>On Mon, 26 Jan 2004, Julie Alipaz wrote:
>
>> there are two schools of thought 1) females are the bigger because of
>> reproductive issues. They must feed a brood--take bigger prey, they
>> must incubate the clutch--bigger = more body heat. that said, they
>> also believe 2) that it is a historical artifact of early avian
>> divergence. But the first theory gets more respect.
>
>I'm trying to understand why 1) wouldn't apply more generally in
>predators.
The reason why number 2 is still discussed at all is because we
cannot explain why the female male raptor size difference is not more
general--so number two suggests that female large size is costly and
therefore eliminated. That said, i am sufficiently unfamiliar with
the literature that i actually went to and looked in my book on
sexual selection--there is a whole chapter on reverse sexual size
dimorphism (RSSD):
First RSSD is not limited to birds: marine mammals--some
seals; those that are monogamous tend to have RSSD, this is thought
to be because females need more fat stores to successfully birth and
raise a pup--males have no need to be large as they do not have to
compete for mates, protect a harem and or compete for territory.
Also true for seals that do not aggregate at breeding beaches for
many of the same reasons. In the Wendell seals, they believe that
because mating is underwater and because males are judged on agility
selection may favor smaller males. Other possible reasons: females
are selected for extreme fatness, or that males on average are
younger, as females survive better.
Males tend to be larger in polygamous situations, situations where
there are contests for mates, violent mating and agility displays.
Other examples of RSSD: lizards, turtles (the norm here, 31 out of 32
species) mustelids, insects and baleen whales show RSSD; jacanas
(another bird)
In deep-sea fishes many males have been reduced to being almost
parasitic on females; they attach themselves to a female, going so
far as to tap into her digestive tract, so the female actually feeds
them. The only thing the female needs from the male is sperm--they
are called parasite males--and possibly evolved due to the above
competition issue.
Lastly females are often believed to be larger because then they can
lay more eggs (birds, fishes, and insects) or the young they do birth
can be larger and therefore have a better chance of surviving.
All that said, the chapter ends by saying that there is no consensus
of opinion.
>
>Would this mean female raptors are scraping at the upper range of
>useful size, and male raptors are less likely to starve or break
>wings or graze trees by accident or something? I guess birds have
>a more difficult design job than say cats...
no idea