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Pygmies developed their small size to allow them to travel through the rainforest easily, researcher have claimed.

They say the small body size associated with the pygmy is probably a selective adaptation for rainforest hunter-gatherers.

However, they say that all African pygmys do not have the same genetic change, suggesting a more recent adaptation than previously thought.

A Ugandan pygmy hunts monkeys in Semili national park in Bundibugyo district. Researchers say that all African pygmys do not have the same genetic change, suggesting a recent adaptation.

'I'm interested in how rainforest hunter-gatherers have adapted to their very challenging environments,' said George H. Perry, assistant professor of anthropology and biology, Penn State.

'Tropical rainforests are difficult for humans to live in. It is extremely hot and humid with limited food, especially when fruit is not in season.'

Perry and colleagues looked at the genetics of the Batwa rainforest hunter-gatherers of Uganda and compared them to their farming neighbors, the Bakiga, with whom they traditionally traded forest products for grain, and sometimes intermarry. 

The researchers also looked at the Baka rainforest hunter-gatherers and their farming neighbors the Nzebi/Nzime in central Africa. 

They report their results in the Proceedings of the National Academies of Science.

The average height for Batwa men is five foot and for women it is four foot eight inches. 

Their short stature is not caused by a single genetic mutation as occurs in many forms of dwarfism, but is the result of a variety of genetic changes throughout the genome that influence height.

Researchers say the small body size associated with the pygmy is probably a selective adaptation for rainforest hunter-gatherers

The researchers investigated 16 different genetic locations that were associated with short stature when they looked at individuals who were an admixture of Batwa and Bakiga. 

Several of these regions contained genes known to be involved with growth in humans. 

They then studied these regions to look for indications that the changes were ones that persisted because they were adaptive. 

Short stature may be adaptive for rainforest individuals for a variety of reasons, according to Perry.

Small bodies require less food, which is adaptive for a food-limited location like the rainforest. 

Small bodies also generate less heat, which, in the heat and humidity of the rainforest, is adaptive. 

It is also easier for small, agile individuals to move through dense undergrowth and to climb trees.