How To Sex A Sperm Whale

A few people have written me to ask how I know if a sperm whale is female or male.

In general, family units of sperm whales comprise mature females together with juvenile and baby whales. According to everything I've read about sperm whales, such family units tend to be found in temperate and tropical regions.

When little boy sperm whales get bigger and become young men, they leave these groups and travel to colder climates (north in the northern hemisphere, south in the southern), where they presumably go to eat a lot, hang out with the other boys, and get ready to return at some point to seek out mate(s).

By the time male sperm whales return to lower latitudes for reproduction, they're big. Absolutely humongous. Think of it this way:

Mature bull sperm whale: female or immature sperm whale
Sumo wrestler: figure skater

So from a practical point of view, when you see a big boy, there's no mistake.

It's also possible to differentiate by "looking under the skirt", so to speak, as pictured below:

female sperm whale with mammary glands visible
See the whale's mammary glands adjacent to the genital slit?

You can see two small mammary slits adjacent to the larger genital slit. Boy whales don't have those.

This is a pattern common to many (perhaps all?) cetaceans. Dolphins and humpback whales I've been in the water with, for example, have a similar morphology, making it easy to identify a given whale's sex...provided that they're not shy, of course.