When it comes to reproduction, most fish are external fertilisers, crop dusting eggs in a cloud of sperm. But swordtails (Xiphophorus) aren't like most fish. These fish fertilise eggs internally and "give birth" to live young. To help this whole operation, males have evolved external genitalia for transferring sperm - a tool not typical among fish. Naturally, the next question would be - for swordtails at least - is bigger better? After all, they went through all the trouble of evolving penises in the first place. New research on the matter of female swordtail preferences towards their males' members provides an answer: Not necessarily. Yes, size is important, but so is how the males use it - and only when females are healthy enough to be in a discerning position.