Size of the Female Reproductive Tract Influences the Evolution of Sperm

Sperm size varies between animal species, but it has nothing to do with how big the animal is. Researchers have revealed that the sperm of rodents is at least twice as long as sperm of large mammals like primates, tigers, and even whales. John Fitzpatrick, associate senior lecturer at Stockholm University, said, "We wanted to know why sperm sizes differ so dramatically when they all have the same task - to fertilize eggs."

The researchers 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. This is important because the resources used to produce sperm must be shared between number and size. In other words - the longer sperm, the less can be produced.

Fitzpatrick said, "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."

The researchers have 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 their 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.

Fitzpatrick said, "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."

Source-Eurekalert