DW>> [snip] 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.

>>

AJ> 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.


Actually, it fits more than one pattern. What I described is a pattern,
after all, and it is supported by critical data at hand -- when
dichromatism/monochromatism are mapped onto a cladogram of orioles
(Freeman 1991), the result matches the pattern I described, i.e., it meets
the predictions derived from the SS model (the ancestral condition for
bright plumage in these these orioles is for it to be present in both
sexes; the sexually dichromatic condition is novel or "derived" for this
character). Note that if dichromatism (rather than bright monochromatism)
had turned out to be the ancestral condition the SS explanation would have
been rejected. But it did pass this test. How has the territory defense
explanation been tested? I note that at least some species apparently do
not fit the territorial explanation, as Chris Hill has indicated that
female Red-winged Blackbirds defend territories in this sexually
dichromatic species. Has anyone experimentally dulled female tropical
orioles to see if they are really less successful at defending
territories? (I do recognize that we need evidence as to whether or not
bright colors in orioles are favored by one or both sexes to better
evaluate sexual selection as an explanation.)


AJ>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.


Not to put too fine a point on it, but if both sexes develop bright colors
for territorial defense is that really sexual selection?


David Wright
dwright at u.washington.edu