Social Relationships: Adults and Infants

This is the theme for three lectures- including parent-offspring conflict and infanticide. See Primate Societies book in ch 27, 28 for more articles on this topic.

We've touched on some things about adult-infant contacts; remember Hamilton's rule: rB>C. This is the basic theoretical framework for understanding why an adult should ever take care of any infant. We've also seen how maximizing inclusive fitness explains males taking care of infants in cooperative polyandry, we've also seen male relations with infants in baboons- part of mating effort really, not parenting effort. We've also spoken of adult females and infants in the context of allomothering.

Adult females and infants

Variations in mother-infant relations: Maternal styles in baboons People really think it's interesting, the variation between individuals in this relationship between moms and infants. Some of the best studies are from savannah baboons. Especially the yellow baboon- mostly by Jean Altmann, who also wrote our coursepack article. She basically found that there were two types of mothers: permissive and restrictive mothers.

Characteristics

Permissive mothers stop restraining their infants at a younger age than restrictive mothers- when the infants are small, the mother is manly responsible for keeping the infant with her, grabbing him if he wanders off. At some point, the mom stops grabbing it and it's the infant's responsibility to keep up. Permissive mothers stop grabbing their wandering babies at about .5 months vs. 2.25 months for restrictive mothers.

Permissive mothers stop following their infants earlier than restrictive mothers. Early in life, mom is responsible for keeping the pair together, but later on the infant maintains responsibility. Permissive mothers stop following their kids at around 1mo vs. 3.25 months for the restrictive mothers. (This is using the Heinz index of approaches vs leaves to determine who is doing the work of maintaining the proximity.)

Anything else you look at, such as rejection of nursing or refusals to carry, happens earlier in permissive mothers than in restrictive mothers.

Relationship between maternal style and rank

It turns out that permissive mothers tend to be higher ranking than restrictive mothers. Permissive mother's rank in her group was an average of 6, while restrictive mothers' average rank was about 11- much lower. It's not too clear why this is- could it be that he high-ranking mothers are more secure that their kids'll be safe since they're high-ranking? But nothing people have come up with has seemed to work perfectly- for instance, in macaques it's the opposite.

Evolutionary consequences for mothers and infants

One result is that permissive moms wean their infants earlier. So, because permissive mothers stop lactating earlier, you might think they have a reproductive advantage because they could conceive sooner. However, this does not translate into a reproductive advantage, because permissive mothers do not show shorter birth cycles than restrictive mothers; the former take more cycles to conceive than the latter.

Some evidence suggests that restrictive motherhood increases infant survival. Ill health and infant death occurred at a slightly higher rate among infants of permissive mothers (5 of 7; 71%) than among those of restrictive mothers (2 of 5; 40%). Note from the small sample sizes that these aren't statistically significant.

So, given a choice of mothering style, why would mothers pick a more permissive mothering style if it increases their kids' mortality? We'll devote a whole lecture to this in the lecture on parent-offspring conflict, later on.

Adult males and infants

Intensive caretaking

The male does a lot of parenting, such as in the callitrichids. In monogamous species, males are also doing a lot of carrying and protecting. Males should be sure of paternity to benefit from taking care of infants, so it's usually monogamous species like aotus, the night monkey. Most monogamous species have intensive care from the dads.

Affiliations

The male forms social bonds with the infant; they interact sometimes, playing together or maybe sharing food. You see these types of interactions in black howlers, baboons, and gorillas.

Tolerance

The male and infant don't have much interaction. This is most common in solitary species, such as orangutans. Males show no attractions to infants. They don't seek them out, but don't avoid them either.

A controversial aspect of male relations with infants- Triadic interactions

This was first documented in barbary macaques. They're noted for males doing a lot of infant carrying and holding, but it was particularly Deag and Crook who noticed that males were particularly likely to pick up and carry a baby when they were in a conflict with another male. There are two hypotheses to explain this behavior.

Agonistic buffering hypothesis

The holder is using the baby to protect himself from the other male, who doesn't want to risk hurting the baby because then all the female baboons will get on his case.

Parental care hypothesis

This was suggested by Busse and Hamilton. When studying chacma baboons, they noticed that infants are most often picked up by a long-term resident male, while the male he's having the conflict with is usually a recent immigrant. They guessed that it might be a form of parental care- the holder could possibly be a father of the infant and he is protecting it from the other possibly infanticidal male; so it's a form of parental care by the males.

Evidence and predictions-- Agonistic buffering

Prediction

Males who carry infants receive less aggression than males who don't carry infants.

Evidence

• Among the baboons at Gombe, a male was less likely to be threatened by another male when he was carrying an infant than when he was not carrying an infant. (20 of 22 dyads (91%))
  
• In a study of captive bonnet macaques, 6 of 7 males were harassed by other males less often when holding an infant than when engaged in other activities. (Silk and Samuels did this study.)

Another prediction is that the male who is the recipient of the behavior (i.e. the one who is not carrying the infant) should be the father of the infant- the infant is being used like a hostage against him- but no one has found evidence of this.

So, a correlation has been shown, but causation has not. In other words, maybe because he is holding infant he is not attacked, but it also could be that when he is holding the infant, he is trying harder not to provoke an attack since he is supposed to be protecting it.

Evidence and predictions-- Parental care

Prediction

Male carrier is a probable father, while recipient is not a probable father.

Evidence

(Remember that males often carry their friend's babies- also remember that a lot of the time, males are likely to be the father of their friend's babies.)

• Among chacma baboons, male carriers were high-ranking during the time at which the carried infants were conceived, and were therefore probable fathers.
  
• In contrast, most recipient males were not in the group when carried infants were conceived (42 of 45 infant-recipient combinations). This is particularly interesting because recent immigrants or males who have just recently reached high rank are the ones who are the most likely to practice infanticide- so they are potentially threatening to the infants.
  
• Male barbary macaques frequently care for infants. Recent genetic studies indicate that males do not show preferential treatment to their own infants. They found no particular correlation between care and paternity. This may indicate that the researchers knew more about paternity than the monkeys themselves did. Remember that mating is fairly promiscuous in barbary macaques so the males wouldn't have a lot of information available to guess which infants were their own.