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Cetacean Social Structure

Cetacean Social Structure

  • Source slide deck by Dr. Joana Augusto for MARI 3090 (Fall 2025).
  • Focus: how social structure is defined, measured, visualized, and exemplified across cetaceans (baleen and toothed whales) with emphasis on methods, units, and case studies.

Sirenian Habitat

  • Sirenians can inhabit freshwater, brackish, and saltwater environments; habitat suitability depends on the species.

Paedomorphosis and Evolutionary Considerations (context from last class)

  • Evolutionary advantage of paedomorphosis in porpoises discussed.
  • Argument: Paedomorphic (truncated ontogeny) development allows earlier sexual maturity, increasing reproductive potential.
  • Reference: Galatius, A. (2010). Paedomorphosis in two small species of toothed whales (Odontoceti): how and why? Biological Journal of the Linnean Society, 99(2), 278–295. https://doi.org/10.1111/j.1095-8312.2009.01357.x

Assignment and Coursework Timeline

  • Assignment topic availability: Brightspace.
  • Assignment 1 deadline: September 19 (end of day).

What is Social Structure? (Definitions and scope)

  • Cetaceans are commonly found in aggregations and groups:
    • Aggregations: tied to environment (feeding, mating).
    • Groups: actively maintained; benefits from interacting with conspecifics.
    • Implication: there is a broader social structure beyond mere aggregation.
  • Social structure definitions vary depending on the study focus; in marine mammals, emphasis is on interactions between individuals.

What is Social Structure? (Key definitions)

  • Hinde (1976) framework:
    • Interactions: direct behaviours; content (what) and quality (how).
    • Relationships: interactions that are integrated over time (content and quality).
    • Social structure: the relationship manifested across time, with content and quality
  • Core idea: social structure emerges from structured relationships over time, not just isolated interactions.

Methodology: Why we study associations

  • Cetaceans live underwater, making direct observation of interactions difficult.
  • Proxy: associations – individuals within the same group are considered associated.
  • Use associations to estimate relationship measures (second level of framework): pairwise relationships.
  • Third level: broader framework incorporating permutation tests, mathematical modeling, social network analyses, etc. (Hinde, 1976).

Methodology: Matrixes of Associations (example data)

  • Concept: a matrix of association indices among individuals (e.g., AGU, APA, BUM, CAL, DAR, CLU, ELE, FAC, GOR, LAM, LUA, MED, MID, MUR, QUA, RED, SPI, TAL, THO, TIP, TRU, TUD, WAL, ZOE).
  • Nature: pairwise values typically range from 0 to 1, with 1.00 on the diagonal.
  • Example entry structure (from Augusto, 2007):
    • AGU – APA: 0.81, APA – AGU: 0.81, … , APA – APA: 1.00
    • CAL – DAR: 0.60, etc. (matrix shows many pairwise values such as 0.69, 0.73, 0.75, 0.61, 0.49, 0.59, 0.60, 1.00, …)
  • Notable rows/columns (illustrative):
    • CAL row includes values against AGU, APA, BUM, etc., culminating in CAL–CAL = 1.00.
    • MID row includes a mix of high and near-zero values, e.g., MID–GON (example) ≈ 0.99, MID–APA ≈ 0.04, MID–LUA ≈ 0.00, etc.
  • Purpose: to quantify the strength of associations that feed into social structure analyses.
  • Important takeaway: higher association values indicate stronger association between dyads; the matrix is often used to infer networks and prioritizes long-term relationships.

Methodology: Visualization – Dendrograms

  • Dendrograms show clustering of individuals by association indices.
  • Example ordering (from slide): TIP, TAL, WAL, ELE, ZOE, SPI, CAL, QUA, THO, MUR, LAM, APA, LUA, CLU, FAC, BUM, HUX, TRU, AGU, MID, MED, GOR, DAR.
  • Axis: association index ranging approximately from 0.35 to 0.80 (higher means stronger similarity/association).
  • Use: visual representation of social structure and relatedness among individuals.

