In a study that shows just how cool giraffes can get, researchers have tested a hypothesis that the giraffe's long neck actually helps regulate their body temperature.

At up to 5.8 metres tall (19 feet), giraffes are the tallest animal on land, thanks to their unusually long necks. But although their necks can measure up to 1.8 metres (6 feet) alone, they have, like most mammals, just seven neck vertebrae.

Fossil evidence shows that, once upon a time, giraffes had much shorter necks. But how and why they grew longer over millions of years is still a mystery.

There are two main camps. The first is what you're probably thinking - that a long neck helps a giraffe reach higher foliage than its competitors.

This idea has been around since 1809, when French naturalist Jean Baptiste Lamarck suggested that the giraffe's long neck evolved from its continual striving to reach food.

Lamarck's idea suggested they stretched their necks and passed the stretching down through generations. A modern genetic version of the idea suggests natural selection for better height and reach was at play.

But not all giraffes favour high foliage for grazing.

The other camp believes that the neck is sex-selected. Male giraffes compete for females by whacking each other with their necks, or "necking". The giraffe with the longer neck tends to be the winner in these fights, which means they're more likely to pass down their long neck genes.

But now we meet a different hypothesis. Giraffe necks help them regulate their body temperatures in hot African climates by increasing the surface area of their bodies, through which internal heat can escape.

The idea that giraffes' necks help them regulate their body heat is actually not particularly new. A. Brownlee put it forward as an idea in 1963.

Researcher Graham Mitchell of the University of Wyoming has been thinking about it at least since 2009, but now he and a team have put it to the test for the first time by looking at the body measurements of dozens of giraffes in Zimbabwe.

What they found was that a giraffe's surface area is about what you'd expect for any other animal of the same mass. However, the shape of their bodies could help them stay cool.

When you look at a giraffe from the front, its profile is long and narrow, or dolichomorphic.

"By having a dolichomorphic shape and pointing their heads towards the sun," the researchers wrote in their paper, "giraffes can reduce the proportion of body surface intercepting incident solar radiation to well below the proportion in a 'cylindrical' animal."

In other words, they can turn their "flat" sides away from the direct sunlight, reducing the amount of heat on their skin.

It's just one possibility, though, and gives another intriguing angle for ongoing study. But it's got plenty of study ahead of it to build its case, so for now, the jury is still out on the exact reason why the giraffe's neck is so long.

The paper has been published in the Journal of Arid Environments.