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Swimming against a current can be tough,

but imagine having to do it in a fluid with the consistency of corn syrup.

That's more or less the challenge faced by mammalian sperm as they race to reach an egg.

That's such a hard race for sperm.

It's like a very tough marathon.

Resa Nausradi is a mechanical engineer at Monash University in Melbourne.

He says that sperm are not deterred by tied or averted by viscosity.

In fact, Resa and his team have found that having to navigate the physiological slalom of the female

reproductive tract actually helps sperm swim with optimal efficiency and may guide them to their desired destination.

The results appear in the journal, Cell Reports Physical Science.

When you look at the female reproductive track,

I think it's been known for such a long time

that it's a very complicated environment.

As sperm swim upstream, they make their way through secretions that vary in thickness and flow rate.

One of the first hurdles comes at the cervix, the gateway to the uterus.

Right at the port of entry to the cervix, there is a very strong flow because of mucous

secretions by the cells within the cervix. That current helps to flush out any harmful

disease-causing bacteria, which are not known to be strong swimmers.

It's kind of like a barrier in a filter to keep the environment essentially favorable for fertilization.

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