Orcas are easily identified by their striking black and white coloring. Other dolphins, such as Dahl's porpoises and false killer whales, show similar coloring but are significantly smaller.
Two features are used to identify individual whales; the gray saddle patch at the base of the dorsal fin, and nicks and scratches in the fin itself. Photographic records of these markers, together with observation of the animals over many years, have allowed scientists to assimilate genealogical charts for certain pods.
These markers have also been shown to be useful in discriminating between the three races of orca, with residents showing more rounded dorsal fins and mottled ('open') saddle patches, and transients displaying pointed dorsal fins and uniformly gray ('closed') saddle patches. Offshore orcas have dorsal fins that are rounded, yet they lack the sharp rear angle of the resident fins. Their saddle patch can be open or closed.
Adult males and females are relatively easy to distinguish by fin size. Male 'bulls' have a tall triangular dorsal fin up to 6 feet in height, while female orcas have a curved or sickle shaped dorsal fin 3 feet in height. Young males and female adults can often be confused as male fins take up to 20 years to grow (or 'sprout'). Eventually their height will be ten times that at birth.
Males are also larger than females; up to 32 feet and 11 tons compared to 28 feet and 7 tons for a female. Calves are just 7 feet at birth and weigh 400 pounds, growing to 10 feet in their first year. The flippers of adult males are long and broad and generally more pronounced than those of the female, and the tail flukes are curled downwards.
Males and females can also be easily distinguished by markings on their belly. Females have an oval genital patch marked with three black spots signifying a central vagina and two retractable nipples, whereas the male orca has an elongated white patch with a single black slit covering the penis. The other significant difference between males and females is that of longevity, with females living on average 50 years and males just about half that time. Some females have been estimated to live over 80 years.
Observations of the degree of respiratory synchrony between whales can also be used to infer relatedness between whales, since the blows of mother and calf remain tightly linked from birth.
Orcas can also be identified by their dialects, a collection of discrete calls that are attributable to certain pods and races. All transient whales share a common dialect - in fact this is the primary method of categorizing them. Dialects have also been used to create an evolutionary tree of resident pods, correlating the degree of similarity in their calls to how closely they are related.
