Direct estimation of within-group heterogeneity in photo-identification ofsperm whales

Heterogeneity in photo-identification rates among individuals is a potentia lly serious problem in many studies of cetacean biology, especially the ana lysis of populations. However, this heterogeneity is usually difficult to i dentify or measure. Two instances in which closed groups of female and imma ture sperm whales (Physeter macrocephalus) were tracked and identified usin g fluke photographs over periods of days off the Galapagos Islands allowed direct examination of heterogeneity in identification rates. A group of nin e animals followed in 1999 provided almost no evidence for heterogeneity (p ermutation test for heterogeneity, P = 0.48), with an estimated coefficient of variation in identification rates of 0.03 (95% CI from 1,000 bootstrap replications: 0.00-0.10). In contrast, the identification rates of a group of 22 animals followed in 1995 seemed to show potentially important differe nces (P = 0.058, CV = 0.20, 95% CI = 0.07-0.28). These differences were not related to the internal social structure of the group or to differences in numbers of markings on the flukes, but smaller whales had lower identifica tion rates. Thus, young sperm whales may be underrepresented in photo-ident ification studies, but adults within groups seem to have similar identifica tion rates. Situations in which animals are photo-identified from closed po pulations of known size are particularly useful For examining heterogeneity . They should be vigorously exploited by those who use photo-identification to examine population or behavioral biology.