next message in archive
no next message in thread
previous message in archive
previous message in thread
Index of Subjects
young).&nbsp; So there in strong selection for females to stay, and no selection for males to leave.&nbsp; 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!&nbsp; Males present in Canada could consume food supplies and reduce the survivorship of juvenile birds.&nbsp; clearly this selection pressure is not strong enough to cause dimorphic behavior, or the males would be gone alrea
This is a multi-part message in MIME format.
------=_NextPart_000_002E_01C7B710.79706FC0
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
There is an example of the reverse - females leaving early - in seabirds =
with phalaropes. The females are the most brightly coloured in these =
species. Fall migration has already started with female phalaropes =
starting to migrate to non-breeding areas off South America after laying =
their eggs. The males remain on the nest until the chicks are hatched =
and then leave once the juveniles are independent, with the juveniles =
following still later. With limited food resources on the migration =
route it is suggested that a staggered migration is beneficial to reduce =
competition for food resources which would happen if all phalaropes =
migrated at the same time.
Laurie Murison
Grand Manan, NB
----- Original Message -----=20
From: Andrew MacDonald=20
To: naturens@chebucto.ns.ca=20
Sent: Monday, June 25, 2007 9:48 AM
Subject: Re: [NatureNS] Why do male hummers...
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!=20
----- 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.=20
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
-------------------------------------------------------------------------=
-----
Be smarter than spam. See how smart SpamGuard is at giving junk email =
the boot with the All-new Yahoo! Mail
------=_NextPart_000_002E_01C7B710.79706FC0
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=3DContent-Type content=3D"text/html; =
charset=3Diso-8859-1">
<STYLE type=3Dtext/css>DIV {
MARGIN: 0px
}
</STYLE>
<META content=3D"MSHTML 6.00.6000.16481" name=3DGENERATOR></HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>There is an example of the reverse - =
females=20
leaving early - in seabirds with phalaropes. The females are the =
most=20
brightly coloured in these species. Fall migration has already =
started with=20
female phalaropes starting to migrate to non-breeding areas off South =
America=20
after laying their eggs. The males remain on the nest until the =
chicks are=20
hatched and then leave once the juveniles are independent, with the =
juveniles=20
following still later. With limited food resources on the =
migration route=20
it is suggested that a staggered migration is beneficial to reduce =
competition=20
for food resources which would happen if all phalaropes migrated at the =
same=20
time.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Laurie Murison</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Grand Manan, NB</FONT></DIV>
<BLOCKQUOTE=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV=20
style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
<A title=3Dcb_andrew@yahoo.ca =
href=3D"mailto:cb_andrew@yahoo.ca">Andrew=20
MacDonald</A> </DIV>
<DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3Dnaturens@chebucto.ns.ca=20
href=3D"mailto:naturens@chebucto.ns.ca">naturens@chebucto.ns.ca</A> =
</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Monday, June 25, 2007 =
9:48 AM</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Subject:</B> Re: [NatureNS] Why do =
male=20
hummers...</DIV>
<DIV><BR></DIV>
<DIV=20
style=3D"FONT-SIZE: 10pt; FONT-FAMILY: times new roman, new york, =
times, serif">
<DIV=20
style=3D"FONT-SIZE: 10pt; FONT-FAMILY: times new roman,new =
york,times,serif">Hi=20
everybody,<BR><BR>Perhaps this can be considered a case of a sexual =
dimorphism=20
that has NOT developed. Sex-specific traits can evolve (as we =
all know,=20
because we're interested in birds) but often one sex is =
constrained by=20
the other. That is, sometimes the optimal value for a trait lies =
far=20
apart for males and females (height, color, hairiness, departure date =
for=20
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=20
individual female, and she passes this trait on to her offspring, it =
may=20
increase the fitness of her daughters but lower the fitness of her =
sons. =20
For example, the constraints on the evolution of dimorphism are easy =
to see=20
for sexually-selected traits: if facial hair is considered attractive =
on males=20
and unattractive on females, an version of a gene that increases =
