Got balls? Not like the Southern Right Whale!

In our world, we say size does not matter. However, we find animals in the wild kingdom with various large parts: elephants have large trunks, baboons have oversized butts … and male southern right whales have massive testes. This article will explain the unique features of whales’ reproductive systems and explore the potential reasons for the right whale’s bigger-than-average organs.

Southern right whales
A southern right whale with her calf — Credit: Lee Dingain

What makes whales’ reproductive systems unique

For females, the reproduction system is very similar to what humans have, apart from a few key differences. First, the right whale’s uterus is bicornuate, meaning it’s more heart-shaped than pear-shaped. It has a single cavity and the fetus will develop in only one of the uterine horns. Second, whales do not have their period! Instead of shedding their uterine walls, their body will reabsorb the lining. Finally, their vaginas have multiple folds (the shape and number depend on the species). Scientists think these folds prevent excess water from entering the vaginal area.

Here is a rendition of the differences between a male and female southern right whale — Credit: Naomi Mathew

The whale male reproductive system is completely different from humans’ because it is all internal! The main evolutionary advantages of having an internal reproductive system are better hydrodynamics and minimizing heat loss (since testes have a lot of blood vessels). Right whales’ pelvic bones are not used for movement but for penile erection and control. In fact, their penis originates from two fused structures attached to a vestigial pelvic bone.

Having their reproductive organs stay inside their body could lead to testes overheating. But whales have developed a cool trick. To keep their testicles cool they utilized a network of blood vessels known as the rete mirabile. This network consists of veins and arteries stacked very tight to promote countercurrent blood flow keeping certain body parts cool or warm.

“Came in like a Wrecking Ball…”

Testes come in all shapes and sizes in the animal kingdom. However, nothing can compare in size to the southern right whale’s testes. Each testicle can weigh up to 500kg- 900kg. Each! That’s about as heavy as a newborn gray whale or an adult horse! This weight also accounts for about 2% of an adult male southern right whale’s body weight. So why on earth would they need such massive testes? Scientists think it has to do with how southern right whales mate.

Unlike other whale species, male southern right whales do not aggressively compete with each other for a female’s attention, even though it is still not a walk in the park… Multiple males usually surround a female and rub themselves as close as they can to the female’s body. Using their long prehensile (movable) penis, they then try to mate with the female. Experts have reported that females can sometimes have up to 7 mates in one session! Scientists hypothesize that with larger testes, males can produce more sperm, allowing them to increase their chance to pass on their genes. Talk about a heavy load to carry!

Credit: Anaïs Remili

Sources and further reading

Did you enjoy this post? Check out our post about the North Atlantic right whales:

Naomi Mathew is a PhD student at University of Louisiana at Lafayette. She works on bioacoustics in marine mammals from the Gulf of Mexico. She is the co-founder of Whale Scientists. You can read more about her here

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