Sperm need to delay their frantic dash until they are close to the egg
A mechanism which starts sperm swimming when they get near the egg could one day lead to new forms of male contraception, scientists have said.
Tiny pores on the sperm's surface allow it to change its internal pH, which in turn starts its tail movements.
Researchers from the University of California in San Francisco said the study might also explain why marijuana makes men infertile.
A UK expert said it could have a big effect on men's fertility.
Sperm do not start swimming from the moment of ejaculation - they have only limited resources, and to stand much chance of reaching the egg, need to delay their frantic dash until they are closer to the egg.
Scientists have long known that a sperm's level of activity is governed by internal pH - how acid or alkaline its contents are - but the method which alters this at the right moment has proved elusive.
In order to increase its pH and become more alkaline, the sperm needs to jettison protons, and the US scientists have found pores on its surface which allow it to do precisely that.
Dr Yuriy Kirichok, who led the research, said: "The concentration of protons inside the sperm cell is 1,000 times higher than outside.
"If you just open a pore, protons will go outside - we identify the molecule that lets them out."
These pores, or Hv1 proton channels, as they have been termed, seem to be primed to open at precisely the right moment.
They respond to a substance called anandamide, which is present in the female reproductive tract, and in particularly high levels near the egg.
Anandamide is an "endocannabinoid", a natural substance which can affect neurons, but it is possible that the cannabinoids found in marijuana may mimic some of its effects - which could explain why cannabis use has been associated with subfertility in men.
Dr Kirichok said: "Marijuana likely activates sperm prematurely, leaving them burnt out in a matter of hours."
He said that it was possible that the newly discovered channel could be exploited by a drug which hampered the proton release, leaving sperm stranded.
He said: "All of these events are essential to fertilisation - you can imagine now that we know the molecule responsible we could block it to prevent activation and fertilisation as a kind of male contraception."
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: "Although it seems obvious that sperm have to swim in order to find and fertilise an egg inside the female body, or in the dish during IVF, we are only just uncovering some of the molecular details that the sperm use when swimming"
"Now that we know what this ion channel is, then it could lead us to either develop a novel contraceptive for men, or perhaps find a way to improve the sperm motility for men whose sperm don't swim as well as they should"