Motility is a biological term which is used in reference to the capability to move actively and spontaneously, while consuming energy. While motility can be used to describe most animals, it is usually used to refer to unicellular and simple multicellular organisms. This is why the term is used to refer to the ability of sperm to move correctly and successfully towards a female egg, thus fertilizing it. However, sperm motility is a term also used to describe the quality of sperm, meaning its competence in fertilizing eggs. No matter the quantity of the sperm, the motility is vital because a sperm that cannot swim all the way to an egg will not be able to fertilize it and thereby pregnancy will never occur. In layman’s terms, this is usually called lazy sperm, and it derives from the fact that the sperm lacks motility, or the motility is not enough for successful reproduction.
With mammals, sperm motility is also necessary in helping the sperms pass through cumulus oophorus and zona pellucida, which are a sort of layers that surround the ovulated egg-cells. An interesting example is the Apodemus sylvaticus, a wood mouse whose reproductive processes are, for lack or better words, surprising. Within the male wood mouse, the sperm aggregates shape mobile trains that are endowed with consolidated fertilization capabilities because they can traverse the female reproductive tract easier. Individual spermatozoa have little if any chance of fertilizing female eggs, seeing how it is very difficult to navigate through the viscous and gluey environment from the female reproductive area.
Therefore, a successful fertilization occurs when the sperm manages to go through all the obstacles in the female reproductive tract and reach the ovulated eggs and penetrate them. But like we’ve seen, there are several environments and substances that the sperm needs to overcome, such as extracellular matrixes surrounding the eggs. These characteristics differ, of course, from species to species. Sperm motility is dependant on a number of factors, the direct one being intracellular ion concentration. Varying from species to species, a certain value of ion concentration will signal the mechanisms and sperm motility will be activated.
Nevertheless, sperm motility plays a huge role in reproduction, and many men are affected by subfertility and infertility due to insufficient sperm motility. Luckily, there are medical treatments, sometimes surgery, which can help increase sperm motility and thus the chances for reproduction. Though this may seem strange, sometimes the cause of subfertility is actually abnormally-motile sperms, which thereby miss the target.