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Primate Social Systems - Diversity

Extant Primate Taxonomy

  • Strepsirrhini
    • Lemuriformes
      • Lemuridae: ring-tailed lemurs, true lemurs, ruffed lemurs, bamboo lemurs
      • Indriidae: indris, sifakas, woolly lemurs
      • Lepilemuridae: sportive lemurs
      • Cheirogaleidae: mouse lemurs, dwarf lemurs, fork-crowned lemurs
      • Daubentoniidae: aye-ayes
    • Lorisiformes
      • Lorisidae: lorises, pottos
      • Galagidae: galagos
  • Haplorhini
    • Tarsiiformes: tarsiers
    • Anthropoidea
      • Platyrrhini
        • Cebidae
          • Pitheciidae: titis, sakis, uakaris
          • Atelidae: spider monkeys, muriquis, woolly monkeys, howlers
          • Callitrichinae: marmosets, tamarins
          • Aotinae: owl monkeys
          • Cebinae: capuchins, squirrel monkeys
      • Catarrhini
        • Cercopithecoidea: Old World monkeys
        • Hominoidea: apes, humans

Archonta

  • Order Primates
    • Semiorder Euprimates (primates of modern aspect)
      • e.g., Omomyidae, Adapidae
    • Semiorder Plesiadapiformes (archaic primates)
      • e.g., Carpolestidae (Carpolestes), Paromomyidae, Micromomyidae, Plesiadapis
  • Order Scandentia (tree shrews)
    • e.g., Ptilocercus
  • Order Dermoptera (flying lemurs)
  • Order Chiroptera (bats)

Primate Social Systems

  • Definition: Social organisation + social structure + mating system.
  • Key Question: How many different types of social systems do primates have?

How are primate groups different?

  • Individual recognition: The basis for complex social interactions.
    • Birds and mammals recognize individuals.
    • Many species without parental care or bonding probably lack this.
    • Other species only recognize categories (e.g., colony member or not).
    • Anonymous societies.
  • Stability of association: Membership changes through births, deaths, immigration, and emigration.
  • Individual recognition + stable association --> individualised societies (= fundamental social units).

Components of a Social System (Society)

  • Social organisation (group size, composition and stability)
  • Social structure (patterning of social relationships)
  • Mating system (has both a social and genetic element)
  • Spatial association
  • Social interactions

1. Social Organisation

  • Size, sex composition, and spatio-temporal cohesion of social units.
  • Spatial overlap has to coincide with social boundaries --> Social unit.
  • Variability in a population or species:
    • The extent of variation in spatio-temporal dynamics.
    • The degree to which the modal social organisation is realised within a population.
      • Uniformly: > 90% of social units.
      • Variably: <90% of social units.
  • 5 main types of social organisation:
    1. Solitary
    2. Pair
    3. One-male-multi-female group
    4. Multi-male-multi-female group
    5. Multi-male-one-female group

Spatio-Temporal Dynamics (in group-living primates)

  • Troops:
    • Predictable membership; all together all day.
    • E.g., gorillas, baboons, vervets
  • Fission-fusion societies:
    • Predictable (stable) community membership
    • Unpredictable party membership
      • (a) ’Atomistic’ parties - down to 1; E.g., chimpanzees, spider monkeys
      • (b) ’Molecular’ parties - down to predictable subgroups; E.g., hamadryas baboons, geladas

2. Social Structure

  • Emergent property of dyadic relationships.
  • Relationship = repeated social interactions between individuals.
  • Types of social interactions:
    1. Affinitive - indicate a spatial association
    2. Affiliative - friendly
    3. Agonistic - aggression and submission
  • Dispersal patterns - strong influence on social structure: They determine the availability of social partners!

3. Mating Systems

  • Some overlap with social organisation (but not always).
  • Social mating system (mating behaviour) vs. genetic mating system (the reproductive outcomes).
    1. Monogamy
    2. Polyandry
    3. Polygyny
    4. Polygynandry
  • Mating also occurs outside the social unit (extra- pair or extra-group paternity).

