VARIATION IN THE VISUALLY OBSERVABLE BEHAVIOR OF GROUPS OF GALÁPAGOS SPERM WHALES1

Abstract

AbstractThe behavior of groups of female and immature sperm whales (Physeter macrocephalus) was measured on 117 d within an 11‐yr period off the Galápagos Islands, Ecuador. On each day, up to 18 measures of visually observable behavior were calculated. These concerned speeds, headings, movement patterns, diving synchrony, foraging formations, time spent socializing, and aerial behavior. The measured behavior of the sperm whales was considerably more variable when they were socializing than when foraging. None of the measures showed much correlation with sea‐surface temperature, and only measures of consistency of movement were significantly correlated with defecation rate, an indicator of feeding success. However, month‐long time periods accounted for over 50% of the variance in eight of eighteen measures, and, in the cases of surface speed and dive synchrony, the effects were statistically significant. In contrast, there was no autocorrelation with lag of one day in the residuals of any of the measures. Thus, behavior may be tracking substantial temporal variation in the whales' environment over scales of about several months. Groups of whales had significantly different travel patterns, but there was little other evidence for group‐specific behavior, perhaps because tests of group‐specific effects were not of adequate statistical power. Variation in sperm whale behavior, especially over time scales of a few months or longer and spatial scales of a few hundred kilometers or larger, should be considered when estimating densities from sighting surveys.