How to Tell the Gender of a Killer Whale by Its Marks

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If you've only seen orcas in marine parks, you might assume that whoever dreamed up the name "killer whale" to describe such friendly, playful, dazzlingly intelligent mammals didn't know them very well. In fact, Expert Robert L. Pitman of the U.S. National Marine Fisheries Service says that the orca's "killing power hasn't been rivaled since dinosaurs quit the earth 65 million years ago." Orcas are sexually dimorphic, meaning that adult males and females differ not only in the external appearance of their reproductive organs but also in many other ways.

Genital Markings

When killer whales jump into the air from the water, called breaching, they expose their undersides to view, thereby presenting one way for observers to distinguish males from females. Down towards the tail flukes, between the navel and the anus, a female orca has a white, oval-shaped genital patch with three black spots. The largest one, in the middle, is her vagina, and the two on either side of it are the mammary slits containing the nipples she uses for nursing her calves, which are retracted into her body when not in use. The genital area of the male orca is in the same region of his body but more elongated, with a single black slit covering his penis.

Other Differences Between Males and Females

Male orcas are much larger and heavier than females, growing as long as 32 feet and weighing 11 tons, while females grow up to 28 feet in length but tip the scales at 7 tons. The size of the dorsal fin is also a strong indication of gender. Adult males, called bulls, have dorsal fins about 6 feet high and triangular in shape, but the dorsal fins of females are only half as tall and more sickle-shaped than triangular. Since it takes up to 20 years for the dorsal fin of males to "sprout" to full size, a young male can be mistaken for an adult female. The flippers of adult males are also longer and broader than those of females.

"Race" Differences

Marine scientists have identified three distinct "races" of killer whales, naming them resident, transient and offshore orcas. An untrained eye might not notice many visible differences, but all three groups have subtly different markings and dorsal fin shapes, and even when they inhabit the same waters, they aren't known to hunt together, socialize or interbreed. In fact, a DNA analysis published in the July 2010, issue of "Genome Research" found that these groups began evolving in their own ways between 150,000 to 700,000 years ago. The study's authors conclude that these races are distinct enough from each other to be reclassified as separate species, but other scientists disagree, saying that the different types of orcas only warrant reclassification as subspecies.

Dialect Differences

When different countries speak the same language in different ways, we call the various versions dialects. Experts can't interpret what killer whales are saying when they communicate with each other, but they also describe regional variations in their vocalization patterns as dialects. Clicks are a type of sonar used mostly for navigation and locating prey, but orcas also click during social interactions. Not all the other sounds they make when communicating with each other, whistles and pulsed calls, are audible to human ears as anything except squeaks, squawks and screams, but that's because they're able to hear a greater range of frequencies than we can. Dialects not only vary regionally but also among different pods living in the same part of the ocean.

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