Abstract
The aim of the present study was to describe the characteristics of common bottlenose dolphins ( Tursiops truncatus ) group size and composition in the Cagarras Archipelago (23°01'S, 43°12'W), Rio de Janeiro, Brazil during a six year period study (2004 and 2006 to 2010). Group size (n = 51) ranged between three and 30 individuals (mean = 13.7 ± 7.1) and their frequency distribution showed two modes: one at the seven individuals group and the other at the 11 individuals group. The largest average group size occurred in 2004 (mean 21.4 ± 3.3) and the smallest in 2008 (mean 4.4 ± 0.8). The number of individuals/group decreased over the years which could be correlated with habitat quality indicating that common bottlenose dolphins could play a valuable role as sentry species. The average number of immature individuals (neonates, calves and juveniles) in relation to the total group size varied between 0.31 (2006) and 0.4 (2010) and differences between years were non significant. These findings suggest that Cagarras Archipelago should be considered as critical habitat for the survival and /or population growth of T. truncatus in southeastern Brazil.
Highlights
Gregarious species often form highly complex societies
During 51 (78.5%) of these censuses, groups of common bottlenose dolphins were sighted in the study area
Barbosa et al (2008) reported that in 18 observations made between June and December of 2004 and 2005 in the Cagarras Archipelago, the group size ranged from three – 20 individuals
Summary
Gregarious species often form highly complex societies. This societies offer increased protection, improve the utilization of available resources and allow the maximization of reproductive success (PARRISH; EDELSTEIN-KESHET, 1999).Animal groups may vary in form, size, extent and complexity. Dolphin group sizes in coastal areas may vary because the animals live in an open society characterized by fission-fusion, i.e., the grouping patterns change constantly in space and time (CONNOR et al, 2000). This social structure facilitates important learning abilities such as those related to feeding, reproduction, defense, and, communication (BRÄGER et al, 1994). The basic social unit consists of the following types of groups: 1) immature individuals, including subadult males and females and maturing males; 2) adult males (showing different levels of alliance formation); 3) adult females without calves and receptive females; and 4) pregnant females and females with calves (WELLS et al, 1987; WELLS, 1991; CONNOR et al, 2000 and references therein)
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