Love In The Ocean

Curiosity About Sperm Whales

For an insight into the fascinating creature that is the sperm whale, how they interact, with each other and their environment, you have come to the right place!

By Patrizia Stipcich

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Sexual dimorphism (the systematic difference in form between individuals of different sex in the same specie) is very common in the underwater world.

Among the most sexually dimorphic of all cetaceans we find sperm whales, where a mature male can be 40% larger and three times heavier than a female. Size is not the only thing that highlights this dimorphism but also their behavior and way of life are different: while females live in groups with communal care of calves and they are more social than males, male sperm whales are solitary nomads that occupy water from the equator to the polar regions.

Females reach sexual maturity at 9 years, and they can conceive right after reaching the mature age. They normally reproduce once every five years, with 14-16 months of gestation. When it’s time to mate, the male has to cruise the open sea in the hope of meeting a group of females somewhere, one of which is ready to breed. 

Why is the sperm whale's head so big?

Looking for a partner to mate with is not an easy task for someone who is travelling 5000 km a year on average, but here comes the usefulness of the very big head that males have: this other important sign of sexual dimorphism has been used especially as acoustic organ. Through the echolocalization, sperm whales can easily communicate and find other individuals of the same species. When a sperm whale is feeding, it is constantly emitting clicking sounds to find prey, but in this way also another sperm whale can understand where the first one is located.

Thanks to the size of the spermaceti organ (organ contains a waxy liquid called spermaceti, involved in the generation of sound) and the junk, the sound production and processing is facilitated. The spermaceti organ in males can be up to 30% of its body (only 20% in females) and thanks to the size of it, males can produce very loud sounds (louder than those produced by females), used for attracting mates, repelling competitors and stunning prey.

The sound is firstly generated by phonic lips (called monkey’s lips) located at the upper part of the head and it deflects to the back of the skull through the spermaceti, where it bounces, passing through the junk into the sea (Fig. 1). The head full of fats and oils help to focus the sound. Interesting to know that the sounds can bounce back and forth within the skull and the time it takes to travel from the front to the back of the skull and then back again is directly related to the length of the nose, which might indicate a sexual success: the bigger the nose, the louder the sound, and the higher the probability to find a female and impress her with a loud tone of voice! In fact, females can assess the quality of potential mates from the power of the sound emitted by the males.

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For those who have always thought that the ocean is a silent place, this is an example of how this is not true. Nowadays, oceans are also far noisier that they were in the past: ship traffic, gas drilling offshore, increased naval activity relying on sonar, have generally increased and organisms like sperm whale that are sensitive to sounds may be negatively affected by sound pollution.

To see how these great creatures behave underwater, and to hear a snippet of what it sounds when they are looking for love, check out the video below, featuring Steve Backshall:

Fig. 1 Sperm whale’s nose anatomy. Eguiguren, A., Konrad Clarke, C. M., & Cantor, M. (2023). Sperm whale reproductive strategies: current knowledge and future directions. Sex in Cetaceans: Morphology, Behavior, and the Evolution of Sexual Strategies, 443-467.

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