In famous triathlons epitomizing human endurance, such as Ironman or Norseman, competitors are undertaking nearly 150 minutes of running, swimming, and cycling in grueling conditions.
By analyzing almost 200 Ironman participants over two decades, Ryan Calsbeek – a professor of Biological Sciences at Dartmouth College and former Ironmen contestant himself – has found that performance in such competitions is not only driven by training and resilience, but also by how athletes’ physiques are adapted to shedding or retaining heat in specific climates.
According to Calsbeek, taller and leaner runners with long limbs tended to excel in Ironman events held in warmer climates, while athletes with stockier builds and shorter limbs were more likely to fare better in colder climates. Interestingly, these physiological links were detected in men, but not in women.
This study is among only a few connecting human physiology to what are known as Bergmann’s and Allen’s rules (named for the 19th century biologists Carl Bergmann and Joseph Asaph Allen), which argue that the overall body size of animals, along with the length and thickness of their limbs, correspond to climatic particularities.
More specifically, the rules state that, while cold-adapted species tend to be burlier with thicker and shorter limbs to avoid heat loss, species adapted to hotter climates are sleeker with thinner and longer limbs that help them keep cool efficiently.
Although these rules are generally accepted in the case of animals, research studying them in humans has been scarce, with only a few studies from mid-20th century suggesting a link between human physiology and climate.
In the current study, Calsbeek examined 171 triathletes who competed in at least two Ironmen events – one in a hot and one in a cold climate – during the past two decades. The wealth of available photos of these events allowed him to use software to measure competitors’ morphology based on pictures from the cycling part of the competition.
Moreover, since swimming and cycling are not evolutionary adaptations in humans, participants’ performance in these events could be weighed against their aptitude to running – an evolutionary adaptation – to clarify the influence of body type on performance.
“A triathlon is a great human system to study because there is one event, running in particular, that we know to be important in the evolution of humans and two events – swimming and cycling – that are not, so there’s a great natural control. Male triathletes exhibited the most significant difference in performance based on physiology and the effect was most prominent in the run,” Calsbeek explained.
In his view, the lack of a significant influence of physiology on the performance of women could be explained by evolution. “This effect is likely seen only in males because natural selection acted on running performance in males to be persistence hunters.”
These findings, published in the journal PLOS ONE, suggest that aspiring endurance male athletes should consider venues in climates for which their physiology is more naturally attuned.
“This study included a broad sample of athletes to account for motivation and training, but the link between performance and physiology was statistically significant in spite of those factors. People attempting a personal best time can think about race locations and average temperatures to pick a venue based on how their body type is adapted to perform,” Calsbeek concluded.
Humans have adapted to different climates and environments over thousands of years through both biological and cultural adaptations. Here are some examples:
These are physical changes in the human body that have evolved over generations to better suit particular environments.
Darker skin, which has more melanin, provides protection against strong UV radiation prevalent in equatorial regions. Lighter skin, found in populations in higher latitudes, allows for more UVB absorption, which is necessary for the production of vitamin D, especially in regions with less sunlight.
The Bergmann’s Rule and Allen’s Rule suggest that populations living in colder climates tend to have larger body masses and shorter limbs to minimize heat loss, while those in warmer climates have slimmer bodies and longer limbs to facilitate heat dissipation.
For instance, the Inuit people in the Arctic regions have stockier builds to conserve heat, while groups in the hot, arid climates of Africa and Australia have leaner bodies to stay cool.
Some populations have developed specific genetic traits to survive in certain environments. For example, some Tibetan people have genes that help them live at high altitudes, where oxygen levels are lower.
These include behavioral changes, inventions, and technologies that have enabled humans to survive in different climates.
In cold climates, people developed clothing using animal skins and fur for insulation. In hot, sunny climates, loose and light-colored clothing is often used to reflect sunlight and allow air circulation.
In hot desert climates, people have built houses with thick walls and small windows to keep out the heat, often from materials like mud-brick that have good insulating properties. In contrast, in colder climates, houses are designed to retain heat, such as the igloos of the Inuit people.
People in different climates have adapted to utilize the resources available to them. In Arctic climates, people rely heavily on hunting and fishing, while in fertile regions, agriculture is common.
Some societies, such as those in desert climates, developed nomadic lifestyles, moving from place to place in search of resources.
From the development of fire and tools to modern heating, air conditioning, and transportation, technological advancements have allowed humans to live in a wide range of climates.
These adaptations have enabled humans to populate almost every corner of the earth, from the hottest deserts to the coldest polar regions.
By Andrei Ionescu, Earth.com Staff Writer
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