6.11: Social Behavior and Intelligence

Social Organization

Most primates live in groups. The best explanation for why animals form groups and endure the costs of feeding competition is to minimize the risk of predation. Grouping patterns are tied to diet and the defensibility of resources. Females are out to maximize resources for themselves and their offspring, so as to maximize their reproductive success. If a species eats grass or leaves, it does not make sense to defend those resources. 

If a species specializes on ripe fruit, they cannot defend them because of the patchy nature of fruit in geographic space and time. In the case of the few primate ripe fruit specialists, such as chimps and spider monkeys, males defend a home range that contains resources that females need, and thus females are attracted to join them. While orangutans are also preferentially frugivorous, they are solitary due to their large size and strict arboreality, which limits resources to those that are accessible from supporting branches.

While we tend to categorize species by their grouping pattern or social organization, it is increasingly apparent that there is variability within primate species. Some species share our pattern of living in multi-male/multi-female groups. Other categories of primate social organization are solitary, male-female pairs, and one-male/multi-female groups. Interestingly, all of the mating systems seen in primates, i.e. monogamy, polygyny (one male mates with multiple females), polyandry (one female mates with multiple males), and polygynandry (both males and females are promiscuous), are also seen in humans. Some men and women marry or mate for life; some men have multiple wives or partners, and the same goes for some women.

Figure \(\PageIndex{1}\): Stump-tailed macaques. “Macaca arctoides” by Frans de Waal is licensed under CC BY 2.5.

Solitary and Dispersed Polygyny

Except for the orangutans, solitary foragers are small nocturnal prosimians that forage primarily for insects and fruit. Examples of solitary foragers are the bushbabies (see Figure 2) and pottos of Africa, most of the nocturnal lemurs of Madagascar, and the lorises of Asia. Prosimian solitary foragers either avoid predation by stealth (i.e. the slow climbers, such as pottos and slow lorises) or a form of locomotion termed vertical clinging and leaping (e.g., bushbabies) that allows for quick getaways.

Figure \(\PageIndex{2}\): Bushbaby of Africa. “Bushbabies” by Wegmann is licensed under CC BY-SA 3.0.

Females usually forage alone and either park their young nearby or leave them in a “nest,” such as a tree hole. Male home ranges often overlap multiple female home ranges, and males monitor female sexual cycles by “making the rounds” and monitoring their scent, hence the use of the term “dispersed polygyny,” i.e. one male and multiple dispersed females.

Territorial pairs and Monogamy

Monogamy begs the question, “why?” While females may benefit from a monogamous relationship, if their mate supports them or their offspring in some way, it is difficult to understand why males would tie themselves to one mate when mating is not costly for them. There are several theories regarding the adaptive significance of pairing in primates. First is the idea that the female needs help defending a territory in order to obtain enough resources for herself and her offspring. Couples may actively and/or passively defend their territories via threats, fighting, and/or duetting, i.e., calling together to indicate that the territory is occupied by a bonded pair.

Figure \(\PageIndex{3}\): Gibbon of Southeast Asia. “Gibbon Hoolock de l’ouest” by Programme HURO is licensed under CC BY-SA 3.0.

The second theory suggests that monogamy is a way for males to protect their offspring from infanticide. In those species that form one-male groups (see next section), when a new male takes over, he may kill nursing infants. Once nursing is interrupted, a female undergoes hormonal changes and may return to estrus (fertile period). It is in the new male’s best interest to impregnate females as soon as possible, in the “hope” that some of his offspring will make it to the juvenile stage before the next male comes in and wipes out the infants. Why would females mate with a homicidal maniac, you ask?

It is not in their best interest to wait to reproduce either. That is the way natural selection works! Those traits that maximize fitness, i.e. reproductive success, are favored. In addition, a male offspring that grows up to be infanticidal will be in a better position to reproduce, if he has what it takes to take over a group.

One-Male groups and Polygyny

In some species, one male with one or a few females is the grouping pattern. However in other species (Hamadryas baboons, geladas, mandrills, drills, and some odd-nosed monkeys, such as snub-nosed monkeys), one-male units (OMUs) congregate into larger and larger groupings, in a multi-tiered or nested fashion, depending on their current activity. In the majority of one-male group (OMG) species, females are related but as groups get larger, they split along matrilines, meaning that a group of closely related females may splinter when competition increases. In addition, females may move between groups, especially in the colobines (African Colobus monkeys). They are small- to medium-sized monkeys and thus can subsist on a variety of foods, primarily insects and fruit, both of which are indefensible food items, from a female perspective. Thus while a group is beneficial, it does not need to be large. It may be a bit of an oversimplification that female resources drive primate social organization, but it is a useful model with demonstrated heuristic value.

One-Female groups and Polyandry

This type of social organization is seen only in the callitrichids, i.e., the tamarins (see Figure 4) and marmosets of Central and South America. Within those groups, there is usually only one breeding female and one or two breeding males. Females gestate as many as five fetuses but on average, only two survive. Hence we talk about “twinning” in the callitrichids.

Figure \(\PageIndex{4}\): Emperor tamarin. “Emperor Tamarin SF ZOO” by Brocken Inaglory is licensed under CC BY-SA 3.0.

Those groups with an extra male have better offspring survival. At birth, the offspring average one-fourth of the female’s weight and thus foraging to support them is a full-time job for the females. The females nurse the young and the males carry and nurture them. After giving birth it is difficult for the female to carry the twins and find food, so the male usually carries the twins on his back and provides food for his mate. It is thought that male relatives and juveniles also help take care of the infants as a way to practice for fatherhood.

Multi-Male/Female groups and Polyandry

There are two types of multi-male/female groups (MMF). The first is the more common. They are medium to large groups of related females (female philopatric) with a sex ratio skewed in favor of females. Outsider males may congregate in all-male bands. Females and males are promiscuous, the mating pattern known as polygynandry. Many New World monkey species and most of the Old World cercopithecines (such as macaques) exhibit this type of social organization.

The second type of MMF is commonly called a community social organization. Species that exhibit this type of social organization are male philopatric ripe fruit specialists. As mentioned, females cannot defend fruit, so they do not band together into matrilines. Related males defend a territory that contains enough resources to attract females. Females and their offspring forage independently but group members come together periodically into larger aggregations, hence the other term for this type of social organization, fission-fusion. New World spider and muriqui monkeys and the chimps and bonobos of Africa are all categorized as community species.

https://milnepublishing.geneseo.edu/the-history-of-our-tribe-hominini/chapter/primate-social-organization/

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