In the animal world, which male marine mammal has the largest testicles, each weighing 500 kg, or 2% of the body weight?

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.