A cute film about chimps in captivity. They live in a zoo in the Netherlands, in a large enclosure with stuff to stimulate them and only natural barriers- moats with water in them is all you need, since chimps can't swim. We were instructed to watch for social interactions between males and females, and also between adults and infants. While watching a chimp determined to get up into a tree and trying to make a ladder out of a stump, we should also think about primate cognition, which is coming up- how intelligent are they? What kind of intelligence are they using?

Remember your inclusive fitness

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

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?

• In sexually reproducing species, parents and offspring are not identically related, but only by one-half.

• Thus, there is an overlap in the genetic interest between parents and offspring, but not an identical interest. So sometimes what's good for the kid is good for the parent, too, but sometimes it's not.

• This partial genetic overlap creates a conflict of interest whose implications are:
  
  • How long should mothers continue to invest int heir offspring?
    
  • How much should mothers invest in their offspring?

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

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.

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

"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

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 nervou,s and when the infant begins throwing a tantrum, she runs to comfort him and then he begins to nurse. So, yes, they generally work.

We have a gruesome lecture topic today! But interesting because it's got a controversial history and it's a pretty strange set of behaviors which we never would have made sense of without thinking of them in terms of modern evolutionary theory. The scientist who has played the biggest role in infanticide, and also wrote the cp article, is Sara Hrdy.

Infanticide among langurs

This is the usual scenario- a male has a group of females and a group of males will drive him out. Sometimes this is drawn out over a period of time; it's a while before they drive him off and a new male is established. At that time, any unweaned infants in the group will usually be killed by the new male. This was first documented in the 60's by Sugiyama.

Taxonomic distribution

There are plenty of infanticidal primates- lemur catta, red howlers, red colobus, silver leaf monkeys. There are several cercopithecine examples- red-tails, blue monkeys among them. A couple of different savannah baboons, and also chimps and gorillas among the apes.

The two main hypotheses

• Infanticide is due to high population densities, and is an aberrant and dysfunctional behavior.

This was the early explanation. It says that infanticide is a pathological behavior. It isn't part of the normal makeup of the species but is because of abnormally high population densities. Like the way when you crowd lab rats together they kill each other. This made sense for the langurs since one of the groups that was studied was being crowded into little areas by deforestation, and in other study areas people were feeding them and this usually makes levels of aggression rise.

Sexual selection hypothesis

• Infanticide is a male reproductive tactic:
  
  • Loss of suckling infant leads to the onset of estrous in the mother
    
  • Males gain a reproductive advantage through earlier conception by females.

Some examples from chimps- when a new female with an infant comes into the group, usually the infant will be killed by the males in the group. As a result, soon the female is in estrous again, which she wouldn't have been for years- and so one of the males in group can have a child by her.

The Data: Circumstances and victims of infanticide

• The majority (67%) of all infanticides occur in one-male groups, (n=23)
  
• Most (21 or 91%) were committed by strange males
  
  • 17 (74%) were committed by immigrant males
    
  • 4 (17.4%) were committed by extra-group males
    
• Only 2 (< 10%) were committed by a male within the social group, but in both cases it was a male who had just increased his dominance rank. This is significant because only a higher-ranking male can benefit from a female coming into estrous sooner.
  
• 13 cases (=57%) occurred after takeovers by males
  
• All 23 of the infant victims were still unweaned

Evaluating the hypotheses: predictions and tests

Predictions of high population density hypothesis

Infanticide occurs as high population densities
Infanticide will not necessarily benefit the killer

Does infanticide actually occur at higher population densities? When you plot data matching infanticide occurrences and the population densities, the data points are pretty scattered. However, when you separate them out by one-male groups and multi-male groups, you see that infanticide is a lot more common in one-male groups than in multi male groups. This is consistent with the sexual selection hypothesis because it's the males coming into the groups who are doing the killing when they take over.

