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

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.

Strong bonds and small home range in a resident bottlenose dolphin community in a Marine Protected Area (Brittany, France, Northeast Atlantic)

“…, Augusto et al . ). Such social structure is uncommon for these species and may be shaped by resource availability (Lusseau et al .…”

“…, Augusto et al . ). Studying the social structure of additional bottlenose dolphin communities may help to better understand its drivers in this species.…”

“…Benefits of living in a group may therefore outweigh the cost, promoting strong social bonds between individuals and the use of a small core area as suggested for the Sado estuary bottlenose dolphin community (Augusto et al . ). In addition, winter conditions can be harsh as this open habitat is regularly exposed to strong storms and currents.…”

“…, Augusto et al . ) while the core home range is one of the smallest. Although no significant community division has been detected, both field observations and social structure analyses indicated one main social group composed of the majority of the individuals that were tightly associated (HWI ≥ 0.5) and individuals that were more solitary or observed in smaller groups.…”

A long‐term study of bottlenose dolphins (Tursiops aduncus) in an Australian industrial estuary: Increased sightings associated with environmental improvements

“…In some habitats, a demonstrable decline in dolphin abundance has been attributed to general environmental deterioration (Augusto et al . ) or more specifically, to tourism impacts (Bejder et al . , Tezanos‐Pinto et al .…”