Michael Weiss & Monika Wieland

Pod Rearrangement in Southern Resident Killer Whales (Orcinus orca)

Question: Do the current matriline associations still align with pod identity? What are the characteristics of the current pod level social groups?

Southern Resident Killer Whales (SRKWs) !• Closed population with no dispersal. • Stable, highly connected matrilineal population.(1)

• Two levels of relation: • Matriline (17 units) • Pod (J, K, and L)

• Matriline constant unit, pod association is less stable. • Changes in social network structure may have

consequences for disease transmission, mating, and information transfer.(2)

Orca Behavior Institute

MethodsObservations !• 59 research encounters with SRKW groups • Individual killer whales identified by dorsal fin and

saddle patch. • Individuals identified within acoustic range (~10 km)

of one another were considered to be in the same group.(3)

Social Network Analysis !• Social network model with matrilines as the nodes.

Edge weights were calculated using half weight index (HWI).(2)

• Communities identified using algorithm designed to maximize modularity.(4)

• Mean internal HWI used as a measure of each community’s internal cohesion.

K20 Spock

J22 Oreo

J34 Doublestuf

0

0.2

0.4

0.6

0.8

1

1.2

Mean(HWI)

A B C

Community

.��V

.��V /�V

/��V

/��V

/��V

/��V

/��V

.��V

.��V

-��V

-��V

-��V

-��V

-�V

-��V/��V

.��V

.��V /�V

/��V

/��V

/��V

/��V

/��V

.��V

.��V

-��V

-��V

-��V

-��V

-�V

-��V/��V

Figure 1. SRKW social network. Matrilines are arranged using ForceAtlas2 to maximize clustering based on edge weights, and sized based on their weighted degree. Left: Matrilines are colored according to pod identity. Right: Matrilines are colored according to community assigned by community detection algorithm designed to maximize modularity(Resolution = 0.85, Modularity= 0.054).

Community Detection

Figure 2. Connectedness of identified communities. Internal HWIs were significantly different between communities (ANOVA, p<0.001). Stars indicate statistically significant differences (Tukey HSD post hoc test). Error bars are 95% confidence intervals.

Community

Inte

rnal

HW

I

*

Pod Identity

Results

References: (1) Bigg, M. A., Olesiuk, P. F., Ellis, G. M., Ford, J. K. B. & Balcomb, K. C. 1990. Social organization and geneology of resident killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Report of the International Whaling Commission, 12, 383-405. (2) Croft, D. P., James, R. & Krause, J. 2008. Exploring Animal Social Networks. Princeton, New Jersey: Princeton University Press. (3) Parsons, K. M., Balcomb, K. C., Ford, J. K. B., & Durban, J. W. 2009. The social dynamics of southern resident killer whales and conservations implications for this endangered population. Animal Behavior 77, 963-971. (4) Blondel, V. D., Guillaume, J., Lambiotte, R., Lefebvre, E. Fast unfolding of communities in large networks. Journal of Statistical Mechanics: Theory and Experiment 2008, 10.

ConclusionThese results demonstrate a possible shift in the pod-level social organization of the southern resident killer whales. We found three novel units, each with significantly different levels of internal cohesion. This information could provide a starting point for further understanding the changing social dynamics of the SRKWs, which may aid efforts to recover this endangered population.

Further Questions:• What intrinsic and external factors determine a matriline’s position and connectedness within the

network? • Food availability • Genetic relatedness • Effects of human activity

• How do these different groupings effect the whales’ behavior and vocal communications?Acknowledgements: Individual IDs along with pod identification made possible by the Center for Whale Research Orca Survey. Funding for this study provided by generous private donations to the Orca Behavior Institute.