Parent-Offspring Conflict


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

Examples of parent-offspring conflict

Parent-offspring conflict is easy to observe in humans. It takes all sorts of forms. It's common enough that millions of theories have been developed to explain it, particularly in humans. For example, there's freudian theories and theories about socialization of infants (humans are born savage and greedy and the parents' job is to teach them to rein in their selfish drives and to become a part of society.) However, parent-offspring conflict is not specifically human- it's very widespread in the animal kingdom. Anyone who has looked at parents and offspring in primates has reported parent-offspring conflict of some sort or another.

Weaning conflict
Think about how in the first stage, the mother is maintaining proximity to her offspring, following him, restraining him from leaving her, and initiating most nursing bouts. In the second stage,the infant becomes more independent and initiates most nursing. In the last stage,the m other trails off efforts to maintain contact, and she stops wanting to nurse. In this later stage, the mom will begin rejecting the infant, gently at first and then more strongly. The infant spends a lot of time whining and crying. Trivers once said that if you want to find baboons in the morning, listen for the sound of infant weaning; crying and tantrums.
So all this hullabaloo costs the kid a fair amount of calories, and it also could attract predators or enemies. There must be some benefits that come with it. For about the first two months, baboon moms don't reject at all. At around 2 months, the infants get their first rejection, and by about 3-4 months old, the moms are resorting to pushing, hitting, and biting to get rid of the kid. The question arises, why don't infants wean themselves? Obviously they need to be weaned if they're going to be independent, right?

Carrying conflict
Another conflict which is common is over riding/carrying. It is pretty similar to weaning conflict; the mom does everything at first, then less, and the kid gets upset about it. So the parent doesn't want to carry the kid and the kid still wants to ride. The kids obviously need to learn to walk for themselves, so why is there a conflict when they should both have the same goal?

Temper tantrums
Another manifestation is temper tantrums. As babies get older and the mom begins rejecting the baby, it begins throwing tantrums- screaming and yelling, flailing arms, pulling hair etc. This is not specific just to orangutans, but happens a lot in many primates such as chimps, and humans, especially at the height of weaning.

Why is there conflict?

Kinship theory says...

Remember that we're talking about investments which are limited- time is limited, food is limited, energy is limited. Mom has to choose whether to continue lactating her present child, or she stop and begin preparations for a new baby. Mom might have to decide whether to give her energy to an older or a younger sibling or maybe even to an as yet unborn one.

Conflict over how long to invest: A cost-benefit analysis

Natural selection will act on parents and will shape them to act in a way that maximizes the total number of offspring. From the point of view of any one of those offspring, the other ones aren't as important as him, so the interests of the parent and any one offspring can collide.

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

Parents are expected to invest when:
benefit(to child) > cost (to parent)
or to put it another way, when benefit/cost>1

Offspring are expected to continue to demand investment when:
benefit > 1/2 cost

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.

Offspring are expected to demand investment over a longer period than parents are willing to invest.

We must remember that the costs and benefits change over time- an ounce of milk to a newborn will be a great benefit, but to an already-established child, an ounce of milk will not provide too much benefit. However, the cost to the mother of producing an ounce of milk will not go down. It may even go up because the mom may be getting run down during this high-cost lactating time. So, the benefit:cost ratio will drop over time- the shape of the line/curve will vary depending on the ecological environment, species etc., but there will be a point at some time where the costs will outweigh the benefits. (See fig 11.1 from coursepack article.)

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.

Conflict over how much to invest

The same kind for reasoning defines how much a parent should invest while they're still doing so.

"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?

Manipulation by offspring

Some people might assume that parents always winin a parent-offspring conflict. The parents are bigger, right? So what can the kids possibly do to win?

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.