We will begin by refreshing our minds on inclusive fitness since parent-offspring conflict is something that can be explained by it. Remember that it can explain apparently altruistic actions towards relatives. It includes both offspring that you can have plus the offspring that your relatives can have, but the offspring of relatives are weighted less since they're not as related to you as your own offspring. So remember that
inclusive fitness = individual fitness + fitness of
others devalued by their relationship to you.
Also remember Hamilton's equation, Cr>B
In these formulas,
Benefits to both parties=survival of the present offspring
Cost to both parties= decrement in ability of parent to produce future offspring
This is because each offspring is related to itself by at least twice its
relationship to its siblings- so to be worth it to the kid for the parent
to save her energy for someone else, the energy she could have spent on
him will have to make two siblings for his genes to carry on.
(Can also be expressed as Benefit/cost > 1/2)
So, like, if all the offspring were always identical twins, then the situation would be different. All the genes would be the same, so as long as the benefit to my sibling was better than the costs to me, I wouldn't mind 'cause my genes would still get passed along. However, since they're usually just full siblings, only half of their genes are the same as mine and so it's got to help them twice as much to be worth it to me to give up my mom's resources.
From the parent's point of view, as long as B:C >1, then it's good to continue to invest in the present kid. However, from the offspring's point of view, it's still beneficial to him (and his genes) for the parent to continue investing until b:c=1/2. This is assuming that the future siblings will be full siblings with r=.5. If the future siblings are only half siblings, then their r is .25 so the present offspring won't want to stop taking resources from the mom until the b:c is at .25.
Now, note that this engenders some predictions- at first, the mom and the kid should be in agreement that nursing is necessary. If the kid is likely to have full siblings, it should resist weaning for a shorter period than if it's likely to have half siblings. Also, in multimale groups where the highest male gets all the matings, a kid should stop fighting the weaning earlier if the same male is in residence as was when he was conceived, but should want to nurse for longer if a new male has arrived. (Because with a change in alpha male, any kids his mom has now will only be half siblings, whereas they would be full siblings if his father was still the alpha male who gets all the matings.) No one has really done any experiments or collected any data on these things, so we don't know how well the predictions hold up.
Note that the other siblings don't already have to be around for this to apply-because the mom will have to stop lactating for a while and build up resources to be able to have another kid, thus witholding resrouces from her present kid.
REMEMBER: Primates are not sitting around drawing cost:benefit graphs in the dirt and doing all this math- natural selection just only lets the ones reproduce who follow these formulas. The ones who acted otherwise didn't cause their genes to survive as well and so their genes are not represented.
"Since the costs of parental expenditure to the parent will be double the cost incurred by the offspring, offspring will be selected to favor a level of parental investment that maximizes the difference between benefit and 1/2 cost.
In contrast, parents will be selected to favor a level that maximizes the difference between benefit and cost."
Rather than saying that the parent gets twice the costs, we could say that the benefits are only half as important to the offspring since they're only half related to any other siblings the parent might be thinking of having, while they're fully related to themselves.
This is again based on the idea that even if the investment by the parent goes up and up, the benefit of that will level off. If the parent gives the offspring a cup of milk, they'll benefit a lot. However, after five gallons, if they give the kid the same cup of milk, its benefit won't be as great. However, the cost of every cup of milk stays the same or keeps on increasing.
So the benefit levels off, but the costs keep increasing. As long as the costs are less than twice the benefit, the kid doesn't care if it costs the parent more than it benefits him since he's only really half related to the mom or the future offspring- so the parent will only want to give as long as the benefit is greater than the cost, while the kid will keep wanting more and more until the cost is twice as much as the benefit; the cost is devalued by half. See graph in the CP reading.
The bottom line is, offspring will always favor receiving more investment than parents are wanting to give. So there will always be conflict- how will it be resolved?
They can try to get more by psychological manipulation. Even though there's a big asymmetry in power, the infants have a better knowledge of their needs than their parents; The infant knows when it's hungry, cries or otherwise lets the parent know, and the parent responds appropriately. This is how they communicate. It makes a system where the kid gets enough food so that it can live and propagate the family line. It also creates a system which the infant can exploit- the infant can keep insisting that it needs more than it really does. There are a few different ways that kids use to manipulate their parents.
One is regression: An infant who has been rejected will begin to act more young and helpless than it really is. Since younger kids need more investment, the kid might trick its parent into giving it more. A regressing infant who has been riding sitting up will begin riding lying down, acting like a younger infant.
Temper tantrums are another case of psychological
manipulation. In this case, the infant is trying to get more by
threatening to hurt itself. Do they work? Jane Goodall says that in
chimps, the tantrums make the mom tense and nervous and when the infant
begins throwing a tantrum, she runs to comfort him and then he begins to
nurse. So, yes, they generally work.