"The sperm of rodents is at least twice as long as sperm of large mammals like primates, tigers, and even whales. We wanted to know why sperm sizes differ so dramatically when they all have the same task - to fertilize eggs", said lead researcher John Fitzpatrick from Stockholm University, Sweden.
John Fitzpatrick and his colleague Stefan Lupold, from the University of Zurich, examined how competition between sperm from different males and the size of the female reproductive tract influence the evolution of sperm in 100 mammalian species.
Their approach assessed both sperm length and the number of sperm in an ejaculate because the resources used to produce sperm must be shared between number and size.
In other words, the longer the sperm is, the less it can be produced.
"We show that males invest more in sperm size and number in species where sperm from multiple males compete to fertilize eggs compared to monogamous species," Fitzpatrick explained.
They also found that the size of the female influences whether it is better for a male to produce longer or more sperm.
In larger females, with bigger reproductive tracts, sperm can be lost or diluted on its journey to the egg. Therefore, in larger species males produce a lot of tiny sperm.
In contrast, in small species where sperm have an easier time finding the egg, males produce comparatively longer sperm.
"This demonstrates that the location where sperm compete - inside the female's reproductive tract in mammals - determines when it pays a male to produce longer or more sperm," the researchers said.
The study was published in the Proceedings of the Royal Society of London.