When two different species try to mate, they’ll almost always fail. Some, like horses and donkeys, will get lucky and produce hybrid offspring — but those mules are usually sterile. But for some worms ...

It may also be that when the worms live in small mating groups, competition favors the evolution of large male sperm, but when there are a lot of females around, smaller sperm do a better job of ...

Different species are usually unsuccessful at interbreeding; if they do, the hybrid offspring is usually sterile. In this way, species are kept separate and the diversity of life is maintained. In a ...

When it comes to sex, animals have developed a plethora of tricks to avoid mixups with other species. They use sight, smell, and even feel to tell each other apart. But in certain cases, the ...