Xiphophorus helleri (IMAGE) University of Würzburg Caption In several species of the genus Xiphophorus, the males carry a so-called 'sword'. Females prefer to mate with males that carry a long sword. Credit Georg Schneider / Uni Würzburg Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.