hairiness of=20
the face will not spread easily if it is expressed in women. For =
this=20
same reason men have nipples: there is no selection to rid men of =
nipples=20
(they are perhaps of little cost to the male) -- we simply have them =
because=20
women need them.<BR><BR>To hummer behavior: perhaps the males linger =
because=20
males and females use the same cue that says "Time to migrate!!" =
: =20
probably some combination of temperature and duration of =
daylight. It is=20
important for females to remain in Canada for the sake of their =
young. =20
Let's just say for the sake of argument that it is completely =
neutral=20
for the males to remain in Canada (ie an equal number would survive =
migration=20
if they left now or if they leave in September, and their presence in =
Canada=20
does not affect the survival of young). So there in strong =
selection for=20
females to stay, and no selection for males to leave. Therefore =
males=20
end up expressing the same trait that their female counterparts do: =
they=20
remain in Canada for the same reason that male mammals possess nipples =
-- not=20
because there is selection FOR the trait, simply no selection against=20
it.<BR><BR>I Really like your point about density-dependence! =
Males=20
present in Canada could consume food supplies and reduce the =
survivorship of=20
juvenile birds. clearly this selection pressure is not strong =
enough to=20
cause dimorphic behavior, or the males would be gone already. =
Perhaps=20
food is not limiting to hummers. Or perhaps males and females do =
not use=20
the same foods or forage at separate times. Or perhaps males =
leave the=20
female and travel far enough that they don't compete with their own =
mate (its=20
OK if they compete with somebody else's, after all). and =
speaking of=20
somebody else's mate, there's always the possibility of cuckolding the =
"mate"=20
of another male. Who knows, perhaps in very rare and very good =
years=20
more than one brood can be raised per female..<BR><BR>Does anybody =
know of a=20
case where males DO migrate a lot earlier than females? just how =
dimorphic can migration behavior be?<BR><BR>Cheers, and thanks for =
pointing=20
out the fun problem, Peter! <BR><BR><BR>
<DIV=20
style=3D"FONT-SIZE: 12pt; FONT-FAMILY: times new roman,new =
york,times,serif">-----=20
Original Message ----<BR>From: Peter Payzant =
<pce@accesswave.ca><BR>To:=20
naturens@chebucto.ca<BR>Sent: Saturday, June 23, 2007 7:23:48 =
PM<BR>Subject:=20
[NatureNS] Why do male hummers...<BR><BR>
<STYLE></STYLE>
<DIV><FONT face=3DArial size=3D2>Hi, all-</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Here's a question that I ponder from =
time to=20
time: Why do male hummingbirds wait around here all summer before =
migrating=20
south?</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Once they've mated with a female, =
their purpose=20
here is essentially over. They don't help in taking care of the young, =
and=20
there's only one brood per female per year. In fact, you could say =
that the=20
males create a negative reproductive pressure by competing with the =
females=20
and young for food. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Why don't they just go back south =
once the=20
females are sitting on nests?</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I can think of a few =
not-very-convincing=20
reasons:</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>- The environment here (food, =
shelter, weather,=20
predation pressure) might be better than on their southern=20
range</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>- Some resources that they need on =
migration are=20
not yet ready (e.g. some important flowers not generating nectar=20
yet)</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>- There's always a chance that an =
un-mated female=20
will turn up (I call this the "wishful thinking" =
hypothesis)</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Any other thoughts?</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Peter Payzant</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Waverley</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial =
size=3D2></FONT> </DIV></DIV><BR></DIV></DIV><BR>
<HR SIZE=3D1>
Be smarter than spam. See how smart SpamGuard is at giving junk email =
the boot=20
with the <A=20
=
href=3D"http://us.rd.yahoo.com/evt=3D40705/*http://mrd.mail.yahoo.com/try=
_beta?.intl=3Dca"><B>All-new=20
Yahoo! Mail </B></A></BLOCKQUOTE></BODY></HTML>
------=_NextPart_000_002E_01C7B710.79706FC0--
next message in archive
no next message in thread
previous message in archive
previous message in thread
Index of Subjects