Males in a captive pygmy hippopotamus population may be able to adjust the ratio of X- and Y-chromosomes in their sperm, in favour of producing more female offspring, finds a study published in Nature Communications this week. The work reports that this could represent a way for the males to reduce competition for females from other males in this captive population. There is evidence in some animal populations of the ability to shift the birth sex ratio from the expected equal ratio of male and female offspring. Joseph Saragusty and colleagues studied a captive population of the endangered pygmy hippopotamus (Choeropsis liberiensis) and found an excess of females, with only approximately 42% of the offspring born male. This suggests that, while most previous sex ratio shifts have been thought to be due to female choice, the males in this population may also possess a way to actively change the sex ratio in their sperm. In this case, the authors suggest that it may be beneficial for the male to do this in order to reduce future competition for mating partners.