Social structure and residency in aggregations of male sperm whales

Abstract

Sperm whales (Physeter macrocephalus) are sexually dimorphic in several respects, including size, latitudinal distribution, and social structure. Females are known to have complex social structures, including long-term bonds, but the social structures of sexually mature or maturing males have received much less attention. Using data from aggregations of males off Norway, Nova Scotia, New Zealand, and the Galápagos Islands, we examined aggregation size, residence times within aggregations, clustering at the surface, and long-term bonds. Results were generally consistent among study areas. The aggregations found in each area contained around 10-30 males at any time, and were usually a few tens of kilometres across. Mean residence times within aggregations ranged from a few days to a few weeks. Close clustering at the surface was rare, but present at each site. There was no evidence for preferred companionship between individuals at any temporal scale in any of the study areas. The rarity of clustering and the apparent lack of long-term relationships amongst male sperm whales contrast strongly with results of studies on females, suggesting that both close spatial proximity at the surface and permanent bonds between individuals may be a consequence of the need for care of the young.Résumé : Les cachalots (Physeter macrocephalus) présentent un important dimorphisme sexuel, notamment en taille, en distribution latitudinale et en organisation sociale. Les femelles ont une organisation sociale très complexe qui inclut des associations à long terme entre individus, alors que l'organisation sociale des mâles adultes, ou subadultes, reste encore largement inconnue. En utilisant des données provenant de regroupements de mâles au large de la Norvège, de la Nouvelle Écosse, de la Nouvelle Zélande et des Îles Galápagos, nous avons examiné la taille des regroupements, la durée du séjour à l'intérieur de ces regroupement, la manière dont les individus se rassemblent à la surface et les associations à long terme entre individus. Les résultats étaient cohérants d'une région à l'autre. En général, les regroupements avaient quelques dizaines de kilomètres de diamètre et contennaient environ 10-30 mâles. La durée moyenne du séjour à l'intérieur de chacun de ces regroupements allait de quelques jours à quelques semaines. Les individus formaient rarement des groupes compacts de deux ou plusieurs individus séparés par moins de 100 m, mais de tels groupes ont été observés en surface dans chaque région. Rien dans nos données, dans aucune région, ni à aucune des échelles temporelles étudiées, ne laissait croire que certains individus puissent s'associer plus souvent avec tel mâle qu'avec tel autre. Le fait que les individus se regroupaient rarement à la surface et l'absence d'associations à long terme entre mâles présentent un contraste remarquable avec les résultats obtenus chez les femelles. Cela suggère que la proximité spatiale à la surface ainsi que les associations permanentes entre individus servent principalement à assure...

Improved abundance and trend estimates for sperm whales in the eastern North Pacific from Bayesian hierarchical modeling

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“…However, while a trend in total numbers is difficult to assess, there is a high probability that the number of animals traveling alone (or in some cases in pairs) has increased within the study area, the best estimate being more than a 2-fold increase from 1991 to 2008. These are likely to be reproductive adult males, known to roam individually or in very small groups, whereas females and young animals occur in larger familial groups, or in small bachelor herds in the case of younger pre-breeding adult males (Best 1979, Lettevall et al 2002, Whitehead 2003. Whether this trend reflects a population-level increase in adult male abundance or merely in creased use of the study area by adult males is not possible to say from the data.…”

Swimming gaits, passive drag and buoyancy of diving sperm whalesPhyseter macrocephalus

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Population genetics and social organization of the sperm whale (Physeter macrocephalus) in the Azores inferred by microsatellite analyses

“…The sperm whale (Physeter macrocephalus L., 1758) presents a highly complex social structure (Whitehead and Kahn 1992), which includes both stable groups tied with social bonds as well as temporary aggregations of such groups with possibly unrelated individuals (Lettevall et al 2002). The most common social group is the socalled "primary" social group, which is composed of adult females, sub-adults, and calves of both sexes.…”