4.2 Social dynamics
Yet another type of diagram is given in this section. Take a preliminary look at Figure 7, which is a way of illustrating the relationships between individual primates and groups of primates. Some specialist terminology is introduced and symbols and colours are used extensively. You will need to work slowly and carefully through the diagram to make sense of it all. Start by using the key to familiarise yourself with the symbols for a male (circle with an upward diagonal arrow) and for a female (circle on top of a cross). Younger primates of either sex are represented by just a circle. The coloured infill of the male symbol tells you something about the animal's status within the group. Then look at the make-up of each type of society and at the territory occupied. Look first at the simpler groupings shown in the diagram, (a), (b) and (c), to get used to the way territories are represented. You may find it helpful to make a few notes about each society in your own words, as you are referred to it from the text. For example, the first, labelled (a), consists of one adult male, one adult female and their young. Note that these diagrams do not include every individual present in a group but are schematic.
You will be aware from reading LoM that the majority of anthropoid primates, unlike most other group-living mammals, live in very complex societies that typically include adult males and females, subadults (adults that are not sexually active), juveniles and infants. The structure of the group differs between species. The major types of group composition are listed below.
one female and one male (monogamy, Figure 7a);
one female and two or more males (polygamy or more correctly polyandry, meaning more than one male, Figure 7b);
several males and several females (multimale-multifemale); Figure 7e shows such a group, where the females are related (termed matrilineal).
(a) Watch the TV programme from 31.00-48.19 and make notes on what you observe. (You'll be using these notes again in Activity 11.) The sequence illustrates the different social groupings of macaques, baboons and geladas. From your notes and your reading of LoM, identify which of these species form the following types of social groups: monogamous, polyandrous, unimale, multimale-multifemale, referring to Figure 7. (For this activity, ignore the issue of whether the females of a group are related, which I'll discuss later.)
(b) From the description in LoM p. 282, use the conventions in Figure 7 to draw a diagram representing a group of mandrills. Check your diagram with the answer below.
You might like to draw up a table at this point, showing the types of social grouping evident in the anthropoids described so far.
Marmosets form both monogamous and polyandrous groups [p. 264], as do tamarins, although only polyandrous groups are mentioned in the programme at (17.14).
Geladas [p. 278] and mandrills [p. 282] form unimale polygynous groups. In my terminology, this is what I refer to simply as unimale groups. (Don't be confused by the fact that DA calls these gelada and mandrill groups 'polygamous' [p. 282]; my preference is to avoid this less precise term - see the definitions above.) Uakari [p. 248], baboons and macaques form multimale-multifemale groups.
As you will recall from the programme, geladas form very large troops but these troops consist of smaller unimale groups that have banded together to avoid predation (Figure 7f).
(b) The mandrill society described in LoM consists of one male and up to 20 females with a few 'junior' males that have less developed colourings. This is represented below (Figure 8) as a multimale-multifemale (non-kin) group. You will see that it is similar to the multimale-multifemale (matrilineal) group shown in Figure 7e but the females are neither kin nor a matriline. (I have represented the junior males on my diagram using the symbol for subordinate males.)
It is also important to note that in some species the social structure of the group depends also on the number of animals that can be supported by that particular habitat; a poor habitat may support a polyandrous or unimale group, whereas a richer habitat may support a multimale-multifemale group.
Most primate groups are usually formed around related (kin) or non-related (non-kin) groups of females that remain in the area of their birth. One or more males join a group to gain access to the females, and subadult males leave at maturity to find females of their own. In some species, such as red colobus, the groups form around male kin and the females disperse at maturity.
In primates, groups of males and females generally form year-long associations because males are needed to help the females raise their offspring, either through paternal care, as seen in marmosets and tamarins, or through defending the territory from other groups.
As I've mentioned, the simplest society consists of a monogamous pair and their offspring: the current infant and up to three subadults from previous litters (Figure 7a). But monogamy is rare among anthropoid primates, found only in marmosets, tamarins, siti monkeys and gibbons. As you will recall from LoM p. 264 and the TV programme, marmoset and tamarin females usually give birth to twins and the female needs both the male and the subadults to carry the infants so that she can devote some time to foraging. In marmosets and tamarins, these small family groups may include a second male (polyandry, Figure 7b) and when two mature males are present, the female mates with both.
Watch the TV sequence on tamarins (16.41-18.18) and reread LoM p. 264. In a sentence, write down why females are thought to mate with both males in polyandrous groups.
So that each male 'thinks' he has fathered the offspring and willingly stays to help rear them.
Just as with humans, paternity can be determined by DNA fingerprinting. This technique reveals that each male has sometimes fathered one of the twins; you may notice that LoM makes a contrary claim [p. 264], but I'm describing more recent research here. Without the assistance of both males and the older juveniles, the newborn twins are likely to die; thus there is survival advantage in this system of communal care.
You will recall from LoM that young female marmosets remain sexually inactive until they leave the group; the presence of their mother suppresses production of reproductive hormones so the females don't come into oestrus. (This is the stage when females are receptive and can conceive.) There is another species of mammal, the marmot, a squirrel-like rodent, in which young females remain with their mother despite being fully adult. Read Section 4 on the concept of fitness in S182_3 Studying mammals: chisellers and write a brief account, of about 150 words, to compare and contrast the situations in marmosets and in marmots. Include in your answer any benefits to the mother and the young females that arise from this behaviour.
Young female marmosets remain with their mother to carry younger siblings, allowing the mother to devote her energy to foraging. The presence of their mother suppresses hormone production, preventing young females from becoming sexually active. Young female marmosets benefit from this behaviour through the acquisition of good parenting skills, which may enhance their lifetime reproductive success by increasing the viability of their future offspring.
In contrast, young female marmots remain with their mother to provide warmth for younger siblings during hibernation. Young female marmots are sexually active but their mother subjects them to stressful behaviour that prevents pregnancies and causes abortions. The strategies of both mothers achieve the desired result, preventing additional young being born that might jeopardise the lives of current youngsters. In both species, young females enhance their own inclusive fitness by increasing the viability of their siblings, and mothers enhance their own lifetime reproductive success by increasing the likelihood that their offspring (subadult and infant) will survive to reproduce themselves.
In general, primate infants are particularly vulnerable to attack by their own species because they are born singly or as twins and are fed milk for a substantial period. Lactating females are unable to conceive, so when a male successfully ousts a resident male from a unimale group, he generally commits infanticide to bring the females back into reproductive condition. This behaviour is widespread among primates, possibly because a male may only have residency, and therefore the chance to reproduce, for a limited period, typically several years of a long life, although chacma baboons sustain residency on average for only seven months! Infanticide is also seen in multimale-multifemale groups when new males join. Although new males are not readily accepted (recall the difficulties faced by a new male baboon trying to join a baboon troop in the TV programme from 40.26-41.20), take-overs occur sufficiently often for females to have evolved counterstrategies that prevent infanticide. One strategy is paternity confusion; females mate with different males, so none 'knows' if he is the father and all the males help defend infants against new males. As mentioned in LoM [p. 275], female baboons signal their receptivity to mate by developing brightly coloured genitals; but in fact they can produce this coloration in response to a new male before they are actually ready to conceive, which protects their infants from infanticide.