Group structure of Guiana dolphins, Sotalia guianensis (Cetacea, Delphinidae) 

in Ilha Grande Bay, Rio de Janeiro, southeastern Brazil 

Abstract:

ABSTRACT.  Cetaceans  present  a  group  structure  of  great  complexity  and  display  a  wide  behavioral plasticity. Many efforts have been made to understand the group structures of the various species, however, this type of information is still lacking for some species. Therefore, our objectives were to 1) characterize the 

structure of the Sotalia guianensis groups in Ilha Grande Bay, Rio de Janeiro, Brazil, and 2) investigate how both  behavior  and  season  influence  the  group  structure  of  this  population.  This  species  is  considered  “data deficient” by the IUCN. We conducted 28 boat trips using group focal procedures, and a total of 1,314 groups were observed. Of these groups, 1,268 (94.4%) contained calves, the largest percentage ever reported for the species.  Groups  with  calves  were  larger  than  those  without  them,  suggesting  a  strategy  to  protect  these 

individuals with underdeveloped physiology. The mean group sizes reached 17.6 ± 18.3 individuals. Within these  groups,  we  observed  that  both  behavior  (H  =  112.5,  d.f.  =  2,  P  <  0.05)  and  season  (number  of simulations:  10,000;  sample  size  of  fall-winter  =  544;  sample  size  of  spring-summer  =  684;  P  <  0.05), demonstrated  a  statistically  significant  influence.  The  most  common  degree  of  cohesion  was  mixed,  and cohesion  also  varied  with  behavior  (χ²  =  10.1,  P  <  0.05)  and  season  (χ²  =  31.0,  P  <  0.05).  This  paper 

contributes towards understanding the highly variable nature of S. guianensis group dynamics. These data may be important in understanding the structure of groups in a site that is being increasingly impacted by different human activities. Moreover, this area contains the largest aggregation ever observed for this species and may therefore represent an important source of genetic diversity for the species as a whole. 

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Best regards,

Rodrigo