Through clever partner selection, animals can increase the future success of their offspring. With some species, this process continues even after the sex act. Researchers at the Max Planck Institute for Evolutionary Biology in Plön have discovered that among sticklebacks, the egg cells of the fish are involved in the decision regarding fertilisation. An accumulation of genes in the genetic material of the male gamete’s cells determines which sperm is allowed in to the egg cell. Since vertebrates are highly similar in terms of their immune system, including with regard to the gene complex in question, the researchers assume that egg cells of other vertebrates – perhaps even those of humans – are able to control their fertilisation.