Next level fission fusion society: fin whales

Presented at the 2016 European Cetacean Society Conference in Madeira, Portugal. 

Abstract

Baleen whales are generally thought to be less social than odontocetes, particularly outside their breeding season, but remain comparatively poorly studied except for humpback whales.. Here we analysed the social structure of a population of fin whales in the Gulf of St. Lawrence based on 4915 sightings of 2608 groups composed of 292 photo-identified animals (87 males, 72 females) recorded in the summer months of 2004-2010. Fin whales formed groups of varying size (1-15, mean/SD=2.63/1.84) and the process of joining, splitting and re-joining groups was frequent and often accompanied by antagonistic behaviour. Group size and composition changed often between consecutive surfacings, with larger groups being more unstable. Females were observed more often alone than males. The larger the groups, the more male biased they became, although females lead most mixed-sexed groups. Half-weight association indices were higher for males and for male-male dyads than for females, female-female and female-male dyads. Permutation test showed that the association patterns are not random, but failed to show any long-term (>1day) companionship, which was confirmed by standardized lagged association rates. Based on a weighted non-directed network males showed a higher strength, eigenvector centrality and reach than females. Using a directed network analysing the within-group position revealed that a few females possessed the highest strength and lower but similar strength for many males. The resemblance to competitive groups of humpback whales observed during the breeding season is compelling. However, our study was conducted outside the presumed breeding season of fin whales. Targeting large schools of fast preys could justify fin whales’ tendency to form larger groups, even though animals might also face scramble competition within their group. Female’s preference to be alone and if in groups, leading them, could indicate a tendency to reduce feeding competition due to their larger energetic demands.

Christian Ramp1 , Julien Delarue1 , Richard Sears1 , Marianne Marcoux2 , Martine Bérubé3 , Per Palsbøll3

(1) Mingan Island Cetacean Study, Quebec, Canada

(2) Arctic and Aquatic Research Division, Fisheries and Oceans Canada, Winnipeg, MB, Canada

(3) Marine Evolution and Conservation Centre for Ecological and Evolutionary Studies University of Groningen, The Netherlands