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Social structure of the declining resident community of common bottlenose dolphins in the Sado Estuary, Portugal

Published online by Cambridge University Press:  03 August 2011

Joana F. Augusto ,
Patrícia Rachinas-Lopes  and
Manuel E. dos Santos
Joana F. Augusto*
Affiliation:
Unidade de Investigação em Eco-Etologia, ISPA—Instituto Universitário, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal Projecto Delfim—Centro Português de Estudo dos Mamíferos Marinhos, Rua Alto do Duque 45, 1400-009 Lisboa, Portugal
Patrícia Rachinas-Lopes
Affiliation:
Unidade de Investigação em Eco-Etologia, ISPA—Instituto Universitário, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal Projecto Delfim—Centro Português de Estudo dos Mamíferos Marinhos, Rua Alto do Duque 45, 1400-009 Lisboa, Portugal
Manuel E. dos Santos
Affiliation:
Unidade de Investigação em Eco-Etologia, ISPA—Instituto Universitário, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal Projecto Delfim—Centro Português de Estudo dos Mamíferos Marinhos, Rua Alto do Duque 45, 1400-009 Lisboa, Portugal
*
Correspondence should be addressed to: J.F. Augusto, Unidade de Investigação em Eco-Etologia, ISPA—Instituto Universitário, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal email: jaugusto@ispa.pt
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Abstract

The resident population of common bottlenose dolphins (Tursiops truncatus) in the Sado Estuary, Portugal, has been declining at least during the past three decades. A complete photographic census produced a current count of 24 animals—19 adults, three subadults and two calves. It appears to be phylopatric and essentially closed, but given the likely importance that exchanges with neighbouring coastal groups may play, even if rare, the most adequate term to define this dolphin should be community and not population. Large groups with all age-classes are common in the community, possibly as a calf and subadult protection strategy, and this may be related to the fact that these age-classes have had high mortality rates in the last decade. Maternity of two calves was determined, and we found that the two mothers adopted different parenting strategies. While one mother spent more time alone with her calf, the other mother spent more time with her calf in larger groups. The average coefficient of association for this community is 0.45, quite high for this species. Associations and typical group size are similar between all individuals, with no patterning according to age-class or sex, which constitutes an atypical trait for dolphin societies. There are also no clear divisions in this community according to cluster analysis. Associations are preferred and long term, lasting approximately 34 days and fitting a pattern of casual acquaintances, where individuals associate for a period of time, disassociate and may reassociate after that. This reflects the fission–fusion character of the community, but in a more stable manner. We think this is caused by a combination of demographic characteristics and a stable and productive environment, which led to a decrease in competition between individuals.

Keywords

social structurebottlenose dolphinsTursiops truncatusdemographic effectsfission–fusion dynamics
Type
Research Article
Information
Journal of the Marine Biological Association of the United Kingdom , Volume 92 , Issue 8: Marine Mammals , December 2012 , pp. 1773 - 1782
Copyright
Copyright © Marine Biological Association of the United Kingdom 2011

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