A. Solitary Primates

  • Mostly nocturnal (except orangutan).
  • Difficult to study (the nocturnal ones)!
  • Forage alone.
  • Little coordination between individuals in space and time.
  • But they still have social relationships and social networks ( ~ social structure).
  • Solitary \neq asocial!
    • Mothers + offspring
    • Sleeping groups
    • Communication (auditory and olfactory)
  • Goodman’s mouse lemur (Microcebus lehilahytsara)
  • Up to 1/3 of all primate spp. [or only 6%!?!]
  • Most lorisiforms (except Galagoides zanzibaricus)
  • Several species of:
    • Tarsius
    • Daubentonia
    • Microcebus
    • Allocebus
    • Mirza
  • Some Cheirogaleus and Lepilemur
  • Pongo spp. (~ semi-solitary)
  • Hairy-eared dwarf lemur (Allocebus trichotis)

B. Pair-Living Primates

  • The rarest type of social organisation in primates (and mammals).
  • But more common in primates than in mammals! (~15% or 23% VS ~3-4%).
  • Two types of pairs:
    1. Dispersed: M and F not together all the time
    2. Gregarious: permanent association between M and F
  • Male care for offspring is common (but not always obligate).
    • Titi monkeys - obligate
    • Gibbons - no direct care (e.g. infant carrying)
  • Pair-living is not = reproductive monogamy! Extra-pair copulations happen.
  • The adaptive basis of pair-living still debated…

2. Pair-living primates: dispersed pairs

  • Strepsirhines e.g.: Galagoides zanzibaricus, Cheirogaleus medius, Phaner furcifer, Lepilemur edwardsi, Lepilemur ruficaudatus
  • M and F range overlap closely.
  • Defend together.
  • But M & F not consistently associated, or sleeping together….
  • Fork-marked lemur (Phaner furcifer)

2. Pair-living primates: gregarious pairs

  • Pairs with offspring (up to several generations)

  • Extensive paternal care (probably obligate!)

  • Up to 90% of time infant is carried by father

  • Playing, grooming and food sharing with infant

  • Siblings don’t help.

  • Living in pairs: Rare, explanation needed (or is it??)

  • Costs/benefits from the perspective of each partner?

  • Particularly interesting --> understanding the evolution of human pair bonds.

(C) Group-Living Primates

  • One-male, many females (Unimale groups; One- male units = OMUs) ~ polygynous mating system
  • Multi-male-one-female groups ~ polyandrous mating system
  • Multi-male-multi-female groups ~ polygynandrous mating system (but some exceptions, e.g. in the multi-level societies of geladas, hamadryas baboons, etc. with 1-male units as the lowest level or organisation)

One-Male Units

  • Both sexes can disperse
  • Typically, a strong bond between females and the male
  • Weaker bonds among females (but sometimes can be relatives)
  • Bachelor groups common
  • Highly polygynous
  • Intense male-male competition for breeding opportunities

Geoffroy’s tamarin, Saguinus geoffroyi

  • Tamarins: Need help to rear twins (combined weight 20% of mother’s weight at birth and 50% at weaning).
  • Infants carried at all times.
  • Mothers need to feed much more when raising offspring (high cost of lactation!)
  • Groups with more males taking care of offspring have more surviving offspring. --> Facultative polyandry
  • Co-breeding males are usually related (likely brothers)

(C) Group-living primates

  • Troop-living (spatially and temporally cohesive groups), e.g. rhesus macaques
  • Fission-fusion societies (fluid membership) with temporary parties (subgroups)
    • Atomistic parties, e.g. chimpanzees
    • Molecular parties -> multi-level societies (e.g. hamadryas baboon)

Rhesus Macaques

  • Stable groups
  • Female philopatric --> matrilines within groups
  • Males move to new groups when sexually mature; queue for dominance