Methodology: Visualization – Social Networks

  • Social network visualization (Figure 5) shows individuals seen more than 20 times during the sampling period.
  • Key features:
    • Colors denote different units (groups) or NaN for unassigned individuals.
    • Symbols denote sex: circles = unidentified, squares = females, triangles = males.
  • Purpose: to illustrate the network structure, unit boundaries, and sex-based segmentation.

Methodology: Drivers and Consequences of Social Structure

  • Social structure is both influenced by and influences:
    • Demography
    • Genetic population structure
    • Population biology
    • Culture
    • Kinship patterns
    • Fitness
  • Techniques exist to link additional parameters to social structure to investigate drivers and effects.

Methodology: Identifying Individuals

  • Essential step to study social structure: individual identification.
  • Techniques and cues:
    • Photo-identification: dorsal fins, flukes, scar patterns, coloration, man-made markings.
    • Tags: visual or GPS.
    • Acoustic signatures (species-specific or individual-specific).
  • Species-specific considerations apply to how identification is implemented.

Overview: Baleen vs. Toothed Whales – Social Structure Drivers

  • Major drivers of group formation differ between baleen and toothed whales due to ecology (foraging, mating, predation).
  • Baleen whales:
    • Tend to be solitary, with brief and unstructured interactions.
    • Mother-calf pairs are common.
    • Social structure likely linked to food sources and low predation pressure.
  • Toothed whales (Odontoceti):
    • Exhibit a continuum of stability with fission-fusion dynamics and more stable, hierarchical societies.
    • Smaller odontocetes: fission–fusion with frequent associations, many acquaintances, and some long-term preferences.
    • Sperm whales, orcas, and pilot whales: more stable societies, often matrilineal, with pods or social units.

Bottlenose Dolphins: Two well-studied populations

  • Sarasota, Florida – T. truncatus (Common bottlenose dolphin).
  • Shark Bay, Australia – T. aduncus (Indo-Pacific bottlenose dolphin).
  • Group structure: 2–15 individuals in open waters; smaller groups in coastal areas.
  • Key concept: Bottlenose dolphins exhibit fission–fusion dynamics with multiple social layers.

Bottlenose Dolphins (Portugal, Sado Estuary) – A case study

  • Location: Sado estuary, Portugal; port city context.
  • Population: around 25 individuals (2023); historically larger in the 90s but calves struggled to survive then.
  • Population status: philopatric and closed; some coastal interaction with external populations but limited.

Bottlenose Dolphins (Portugal) – Group size and associations

  • Typical group size: about 13 individuals (roughly half of the population size).
  • Age structure: mixed age classes; calf-protection strategy in play.
  • Associations: high average for the population (approx. 0.45)); higher than in the larger 1990s population, reflecting effects of reduced population size and larger typical group sizes.
  • Pairwise associations: generally similar across pairs (homogeneous) with atypical patterns based on sex and age.
  • Non-random associations: long-term preferences; casual acquaintances exist but fission–fusion is more stable than expected (atypical patterns).

Bottlenose Dolphins (Portugal) – Stability and ecology

  • Stability rationale: stable year-round feeding grounds combined with declining population reduce intraspecific competition, increasing group size and stability.
  • Fragility: such stability can be brittle due to limited exchange or immigration; future dynamics are uncertain.
  • Other populations: Doubtful Sound, NZ noted as another very stable population.

Risso’s Dolphins (Azores) – Stratified community

  • Social structure lies between fission–fusion and stability.
  • Group organization by age and sex:
    • Males: highly stable units; access to females and feeding influence structure.
    • Females: stable nurseries in summer (calving season); calves receive support while feeding.
    • Juveniles: form bachelor pods; stay close to natal group for years.
    • Transition to male units and female nurseries as adults occur over time.

