From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: helping at the nest -- corrected version
Date: 19 Aug 92 01:09:05 GMT
Organization: animal -- coelomate -- deuterostome

*** If you saw a previous version of this, sorry. I cancelled it
because I made a serious error in one paragraph. Here is the
corrected version. In case the first version didn't cancel
and you read it, I'm marking the paragraph that I fixed so
you won't have to trudge through looking for the correction.***

In article <713829321.1319@minster.york.ac.uk> cjhs@minster.york.ac.uk writes:

>I read an interesting example of this just today somewhere, about a
>species of bird which frequently prefer not to mate, but rather to stay
>at home and help raise siblings. This is interesting, since it would
>seem to go against expectations suggested by evolution; the
>explaination suggests that this species of bird lives in circumstances
>where natural selective pressure gives rise to the behaviour.

Chris touched on an interesting study, allow me to fill in the
details on this one. This might seem to start out a little off
course, but I'll get around to the point eventually.

In sexual species, parents and offspring are not genetically
identical. Thus, parents and offspring are bound to disagree
over the amount of parental care the offspring recieves. For the
parent there will be a benefit and a cost to parental care. For the
offspring parental care is a cost-free benefit. In this paragraph
cost and benefit refer specifically to a fitness cost and benefit --
not a metabolic one. The benefit to the parent in terms of fitness
is the boost in inclusive fitness it gains when it's offspring
reproduce. The cost is prolonging the time until the next reproductive
effort is made and/or damaging further reproductive potential in
some way. Since there is high mortality in young animals, parental
care in the form of feeding and protection benefits the parents fitness
because it increases the probability the offspring will live to
reproductive age.

(*** Here's the corrected paragraph ***)
Parents should "want" to cease parental care when the cost/benefit
ratio reaches one. Offspring should want it to continue until the
cost/benefit ratio reaches r, the relatedness of future sibs (i.e.
0.5 if the same father will sire the next batch.) At this point it
benefits the offspring to have the parent reproduce because the
care is not raising the offsprings fitness as much as the the
production of new sibs would.

		]*
cost/benefit	] *
ratio		]  *
	0.5	]    *(1)	
		]      *
	r	]        *(2)
		]           *
		]__________________

			time

(1) parent should seek to halt parental care
(2) offspring should want care to extend to here -- parent-offspring
conflict will occur in the time between these two points

(*** end correction ***)

In the case of the White-fronted Bee-eater (a bird), there is
an extra twist thrown in -- sons often forsake breeding attempts
when they are young to stay and help at the nest. Unaided bee-
eaters produce (on average) 0.51 offspring when they breed unaided.
This goes up by 0.47 for each helper at the nest. Thus, a parent
should "want" his offspring (sons in this case - females disperse)
to help at the nest because his fitness is boosted(*). Sons, however,
will lose out slightly if the help at the nest (their fitness --
in offspring equivalents -- will be 0.47 instead of the 0.51 they
would recieve if they mated themselves. But, some sons do help
their fathers -- why?

(*) fathers receive a 0.47 offspring equivalent increase as opposed 
to getting a 0.26 o.e. increase from grandkids if he let his son mate 
on his own.

They do it because the fathers will go and disrupt their sons
courtship attempts until he give up and helps out at "home".
One thing initially seems paradoxical, why shouldn't fathers
harass unrelated individuals to help at the nest? This won't
work because an unrelated individual has more to lose than the
son -- the son is getting almost as much fitness by helping
out dear old dad (because they are related by a minimum of 1/2 --
the number goes up if the species is inbred). So, the son loses,
but only by a little. Older sons are harassed less and the percent-
age of successfull harassments is lower. Likewise, less closely
related individuals (grandkids) are harassed less and are less
likely to be coerced into helping at the nest. (The statistics
are in the reference given below.)

I thought this was a great paper.

AND NOW A MODERATE DIGRESSION... 

Anybody out there with a newborn child? Want an evolutionary 
explanation of why it cries at night? It is trying to stop
you from mating again so soon (of course it is not conscious
of this). Offspring that disrupt mating attempts of their
parents have an advantage over ones who don't because more
parental care is given to them in the absense of new siblings.
The proximate trigger is hunger, the baby feels hungry and cries
to be fed. This wakes the parents in the night and makes them
irritable and not sexually motivated/receptive. Often, the baby will
not need to be fed much, or it won't eat at all. This is because
it does not need to be fed for any physiological reason (although 
it does feel hungry).

I should mention that the above is just a hypothesis, take it
with a grain of salt. I have a hunch it is probably true (or
at least part of the story), but I don't think it has been
tested. Perhaps anyone who is a parent could comment on how
reasonable this sounds -- did "Jr." have a negative impact
on you and your partners "urge to merge"?

REFERENCE
Emlen and Wrege, 1992, Parent-offspring conflict and the recruitment
of helpers among bee-eaters, Nature 356: 331-333

>Felicitations -- Chris

SPECIAL ADMINISTRATIVE NOTE - The author of the preceding message
would like to profusely apologise for the lack of irrelevant
religious special pleading in this post -- I am between deities 
at the current time. Rest assured that once I begin believing 
in a deity (or deities) I will promptly denounce you all as heathens 
and explain in ferverent, heartfelt postings why exactly you will rot 
in hell unless you convert to my way of thinking with only my hardly
disinterested say-so as evidence that what I say is true. Thank you
for your patience.

Chris Colby 	---	 email: colby@bu-bio.bu.edu	---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
 	--Kurt Vonnegut from "Galapagos"



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