Some species of marsupial males are having such intense sex during mating season—lasting up to 14 hours at a time and with as many partners as possible—that it’s resulting in death. Scientists studied 52 different species of small marsupials in Australia to take a closer look at the behavior. Because the females are so promiscuous, the males compete against each other for the best sperm, said lead author of the study Dr. Diana Fisher. The problem is the males become too stressed about reproducing and being a good pick that they’re working too hard during sex, and then they die.