Long-Finned Pilot Whales – Stable social structure

  • Social units average around 7 individuals; units interact in labile, larger-scale groups.
  • Typical group size: 57–62 individuals.
  • Units include both male and female adults; individuals are related both between and within units.
  • Matrilines are not easily identifiable in this species.

Long-Finned Pilot Whales – Fission–Fusion Dynamics

  • Units undergo fission and fusion over time:
    • Fission: difficulty maintaining associations across all individuals; common in other species (orca, sperm whale, elephants); often occurs along matrilines.
    • Fusion: occurs when units are too small (due to strandings, stochasticity, low birth rates) to maintain safety in numbers; fusion can increase protection from predators.
  • Floater individuals exist (solitary animals).
  • Alloparental care is observed in this context.

Alloparental Care

  • Definition: care of an offspring by an individual other than its parent.
  • Benefits the infant and would not occur in its absence.
  • Rarity: relatively rare but can influence social structure.
  • Examples across species:
    • Orcas: post-reproductive females participate in care.
    • Sperm whales: calves receive care from individuals while mothers feed at depth.
    • Pilot whales: both males and females provide alloparental care.

Summary of Key Concepts and Terms

  • Aggregations vs. Groups: environmental vs. interaction-driven social units.
  • Paedomorphosis: truncated ontogeny leading to early maturity and higher reproductive potential.
  • Fission–Fusion Dynamics: fluid group composition over time common in toothed whales.
  • Matrilineal Social Units: lineage-based social organization; common in some large odontocete societies.
  • Philopatry: tendency of an individual to stay in or habitually return to its birthplace or natal area.
  • Alloparental Care: care by non-parent individuals; implications for social structure and infant survival.
  • Association Index: a measure of the strength of association between two individuals; typically bounded by 0 and 1; higher values indicate stronger association.
  • Social Network Analysis: a methodological framework to study relationships and structure within a population.

Notable References and Data Sources

  • Hinde, Kinds of interactions and relationships (1960s–1970s foundational framework; cited as Hinde, 1976).
  • Augusto, Joana (2007): Methodology and visualization approaches (dendrograms, association matrices, and social networks).
  • Galatius, A. (2010): Paedomorphosis in small Odontoceti (toothed whales).

Practical and Ethical Implications

  • Studying social structure informs understanding of demography, genetics, culture, kinship, and fitness in marine mammals.
  • Identifying individuals and maintaining long-term datasets are essential but logistically challenging; methods must minimize disturbance.
  • Understanding stability, fission–fusion dynamics, and alloparental care has implications for conservation, management, and human impacts on social systems.

Connections to Foundational Principles and Real-World Relevance

  • Social structure research in cetaceans ties to broader principles of animal behavior: group living costs and benefits, mate choice, kin selection, and social learning.
  • Real-world relevance includes guiding conservation strategies, predicting responses to population declines, and interpreting social resilience in changing marine environments.

Key Equations and Symbols (LaTeX)

  • Association index between two individuals A and B:
    • Let a{AB} denote the association strength; values lie in [0,1] with a{AA} = 1.0.
  • A representative excerpt from the association matrix (from Augusto, 2007):
    • AGU–APA = 0.81, APA–APA = 1.00, CAL–DAR = 0.60, MUR–MID = 0.99, THO–WAL = 0.53, etc.
  • Descriptive range for dendrogram axis (association index): approximately from 0.35 to 0.80 in the example visualization.

Quick Reference (Study Cheatsheet)

  • Baleen whales: relatively solitary or short interactions; mother–calf focus; ecology driven by food and low predation.
  • Toothed whales: fission–fusion societies; matrilineal tendencies in larger groups; stability varies by species.
  • Bottlenose dolphins: classic fission–fusion with long-term alliances; Portugal population highlights philopatry and stability under pressure from population decline.
  • Alloparental care expands social complexity and infant survival; signals social investment beyond the immediate parent–offspring dyad.
  • Techniques to study social structure: photo-identification, tagging, acoustics, and network analyses; visualization through dendrograms and social networks.