Which whale species has the largest testes relative to body size?

The whale species with the largest testes relative to body size is the right whale. Right whales have exceptionally large reproductive organs: their testes can reach about 2 metres in length and weigh up to 525 kilograms, together accounting for nearly 1% of body mass.

This is remarkable because the blue whale is much larger overall, yet its testes are reported to be roughly one-tenth the size of those of a right whale. The extreme relative investment is linked to sperm competition, because females may mate with multiple males.

Right whales belong to the genus Eubalaena, which includes the North Atlantic, North Pacific, and southern right whales. A common mix-up is choosing the blue whale simply because it is the largest animal known; body size and relative reproductive anatomy are different comparisons.