Some sperm cells are ruthless manipulators that will literally poison their competition in the race to fertilize an egg, new research shows.

In a study published Feb. 4 in the journal PLOS Genetics, researchers from the Max Planck Institute for Molecular Genetics (MPIMG) in Berlin studied mouse sperm cells under the microscope to better understand the effects of a particular DNA sequence known as the t-haplotype. The team knew from previous research that sperm cells carrying this sequence tend to swim straighter (rather than in circles of death) and faster on average than competing sperm without it.