Chimpanzees

  • Fission-fusion society
  • Male philopatric --> coalitions with kin
  • Females leave natal group when sexually mature
  • Adult females (particularly mothers) are much less gregarious than males
  • Male chimpanzees more gregarious than females

Multi-Male, Multi-Female Societies

  • Multilevel societies (down to ‘molecular’ stable parties)
    • Geladas
    • Hamadryas baboons
    • Snub-nosed monkeys
    • Capped langurs
    • Proboscis monkeys

Multi-Level Organization in Hamadryas Baboons

  • Individuals
  • Family
  • Clan : 10-20
  • Band : 30-100
  • Troop : > 600

Multi-Level Societies

  • Multi-level societies: the most complex societies.
  • Primates:
    • Papionins (gelada, Hamadryas baboon, Guinea baboon).
    • Asian colobines (snub-nosed monkeys, proboscis monkey, douc langurs).
    • Maybe also in: uakaris, drills, Angolan colobus…
  • Other mammals: elephants, zebras, giraffes, sperm whales
  • Often in species where food competition is not too high (e.g. grass foraging) - e.g. gelada, zebra…
  • Often pronounced sexual size dimorphism.
  • Exaggerated secondary sexual traits (e.g. chest patches, big nose… ) to act as ‘badges’ in a large society where not everyone knows all other members!
  • Cognitive consequences largely unexplored but could be significant.

What was the social organisation of the ancestral primates?

  • Nocturnal, small, arboreal, solitary?

Phylogenetic analysis of sociality

  • Phylogenetic inertia in behavioural traits --> allows the use of Bayesian statistics to infer changes through time.
  • N = 217 species examined.
  • Classified as: Solitary (purple), Family groups (pink), Harems/one-male groups (orange), Multi-male (red)
  • Analysis showed a strong phylogenetic signal in the data (Pagel’s lambda = 0.983)!

Models for the evolution of sociality

  • Three alternative models for the evolution of sociality:
    • Increasing complexity
    • Equal rates (null) and parameter rich
    • Reversible-jump model
  • Best support: reversible-jump model
  • Estimated transition rates for co-evolution of social living: Switch to social living - alongside switch from nocturnal to diurnal living
  • The model with the highest posterior support for the evolution of stable or bonded social groups implies that stable social groups evolve from sociality through unstable social groups.
  • Strong support for the importance of predation: group living co- evolved with the transition from nocturnal to diurnal life.
  • Sociality based on loose aggregations followed by second shift to stable or bonded groups --> importance for the evolution of cooperation within groups! (implication for the evolution of coalition formation, resource defense and large brains).

Olivier et al. 2024 PNAS

  • 493 populations from 215 species
  • Only field data
  • Explicitly considered intra-specific and intra -population variation in social org
  • High rates of pair-living: primary for 16% of populations and 23% of species
  • Low rates of solitary: primary for 3% of populations and 6% of species
  • 64% of species and 43% of populations had more than one type of social organisation
  • IVSO = intrapopulation variation in social organisation
  • Ancestral state reconstruction: Under the assumptions of small body size (~50 g) + arboreality + nocturnality (based on fossil evidence)
  • Statistical models identified pair-living (MF) to be the most likely primate ancestral state
  • ~15% of social unit in ancestral populations would be solitary
  • Variable social organization with most individuals being pair-living
  • Differences in classifying social systems can influence interpretation of data.
  • Solitary living is a derived state – and an adaptation to specific environmental conditions.

PRIMATE SOCIAL SYSTEMS

  • Complex individualised societies.
  • Three descriptive axes of variation (social organisation, social structure, mating system).
  • Diversity found across all taxa and sometimes within taxa (although some types of social system are restricted in distribution - e.g. solitary are mostly nocturnal).
  • Special interest ~human evolution: pair-living/social monogamy; multi-level fission-fusion societies.Primate social systems