The following may not be suitable for children.
Vaquita porpoise, aka little cow porpoise, aka phocoena sinus, aka gulf of California harbor porpoise. They have a polygynous reproductive lifestyle meaning one male mates with multiple females. They have large testes to compensate for how many females they mate with. Since they are such solitary and shy animals there is little information about their courtship behavior. Females do not produce a calf every year since they have a long offspring creation period and still need to care for their young. Large testes will be selected for because the male with the largest testes can mate with the most females.
