, nor is there any inflexion; instead, distinct names are used.

Why do these different ways for indicating gender exist? Why is it that in some cases the male is differentiated from the female and vice-versa. And why in some cases are both female and male known by words so different from the general term used to describe them?

Solution 1:

To take one of your examples, doe is from old English deon, to suck, and was used to describe a female animal of a number of kinds. buck is from old English bucca, a he-goat. The unisex rabbit is from Flemish. So the words buck and doe appear originally to have meant the male and female of any animal, and have presumably become specialised in referring to deer, rabbits, and certain other kinds of animals.

Ram and sheep are both derived from old English, in the case of ram a word meaning fighting sheep. (Ewe is from Latin.) I'd hazard a guess that in old England there were fighting sheep (males) and sheep (all the others, including wethers).

Hope this helps (all this from the Collins English Dictionary, except the suppositions and guesswork).

Solution 2:

Because they come from a time when people were more intimately involved with animals than they are now. Nowadays most people can't tell the difference between a hen and a rooster, but 500 years ago they sure could. Most of the terms you'll encounter of this nature come either from farming or hunting, which were big parts of the economy of the average village. You won't see this in newly discovered species for the same reason.

Solution 3:

It's not one word became two, it's two became one. Other creatures evolved gender-neutral term over time, while these didn't, somewhy. Lion is by the way, almost did: it can be either he-lion or she-lion, it's not a mistake to say "she's a lion" about lioness.

And we still use different words for humans too — “woman” and “man”. In languages other than English, the “woman” word has no “man” inside it, they are absolutely different words.

In old languages, every noun had gender, even non-living things. Many languages still have this, e.g. Italian “home/house” is female (“casa”) and Russian one is male (“дом”). The language structure is such so even synthetic made-up words are perceived either as male or female.