Data from: A multilevel society of herring-eating killer whales indicates adaptation to prey characteristics

Non-social factors can influence animal social structure. In killer whales
(Orcinus orca), fish- versus mammal-eating ecological differences are
regarded as key ecological drivers of their multilevel society, including
group size, but the potential importance of specific target prey remains
unclear. Here, we investigate the social structure of herring-eating
killer whales in Iceland and compare it to the described social structures
of primarily salmon- and seal-eating populations in the Northeast Pacific,
which form stable coherent basic units nested within a hierarchical
multilevel society. Using 29023 photographs collected over 6 years, we
examined the association patterns of 198 individuals combining clustering,
social network structure, and temporal patterns of association analysis.
The Icelandic population had largely weak but non-random associations,
which were not completely assorted by known ranging patterns. A
fission–fusion dynamic of constant and temporary associations was observed
but this was not due to permanent units joining. The population-level
society was significantly structured but not in a clear hierarchical tier
system. Social clusters were highly diverse in complexity and there were
indications of subsclusters. There was no indication of dispersal nor
strong sex differences in associations. These results indicate that the
Icelandic herring-eating killer whale population has a multilevel social
structure without clear hierarchical tiers or nested coherent social
units, different from other populations of killer whales. We suggest that
local ecological context, such as the characteristics of the specific
target prey (e.g., predictability, biomass, and density) and subsequent
foraging strategies may strongly influence killer whale social association
patterns.