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Hi everybody,
Perhaps this can be considered a case of a sexual dimorphism that has NOT developed. Sex-specific traits can evolve (as we all know, because we're interested in birds) but often one sex is constrained by the other. That is, sometimes the optimal value for a trait lies far apart for males and females (height, color, hairiness, departure date for southern migration, etc) and the actual values for the trait get stuck somewhere in in middle: if a mutation exaggerates a certain female trait in an individual female, and she passes this trait on to her offspring, it may increase the fitness of her daughters but lower the fitness of her sons. For example, the constraints on the evolution of dimorphism are easy to see for sexually-selected traits: if facial hair is considered attractive on males and unattractive on females, an version of a gene that increases hairiness of the face will not spread easily if it is expressed in women. For this same reason men have nipples: there
is no selection to rid men of nipples (they are perhaps of little cost to the male) -- we simply have them because women need them.
To hummer behavior: perhaps the males linger because males and females use the same cue that says "Time to migrate!!" : probably some combination of temperature and duration of daylight. It is important for females to remain in Canada for the sake of their young. Let's just say for the sake of argument that it is completely neutral for the males to remain in Canada (ie an equal number would survive migration if they left now or if they leave in September, and their presence in Canada does not affect the survival of young). So there in strong selection for females to stay, and no selection for males to leave. Therefore males end up expressing the same trait that their female counterparts do: they remain in Canada for the same reason that male mammals possess nipples -- not because there is selection FOR the trait, simply no selection against it.
I Really like your point about density-dependence! Males present in Canada could consume food supplies and reduce the survivorship of juvenile birds. clearly this selection pressure is not strong enough to cause dimorphic behavior, or the males would be gone already. Perhaps food is not limiting to hummers. Or perhaps males and females do not use the same foods or forage at separate times. Or perhaps males leave the female and travel far enough that they don't compete with their own mate (its OK if they compete with somebody else's, after all). and speaking of somebody else's mate, there's always the possibility of cuckolding the "mate" of another male. Who knows, perhaps in very rare and very good years more than one brood can be raised per female..
Does anybody know of a case where males DO migrate a lot earlier than females? just how dimorphic can migration behavior be?
Cheers, and thanks for pointing out the fun problem, Peter!
----- Original Message ----
From: Peter Payzant <pce@accesswave.ca>
To: naturens@chebucto.ca
Sent: Saturday, June 23, 2007 7:23:48 PM
Subject: [NatureNS] Why do male hummers...
Hi, all-
Here's a question that I ponder from time to time:
Why do male hummingbirds wait around here all summer before migrating
south?
Once they've mated with a female, their purpose
here is essentially over. They don't help in taking care of the young, and
there's only one brood per female per year. In fact, you could say that the
males create a negative reproductive pressure by competing with the females and
young for food.
Why don't they just go back south once the females
are sitting on nests?
I can think of a few not-very-convincing
reasons:
- The environment here (food, shelter, weather,
predation pressure) might be better than on their southern
range
- Some resources that they need on migration are
not yet ready (e.g. some important flowers not generating nectar
yet)
- There's always a chance that an un-mated female
will turn up (I call this the "wishful thinking" hypothesis)
Any other thoughts?
Peter Payzant
Waverley
Ask a question on any topic and get answers from real people. Go to Yahoo! Answers and share what you know at http://ca.answers.yahoo.com
--0-1944345611-1182775730=:67205
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<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman, new york, times, serif;font-size:10pt"><div style="font-family: times new roman,new york,times,serif; font-size: 10pt;">Hi everybody,<br><br>Perhaps this can be considered a case of a sexual dimorphism that has NOT developed. Sex-specific traits can evolve (as we all know, because we're interested in birds) but often one sex is constrained by the other. That is, sometimes the optimal value for a trait lies far apart for males and females (height, color, hairiness, departure date for southern migration, etc) and the actual values for the trait get stuck somewhere in in middle: if a mutation exaggerates a certain female trait in an individual female, and she passes this trait on to her offspring, it may increase the fitness of her daughters but lower the fitness of her sons. For example, the constraints on the evolution of
dimorphism are easy to see for sexually-selected traits: if facial hair is considered attractive on males and unattractive on females, an version of a gene that increases hairiness of the face will not spread easily if it is expressed in women. For this same reason men have nipples: there is no selection to rid men of nipples (they are perhaps of little cost to the male) -- we simply have them because women need them.<br><br>To hummer behavior: perhaps the males linger because males and females use the same cue that says "Time to migrate!!" : probably some combination of temperature and duration of daylight. It is important for females to remain in Canada for the sake of their young. Let's just say for the sake of argument that it is completely neutral for the males to remain in Canada (ie an equal number would survive migration if they left now or if they leave in September, and their presence in Canada does not affect the survival of
young). So there in strong selection for females to stay, and no selection for males to leave. Therefore males end up expressing the same trait that their female counterparts do: they remain in Canada for the same reason that male mammals possess nipples -- not because there is selection FOR the trait, simply no selection against it.<br><br>I Really like your point about density-dependence! Males present in Canada could consume food supplies and reduce the survivorship of juvenile birds. clearly this selection pressure is not strong enough to cause dimorphic behavior, or the males would be gone alrea