Subject: ABSTRACT: Ecological and social determinants of group size in tr

Michael Williamson (
Tue, 4 Feb 1997 10:50:37 -0500 (EST)

Subject: ABSTRACT: Ecological and social determinants of group size in tr

Baird, R.W., and L.M. Dill. 1996. Ecological and social determinants
      of group size in transient killer whales. Behavioral Ecology


Most analyses of the relationship between group size and food intake
of social carnivores have shown a discrepancy between the group size
that maximizes energy intake and that which is most frequently
observed. Around southern Vancouver Island, British Columbia, killer
whales of the so-called transient form forage in small groups, and
appear to prey exclusively on marine mammals. Between 1986 and 1993,
in approximately 434 h of observations on transient killer whales, we
observed 138 attacks on 5 species of marine mammals. Harbor seals were
most frequently attacked (130 occasions), and the observed average
energy intake rate was more than sufficient for the whale's energetic
needs. Energy intake varied with group size, with groups of three
having the highest energy intake rate per individual. While groups of
three were most frequently encountered, the group size experienced by
an average individual in the population (i.e., typical group size) is
larger than three. However, comparisons between observed and expected
group sizes should utilize only groups engaged in the behavior of
interest. The typical size of groups consisting only of adult and
sub-adult whales that were engaged primarily in foraging activities
confirms that these individuals are found in groups that are
consistent with the maximization of energy intake hypothesis. Larger
groups may form for: 1) the occasional hunting of prey other than
harbor seals, for which the optimal foraging group size is probably
larger than three; and 2) the protection of calves and other social

Robin W. Baird, Ph.D.
Biology Department
Dalhousie University
Halifax, Nova Scotia
B3H 4J1 Canada
Phone (902) 494-3723
Fax (902) 494-3736