Pod‐Specific Demography of Killer Whales (Orcinus Orca)

Abstract

Killer whales live in stable social groups, called "pods." It has been suggested that the structure of such groups may influence the vital rates, and hence the fitness, of their members. To test this suggestion we used data from a long—term study of killer whales in the Pacific Northwest (Bigg et al. 1990). We constructed stage—classified matrix population models for the entire population, two sub—populations, and individuals pods. The population growth rate for the entire population is λ = 1.0254, with 90% bootstrap confidence interval from 1.0178 to 1.0322. The mean female population stage distribution is not significantly different from the predicted stable stage distribution. Population growth rate is most sensitive to changes in adult and juvenile survival, followed by fertility Factors that cause even small changes in survival will thus have a large impact on population growth. Pod—specific growth rates range from λ = 0.9949 to λ = 1.0498. Most of the interpod variance in growth rate is due to variance in adult reproductive output. Randomization tests show that this variance is not significantly greater than expected on the basis of variation in individual life histories within the population. We conclude that there is no evidence for an effect of social structure on pod—specific population growth rate. The restriction of population growth rates to such a narrow range suggests, but does not prove, a possible role of density—dependent processes.