>
> So crests in birds such as jays may well be the product of sexual
> selection on males, even though they are present in both sexes. (Along
> the lines of burt Guttman's suggestion, if these crests incur little cost
> in females, there will be little pressure for females to "lose" them, and
> that slight negative selection will be swamped by sexual selection for the
> crest in males, and they could be retained by females indefinitely despite
> having no function in females) Finally, groups such as orioles in which
> some species have bright plumages in both sexes, some are strongly
> dichromatic, and some are weakly dichromatic, may represent cases where
> multiple "steps" in the evolution of a sexually selected character have
> been preserved because different lineages are evolving (females losing
> male trait) at different rates. It's fun stuff, but you really need
> phylogenetic evidence to play the game.
>
Actually, sexual dimorphism in Orioles does fit a pattern. The birds
which are dimorphic tend to migrate in to the breeding areas, males
establish and defend a territory and females do not. The species which
are not dimorphic tend to be tropical, and are year round residents in their
territory, both sexes help to defend the territory. This pattern is also
seen in some Agelaius blackbirds, tanagers etc. In these cases it really
does appear that the bright female plumage is selected for, rather than
just a genetic hand me down from the males. One must remember that sexual
selection does not just operate on males as well.

Al Jaramillo
jaramill at sfu.c a
Vancouver, B.C.