Sex-biased summer association patterns in fin whales: An indication of male mate competition?

Christian Ramp, Julien Delarue, Anna Schleimer, Richard Sears, Per J. Palsbøll, Martine Berube

Abstract:

Baleen whales’ social behavior remains poorly studied, particularly outside the breeding season. We analysed 4,915 summer sightings of 2,608 groups involving 292 photo-identified fin whales (Balaenoptera physalus) in the Gulf of St. Lawrence (2004-2010).  Skin biopsies were collected from 85 males and 59 females. Solitary females were more common, and male sex bias increased with group size. Half-weight association indices confirmed the strong sex bias. Permutation tests showed that the association patterns were non-random, but did not reveal any long-term (>1day) associations. Based on the weighted non-directed network, males displayed a higher degree of centrality. A directed network analysis using within-group positioning revealed that most males have higher tendency to follow conspecifics, with only few males and females leading many of these groups. We employed skin biopsy samples of the 144 adults and 29 calves to estimate relatedness from the genotypes at 19 microsatellite loci. We identified a total of 34 parent-offspring, 13 full siblings and 1,183 2nd order relations. Four sampled males sired five calves, two by the same male. The high proportion of related animals could suggest that these fin whales preferentially mate within the Gulf and hence are not randomly mixing with other fin whales stocks in the Northwest Atlantic. This could represent a recent development due to the loss of persistent winter ice in the Gulf over the last decades, thereby opening a new wintering habitat for fin whales. Thus, the strong sex bias observed in the associations could be related to the species’ breeding behavior. Alternatively, cooperative feeding on large schools of fast preys might also explain the large groups, despite increasing competition among group members. Females’ tendency to be found alone might be due to the high energetic demands linked to reproduction and hence avoidance of intra-specific competition.