Short legs, varied topography and locomotor efficiency

Science Daily has a nice story on an article in next month’s American Journal of Physical Anthropology on the relatively short legs of Neandertals. This feature has typically been viewed as an adaptation to the cold climate of Ice Age Europe; shorter legs have relatively less surface area, and therefore more efficiently retain heat. This new study, authored by JHU doctoral student Ryan Higgins, suggests this view might be overly simplified:

“Studies looking at limb length have always concluded that a shorter limb, including in Neandertals, leads to less efficiency of movement, because they had to take more steps to go a given distance,” says lead author Ryan Higgins, graduate student in the Johns Hopkins Center of Functional Anatomy and Evolution. “But the other studies only looked at flat land. Our study suggests that the Neandertals’ steps were not less efficient than modern humans in the sloped, mountainous environment where they lived.”

There are several nice things about this study. First, it highlights the way in which natural selection can simultaneously operate across several systems. Neandertal limbs might be relatively short because they live in a cold environment and a topographically varied terrain. Most patterns of variation we observe in biological system are the result of complex interactions and tradeoffs. This could be a nice illustration of that point.

Additionally, this study gets at one issue that has always nagged me about in-the-lab practical studies of locomotion. Such studies have added a great deal to what we know about the energetics and muscle activation and utilization during walking and running, but they are somewhat constrained in terms of the conditions they can study these activities. Most of this work involves movement on treadmills or across force plates, which limit the study designs to flat surfaces. As anyone who has spent time away from sidewalks or paved roads will know, flat surfaces are actually quite rare in the natural landscape and movement across non-flat surfaces can be quite different than movement across flat surfaces. Depending on the variation in the surface, you might plant your feet differently, stride differently, or utilize your upper body in different ways to ensure balance or move more quickly or efficiently. Since such conditions are hard to replicate in a lab, the authors of this study take the nice approach of looking at a natural experiment in the form of related species of bovids (i.e. goats, sheep, antelope…) that occupy different environments. Sure enough, bovids in more mountainous terrains have relatively shorter limbs, and based on their model, improve their locomotor efficiency.

As it is, this study suggests a plausible additional reason Neandertals might have shorter lower limbs. However it would be great to see a couple of follow-up studies to address this more specifically. It would be challenging work, but it would add a lot if someone went out and actually looked at how individuals in topographically varied terrain move through their environment. I had the experience of climbing Mt. Kilimanjaro a number of years ago, and although it is a very high mountain (19,320 feet), my hike up the mountain involved primarily movement across relatively flat surfaces. This is because as the slope of the mountain became more extreme, the number of switchbacks my path took became more frequent. In other words, just because the terrain is topographically varied does not mean an individual is going to spend their days taking strides up and down steep slopes. It would be great to see an empirical energetic study of human or non-human movements through mountainous terrain.

The second follow-up study I would like to see is one that takes into consideration ecological strategies of populations in mountainous or varied terrains. We know, for example, that Neandertals utilized different topographical regions on the landscape differently across different seasons. Here is a paper by Dan Adler on Middle and Upper Paleolithic populations from the Southern Caucasus on this point. This could potentially create significant differences in energetic demands throughout the year, and therefore change how natural selection might act on this trait. Human behavior is the ultimate mediator of the physical-environmental interaction that shapes our adaptive landscape. How we actually behave, on both an individual and population level, when faced with varied topographic landscapes would be great to know.

It is always a good study if it raises as many, or more, questions than it answers.

UPDATE: I see that Dienekes Pontikos also picks up on the story, wondering if climate does not explain the short legs of Neandertals, maybe it also fails to adequately explain the long legs of people living in the tropics. My bias, as stated above, is to think of it not as an either/or question, but rather as a question of which process, to which degree, under which conditions.

Also, Michigan graduate student Zach Cofran points me to this paper in PNAS from 2008, in which Serrat, King and Lovejoy suggest temperature may play a key role in regulating cartilage growth during development. So perhaps temperature may be an important mediator of the process, even if locomotor efficiency is the primary target for selection.

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1. Ryan W. Higgins, Christopher B. Ruff. The effects of distal limb segment shortening on locomotor efficiency in sloped terrain: Implications for Neandertal locomotor behavior. American Journal of Physical Anthropology, 2011; 146 (3): 336 DOI: 10.1002/ajpa.21575

2. Daniel S. Adler, Guy Bar-Oz. Seasonal Patterns of Prey Acquisition and Inter-group Competition During the Middle and Upper Palaeolithic of the Southern Caucasus. In, The Evolution of Hominin Diets, Jean-Jacques Hublin and Michael P. Richards (eds.). Springer Press, Netherlands. DOI: 10.1007/978-1-4020-9699-0_9