I was just thinking of the word "gander" and how there doesn't seem to be a female equivalent, other than "goose" which can be used for either sex. I realised the same was true of ducks -- a male duck is a drake but a female duck is just a duck, which could be either -- and the opposite was true of a bunch of mammals: a "fox" can be male or female, but a female fox is a vixen, a "dog" can be male or female, but a female dog is called something which might mean I need to put an "adult content" warning on this blog.
Looking at those examples, it seems as if birds are female "by default" and that mammals are male "by default" ... but of course, there's more to it than that.
In an episode of The Simpsons, one of the characters (one of Marge's sisters, Patty or Selma) remarks that "There are no lady goats. A lady goat is a sheep." Obviously the writers intended that as a joke, but it's interesting how the *character*, if not the writer, thought of "sheep" as the name for a female creature.
I guess it's a general rule for farm animals: the farmers breed them to produce more of their kind, as well as by-products of reproduction (milk, eggs) and so they find it more profitable to have more females around. Therefore female farm animals have more of a public presence, and so they colour perceptions of the whole species. Specieses. You know.
Take cows. A female cow is a cow, a male cow is a bull. The situations different from the ducks and geese mentioned above, as nobody uses "cows" as a general name for both sexes. But people, if they don't think about it *too* hard, tend to attribute the female name (cow) and characteristics (milk) to the whole species -- remember a character in an advert for some milk drink, called something like "Hugh Heifer", who was very definitely male but looked like a (female) cow, complete with udder? -- and, according to language enthusiast Bill Bryson, prudish Victorians would sometimes call bulls "male cows" or even "gentlemen cows".
And chickens. Well, seems like we've got some equality here. A female is a hen, a male is a cockerel or rooster. And yet... "hen" and "chicken" are sort of treated as synonyms, and once again, the undeniably female charateristic of laying eggs is generally thought of as an activity of "chickens" as a species. Even though Foghorn Leghorn has often identified himself as a chicken -- rooster, that is!
And how about pigs? Seems about the same -- a female is a sow, a male is a boar. But when do we really hear the word "boar" except when referring to a wild one? And, even more so, a wild pig is always a "wild boar". There must be wild sows out there, wherever there are wild boars... else where would all the wild piglets come from (or "wild boar piglets" as they seem to be generally called... is that why there aren't more of them?)?
But I guess wild animals *are* male by default. Both sexes of lion are lions, and a female is also a "lioness". Hmmm... there aren't too many other examples of that. There used to be of course: it used to be more common for a female tiger to be called a "tigress" or the female of other species to be referred to as a "she-" followed by the species name (e.g. she-wolf). I don't think anyone ever spoke of a he-anything, unless it was a "he-cow".
OK, what about domesticated animals then? Not the ones who are bred out of their natural life-cycle or gender ratio, but the pets, the companions? We've already covered dogs at the beginning, but it goes much further than that, as there are people who call all dogs "he" whether they are actually male or female. Curiously, for such people all cats are "she."
So does that mean that cats, as a species, are identified by the same name which distinguishes female cats from the guys? Well... sort of. A male cat has a special name for him, a "tom cat". But a female cat? Some people say "tabby cat" but of course that's ridiculous -- "tabby" refers to the cat's markings, and there are plenty of "tom" cats who are also "tabby". Heck, where I live, we have one, and Tom is actually his *name*, so you really can't argue with that.
So, what's my point in this digression? Nothing really... but I guess it's part of the human trend to put everything into pairs (I remember from English lectures that there's a word for that -- unfortunately I don't remember what the word is. Once I find out I'll edit this post, so it will look like I remembered all along). A dog is male, so a cat must be female.