Why are pygmies so short?

The small size of many hunter-gatherers living in tropical rainforests is often attributed to their inhospitable habitat, which harbors little food for humans and multiple pathogens. But the adaptive benefits of a pygmy phenotype have not been clearly established. Research by IAST anthropologist and ecologist Vivek Venkataraman is the first to present empirical evidence that short people are better at walking in dense vegetation.

Struggling to keep pace with the Batek foragers through the rainforest of peninsular Malaysia, it wasn’t long before Vivek and his research colleagues noticed how the jungle landscape could quickly throw them off their stride. On a treadmill, shorter people spend more energy than taller people to walk a given distance. But Vivek’s team suspected that the unpredictable rainforest terrain might favor shorter legs by restricting step length. “People tend to walk according to a highly predictable speed-step length relationship,” Vivek explains. “The relationship between walking speed and cost of travel is U-shaped, and the speed at the minimum of this curve is necessarily higher for taller individuals. So we expected taller individuals to have higher preferred walking speeds in an open environment. In the rainforest, they would be forced to walk more slowly, while short individuals would be less constrained.”

The researchers developed a biomechanical model to reflect their hypothesis, before testing it among two rainforest forager populations: the Batek in Malaysia, and the Tsimane of the Bolivian Amazon. Their results show that shorter stature makes foraging more energy-efficient. “Whereas taller individuals took longer steps in open environments, all individuals were generally constrained to a similar (relatively small) step length in the forest. Individuals conformed to the preferred speed-step length relationship across environments, highlighting the applicability of biomechanical models to field settings. Most importantly, we show for the first time in ecologically relevant contexts that constraints on step length generate staturedependent walking speed costs that could make short stature evolutionarily beneficial among forest-dwelling humans.”

Speaking to Newsweek about his findings, Vivek insisted this is just the first step in understanding the benefits of being shorter in a rainforest environment: “There are many more avenues to investigate regarding human physiology in rainforests. For example, does small stature enable one to dissipate heat more effectively in a hot, humid, and windless rainforest? It should, on a theoretical basis, but the idea hasn’t been explored yet.” The researchers’ model could also provide insights about the evolution of short stature in other rugged habitats, such as the Highlands of Papua New Guinea; in other species, such as forest elephants and buffalos; and in at least one human relative, the Late Pleistocene short-statured hominin Homo floresiensis. “Our model could contribute to directional selection for small body-size phenotypes among legged animals whenever environmental constraints cause size-dependent reductions in locomotor performance. Future field studies of naturalistic foraging that combine accelerometry, inertial measurement units, and GPS will improve the study of locomotion in the field.”

 

>See https://www.iast.fr/people/vivek-venkataraman

From IAST Magazine #15, Winter 2019 (read the whole magazine)

 

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