Why do walruses have one of the biggest testicals in all mammals?

Walruses have some of the largest testicles among mammals to enhance reproductive success in their competitive breeding environment. Larger testicles produce more sperm, increasing the chances of successful fertilization during mating, especially when males compete for access to females. Additionally, the size may be a response to sperm competition, where multiple males mate with the same female, making it advantageous to produce a greater quantity of sperm. This adaptation helps ensure that a male walrus can effectively propagate his genes in a challenging social structure.