BDS

Exactly, Jesse.

I agree that wide spread sexual dimorphism would be unlikely if it had no selective advantage, and it's reasonable to speculate about possible adaptive advantages.

However, we cannot logically assume (for example) that tail bones have a selective advantage for humans. This is an example of something other than a spandrel (I'm sure there's a word for it, but I don't know it).

Popular naturalism (as seen on nature shows on TV) makes this kind of logical error all the time. Other errors common on TV:

1) Nature was in a state of stasis before humans ruined it all. (Nature has never been in stasis.)

2) Wolves improve the health of caribou herds by culling the weak. (This may be true, but the nature shows ASSUME it is true based on fallacy #1.) It is certainly possible that some predator would wipe out its prey species, to its own eventual detriment.

SiliconWolf

Female hamsters are, on average, 25-33% larger than male hamsters. I guess it helps to be a bit bigger when you have as many as 16 kids at a time. I think a few, if not most, rodent species follow the same pattern.

Phoenixstar

As several others have already mentioned. The females in some mammal species are larger because of the extra work that comes along with gestation. Prime examples are the whales. Many of the different species of whales have the female larger than the male.

trillian

Slightly OT....
In thinking about the whales, I began wondering about our own human sperm.

Male sperm (XY): smaller, faster, more fragile live 36 hours
Female sperm (XX): larger, slower, hardier, live 72 hours

Here the speed of the male is balanced by the "staying power" of the female and we still get our basic 50/50 sex distribution (or to be exact 51/49).

But why? Any ideas?

trillian