Assessment of the level of surface activity in humpback whales during the breeding season

Abstract

Between 1991 and 2002, the reproductive behavior of the humpback whale was studied during the breeding season off the coast of Ecuador, South America (02°S, 81°W). During the sighting period the level of surface activity exhibited by the whales was assessed according to the type of display performed and its frequency. Five displays that produce percussion sounds and used likely for communication were considered: breaching, head-lapping, tail-slapping, flipper-slapping and tail slashing. A total of 289 selected sightings were included in one of four levels according to a scale of intensity designed for the purpose. To determine what social conditions increased or reduced the activity, groups were organized according to their size and age-class composition. Singletons and groups of three and four whales showed a significantly higher surface activity than pairs or larger than four for the highest intensity level (P<0.05). Differences were also found when age-classes were compared, with subadults showing significantly higher surface activity than other classes (P<0.01). Groups that increased members showed a significant difference respect to decreasing groups at the lowest level of intensity of the scale (P<0.05). Competitive groups showed a significant higher occurrence at intensity level 2 than other groups (P<0.05). No relationship was found between whale speed and surface activity. The possible influence of environment factors such as day hour, sea state, moon phase and 'El Nino', was also assessed; none of which showed a clear trend to affect the intensity of whale surface activity. A discussion about the implications of these findings is provided as well as interpretations of possible meanings of some displays under different social contexts. This analysis indicates that social aspects rather than environment factors drive the surface activity of the humpback whales during the breeding season.