Predictions of sexual selection hypothesis

Infanticidal males will not typically be the fathers of the offspring killed
Mothers will become sexually active earlier than if their infants had lived
Infanticidal males benefit reproductively by killing offspring

Prediction 1- relatedness of infanticidal males an infant victims

• In 22 of 23 cases, the infanticidal male was not the probable father because: he was not in the group at the time the infant victim was born; he was sexually immature; or he was of low rank and probably did not father in the infant.
  
• There are no verified cases of a father killing his offspring.

Prediction 2- the effect of infanticide on interbirth intervals

• In four cases where data exists, infanticide shortened the interbirth interval by 66%.

Note that there are two reasons why a male would want to have the females in estrous quickly- one is just to have more kids in his lifetime. The second thing to consider is that he is going to get overthrown sometime too, so he needs to get kids started early so they're weaned by the time some new male comes in so the new guy doesn't kill them.

Prediction 3- reproductive benefits derived by infanticidal males

• In 8 of 21 cases the infanticidal male mated with the mother after killing her infant
  
• Males may have sired subsequent offspring in 7 of 14 births following committing infanticide.

Response of females to infanticidal attacks

The answer is, they're kind of stuck between a rock and a hard place, being in one male groups. If a female doesn't mate with the new guy, then she decreases her own reproductive success. Females who hold a grudge and don't mate are thus selected against, so females who forgive an forget end up having more kids.

Also, if you mate with the guy who killed your baby, then you know that your future sons will have good genes for getting mates for themselves when they grow up and take over a group.

Data from gorillas: When a usurper comes in and kills a female's baby, even though the silverback was trying to save it, females are more likely to leave the old guy and go with the new guy! This new guy has shown how tough he is, and the guy who tried to protect her obviously wasn't able to do it, so she may as well go with the tougher guy and get his tougher genes for her future kids.

Females do have some ways of responding to this threat to their reproductive success. Although refusing to mate won't really work, there are things they can do.

Female coalitions

One is to form a coalition of females against the infanticidal male. In some cases, this method is effective and together they can protect their babies from the males- see description in CP. This happens in langurs, redtails, and blue monkeys. Note that they are all matrilineal species, so females are living with relatives.

Help from the males

A-- male defense in patrilineal societies
In these female-dispersing species, you don't tend to see coalitions between females- but if there are multiple males in the group they will form coalitions to try and protect the babies against potentially infanticidal males. For instance, a new male who has just joined and couldn't have fathered any of the offspring, or else a male who has just risen up in the hierarchy and hadn't mated before so wasn't anyone's father. (Although if he's related to other males who have mated, then he wouldn't be as likely to commit infanticide.)

B-- male-female coalitions: baboons
When a new male joins a group, he wants to (well really his ancestors have been selected to) kill the babies, but a female and the guy who was likely to have fathered her baby will join together to protect the infant. Sometimes they're effective and sometimes they're not, but it seems when the male tries to help, they are more likely to be successful in protecting the infant. This could be why there's less infanticide in multi male groups.

Post-conception estrus and promiscuity

This is not necessarily a conscious deception- it's just a behavioral trait that has been selected for. Sometimes when a male takes over a group and begins attacking, pregnant females will extend estrous or even back come into estrous even though there's no way they could possibly conceive- he copulates with them when when she later has an infant he figures it's his and so doesn't kill it. This has been documented in colobines, including langurs and red colobus. Females will extend estrous longer into their pregnancy, and they will copulate a lot more, especially with the new male.

Outstanding problems

A female named Passion began killing and eating several of the babies in her community. Together with her daughter Pom, over a period of many years they attacked and killed infants in their group. Usually when males kill a baby, they don't eat it, but these females seemed to be after meat; they'd chase and consume the infant. They were actually seen to eat 3, chase 3 others, and there were 8 others who disappeared under mysterious circumstances. In this period, there were almost no infants weaned successfully.

So this is kind of a question mark because it's only been these two individuals documented- and the daughter probably learned it from the mom- so maybe we can label this one pathological and say that it's not a part of normal chimp behavior.